Referent selection in case-crossover analyses of acute health effects of air pollution

The impact of future summer temperature on public health in Barcelona and Catalonia, Spain

Several epidemiological studies have reported associations between increases in summer temperatures and risks of premature mortality. The quantitative implications of predicted future increases in summer temperature, however, have not been extensively characterized. We have quantified these effects for the four main cities in Catalonia, Spain (Barcelona, Tarragona, Lleida, Girona). We first used case-crossover analysis to estimate the association between temperature and mortality for each of these cities for the period 1983 to 2006. These exposure-response (ER) functions were then combined with local measures of current and projected changes in population, mortality and temperature for the years 2025 and 2050. Predicted daily mean temperatures were based on the A1B greenhouse gas emission, "business-as-usual" scenario simulations derived from the ENSEMBLES project. Several different ER functions were examined and significant associations between temperature and mortality were observed for all four cities. For these four cities, the age-specific piecewise linear model predicts 520 (95%CI  340, 720) additional annual deaths attributable to the change in temperature in 2025 relative to the average from the baseline period of 1960-1990. For 2050, the estimate increases to 1,610 deaths per year during the warm season. For Catalonia as a whole, the point estimates for those two years are 720 and 2,330 deaths per year, respectively, or about 2 and 3% of the warm season. In comparing these predicted impacts with current causes of mortality, they clearly represent significant burdens to public health in Catalonia.

How air pollution influences clinical management of respiratory diseases. A case-crossover study in Milan

Background

Environmental pollution is a known risk factor for multiple diseases and furthermore increases rate of hospitalisations. We investigated the correlation between emergency room admissions (ERAs) of the general population for respiratory diseases and the environmental pollutant levels in Milan, a metropolis in northern Italy.

Methods

We collected data from 45770 ERAs for respiratory diseases. A time-stratified case-crossover design was used to investigate the association between air pollution levels and ERAs for acute respiratory conditions. The effects of air pollutants were investigated at lag 0 to lag 5, lag 0–2 and lag 3–5 in both single and multi-pollutant models, adjusted for daily weather variables.

Results

An increase in ozone (O₃) levels at lag 3–5 was associated with a 78% increase in the number of ERAs for asthma, especially during the warm season. Exposure to carbon monoxide (CO) proved to be a risk factor for pneumonia at lag 0–2 and in the warm season increased the risk of ERA by 66%. A significant association was found between ERAs for COPD exacerbation and levels of sulphur dioxide (SO₂), CO, nitrate dioxide (NO₂), and particulate matter (PM₁₀ and PM_2.5). The multipollutant model that includes all pollutants showed a significant association between CO (26%) and ERA for upper respiratory tract diseases at lag 0–2. For chronic obstructive pulmonary disease (COPD) exacerbations, only CO (OR 1.19) showed a significant association.

Conclusions

Exposure to environmental pollution, even at typical low levels, can increase the risk of ERA for acute respiratory diseases and exacerbation of obstructive lung diseases in the general population.

Short-term effects of ambient particulates and gaseous pollutants on the incidence of transient ischaemic attack and minor stroke: a case-crossover study

BACKGROUND

While several studies have investigated the effects of short-term air pollution on cardiovascular disease, less is known about its effects on cerebrovascular disease, including stroke and transient ischaemic attack (TIA). The aim of the study was to assess the effects of short-term variation in air pollutants on the onset of TIA and minor stroke.

METHODS

We performed secondary analyses of data collected prospectively in the North West of England in a multi-centre study (NORTHSTAR) of patients with recent TIA or minor stroke. A case-crossover study was conducted to determine the association between occurrence of TIA and the concentration of ambient PM10 or gaseous pollutants.

RESULTS

A total of 709 cases were recruited from the Manchester (n = 335) and Liverpool (n = 374) areas. Data for the Manchester cohort showed an association between ambient nitric oxide (NO) and risk of occurrence of TIA and minor stroke with a lag of 3 days (odds ratio 1.06, 95% CI: 1.01 - 1.11), whereas negative association was found for the patients from Liverpool. Effects of similar magnitude, although not statistically significant, were generally observed with other pollutants. In a two pollutant model the effect of NO remained stronger and statistically significant when analysed in combination with CO or SO2, but was marginal in combination with NO2 or ozone and non-significant with PM10. There was evidence of effect modification by age, gender and season.

CONCLUSIONS

Our data suggest an association between NO and occurrence of TIA and minor stroke in Greater Manchester.

The association between enterovirus 71 infections and meteorological parameters in Taiwan

Background

Enterovirus 71 (EV71) infections are a significant cause of neurological disorder and death in children worldwide. Seasonal variations in EV71 infections have been recognized, but the mechanisms responsible for this phenomenon remain unknown. The purpose of this study was to examine the relationship between meteorological parameters and EV71 infection.

Methods and Findings

We analyzed the number of EV71 infections and daily climate data collected in Taiwan between 1998 and 2008 and used Poisson regression analysis and case-crossover methodology to evaluate the association between weather variability and the incidence of EV71 infection. A total of 1,914 EV71-infected patients were reported between 1998 and 2008. The incidence of EV71 infections reflected significant summertime seasonality (for oscillation, p<0.001). The incidence of EV71 infections began to rise at temperatures above 13°C (r² = 0.76, p<0.001); at temperatures higher than approximately 26°C (r² = 0.94, p<0.05), the incidence began to decline, producing an inverted V-shaped relationship. The increase in the incidence with increasing relative humidity was positive and linear (r² = 0.68, p<0.05). EV71 infection was most highly correlated with temperature and relative humidity in the period that likely preceded the infection.

Conclusion

Our study provides quantitative evidence that the rate of EV71 infection increased significantly with increasing mean temperature and relative humidity in Taiwan.

Winter temperature inversions and emergency department visits for asthma in Salt Lake County, Utah, 2003-2008

BACKGROUND

Winter temperature inversions-layers of air in which temperature increases with altitude-trap air pollutants and lead to higher pollutant concentrations. Previous studies have evaluated associations between pollutants and emergency department (ED) visits for asthma, but none have considered inversions as independent risk factors for ED visits for asthma.

OBJECTIVE

We aimed to assess associations between winter inversions and ED visits for asthma in Salt Lake County, Utah.

METHODS

We obtained electronic records of ED visits for asthma and data on inversions, weather, and air pollutants for Salt Lake County, Utah, during the winters of 2003 through 2004 to 2007 through 2008. We identified 3,425 ED visits using a primary diagnosis of asthma. We used a time-stratified case-crossover design, and conditional logistic regression models to calculate odds ratios (ORs) and 95% confidence intervals (CIs) to estimate rate ratios of ED visits for asthma in relation to inversions during a 4-day lag period and prolonged inversions. We evaluated interactions between inversions and weather and pollutants.

RESULTS

After adjusting for dew point and mean temperatures, the OR for ED visits for asthma associated with inversions 0-3 days before the visit compared with no inversions during the lag period was 1.14 (95% CI: 1.00, 1.30). The OR for each 1-day increase in the number of inversion days during the lag period was 1.03 (95% CI: 1.00, 1.07). Associations were only apparent when PM10 and maximum and mean temperatures were above median levels.

CONCLUSIONS

Our results provide evidence that winter inversions are associated with increased rates of ED visits for asthma.

Heat-related mortality in the Florentine area (Italy) before and after the exceptional 2003 heat wave in Europe: an improved public health response?

High ambient temperatures have been associated with increased mortality across the world. Several studies suggest that timely preventive measures may reduce heat-related excess mortality. The main aim of this study was to detect the temporal modification of heat-related mortality, in older adults (aged 65-74) and in elderly ≥75 years old, in the Florentine area by comparing previous (1999-2002) and subsequent (2004-2007) periods to the summer of 2003, when a regional Heat-Health Warning System (HHWS) was set up. Mortality data from 1999 to 2007 (May-September) were provided by the Mortality Registry of the Tuscany Region (n = 21,092). Weather data were used to assess daily apparent temperatures (AT). Case-crossover time-stratified designs and constrained segmented distributed lag models were applied. No significant heat-related mortality odds ratio (OR) variations were observed among the sub-periods. Nevertheless, a general OR decrease dating from 1999-2002 (OR 1.23; lack of HHWS) to 2004-2005 (OR 1.21; experimental HHWS running only for Florence) and to 2006-2007 (OR 1.12; official HHWS extended to the whole Florentine area) was observed when the maximum AT was considered. This modification was only evident in subjects ≥75 years old. The heat effect was higher and sustained for more days (until lag 9) during the period 1999-2002 than 2004-2007. The decrease of the excessive heat effect on mortality between periods with the absence and existence of a HHWS is also probably due to the mitigation of preventive measures and the implementation of a HHWS with specific interventions for safeguarding the health of the "frail elderly".

A systematic review and meta-analysis of alcohol consumption and injury risk as a function of study design and recall period

BACKGROUND

It is well established that alcohol consumption is associated with an increased risk of injury. This systematic review and meta-analysis addresses important methodological issues commonly encountered in the alcohol and injury field by delineating the effect of study design and alcohol consumption recall period on effect size magnitude and by conducting gender-specific analyses.

METHODS

We performed meta-analyses using random-effect models. Data sources were peer-reviewed studies on alcohol and injury from 1970 to 2009 from MEDLINE, PsychInfo, and on-line journals. Case-control or case-crossover emergency department (ED) studies reporting injury risk from alcohol consumption 6 hours before injury were included.

RESULTS

The overall odds of injury were 2.799 (2.214 to 3.538, p < 0.001). For case-crossover studies, the odds were 3.815 (2.646 to 5.499, p < 0.001); for ED case-control studies, the odds were 1.977 (1.385 to 2.821, p < 0.001); and for population case-control designs, the odds were 3.145 (1.583 to 6.247, p < 0.005). The "usual frequency" recall period yielded an odds ratio of 4.235 (2.541 to 7.057, p < 0.001), compared to 2.320 (1.789 to 3.008, p < 0.001) for all other methods. There were significant differences in odds ratio magnitude when comparing studies by design and recall period. Females had higher odds of injury than males, 2.285 (1.361 to 3.836, p < 0.005) versus 1.071 (0.715 to 1.605, p = 0.737).

CONCLUSIONS

Study design and alcohol consumption recall period have significant effects on effect size magnitude in estimating the risk of injury from alcohol consumption 6 hours prior to injury. For the "usual frequency" case-crossover design, significant moderator effects were found, resulting in overestimates of injury risk from alcohol. ED case-crossover designs tend to overestimate risk, and ED case-control designs tend to underestimate. We provide recommendations for future ED research.

Seasonal modification of the association between temperature and adult emergency department visits for asthma: a case-crossover study

BACKGROUND

The objective of this study is to characterize the effect of temperature on emergency department visits for asthma and modification of this association by season. This association is of interest in its own right, and also important to understand because temperature may be an important confounder in analyses of associations between other environmental exposures and asthma. For example, the case-crossover study design is commonly used to investigate associations between air pollution and respiratory outcomes, such as asthma. This approach controls for confounding by month and season by design, and permits adjustment for potential confounding by temperature through regression modeling. However, such models may fail to adequately control for confounding if temperature effects are seasonal, since case-crossover analyses rarely account for interactions between matching factors (such as calendar month) and temperature.

METHODS

We conducted a case-crossover study to determine whether the association between temperature and emergency department visits for asthma varies by season or month. Asthma emergency department visits among North Carolina adults during 2007-2008 were identified using a statewide surveillance system. Marginal as well as season- and month-specific associations between asthma visits and temperature were estimated with conditional logistic regression.

RESULTS

The association between temperature and adult emergency department visits for asthma is near null when the overall association is examined [odds ratio (OR) per 5 degrees Celsius = 1.01, 95% confidence interval (CI): 1.00, 1.02]. However, significant variation in temperature-asthma associations was observed by season (chi-square = 18.94, 3 degrees of freedom, p <0.001) and by month of the year (chi-square = 45.46, 11 degrees of freedom, p <0.001). ORs per 5 degrees Celsius were increased in February (OR = 1.06, 95% CI: 1.02, 1.10), July (OR = 1.16, 95% CI: 1.04, 1.29), and December (OR = 1.04, 95% CI: 1.01, 1.07) and decreased in September (OR = 0.92, 95% CI: 0.87, 0.97).

CONCLUSIONS

Our empirical example suggests that there is significant seasonal variation in temperature-asthma associations. Epidemiological studies rarely account for interactions between ambient temperature and temporal matching factors (such as month of year) in the case-crossover design. These findings suggest that greater attention should be given to seasonal modification of associations between temperature and respiratory outcomes in case-crossover analyses of other environmental asthma triggers.

Seasonality of viral infections: mechanisms and unknowns

Seasonality is a long-recognized attribute of many viral infections of humans, but the mechanisms underlying seasonality, particularly for person-to-person communicable diseases, remain poorly understood. Better understanding of drivers of seasonality could provide insights into the relationship between the physical environment and infection risk, which is particularly important in the context of global ecological change in general, and climate change in particular. In broad terms, seasonality represents oscillation in pathogens' effective reproductive number, which, in turn, must reflect oscillatory changes in infectiousness, contact patterns, pathogen survival, or host susceptibility. Epidemiological challenges to correct identification of seasonal drivers of risk include failure to adjust for predictable correlation between disease incidence and seasonal exposures, and unmeasured confounding. The existing evidence suggests that the seasonality of some enteric and respiratory viral pathogens may be driven by enhanced wintertime survival of pathogens, and also by increased host susceptibility resulting from relative 'wintertime immune suppression'. For vector-borne diseases and zoonoses, environmental influences on vector or reservoir abundance, and vector biting rates, are probably more important. However, numerous areas of uncertainty exist, making this an exciting area for future research.

Assessing the short-term effects of heatwaves on mortality and morbidity in Brisbane, Australia: comparison of case-crossover and time series analyses

Background

Heat-related impacts may have greater public health implications as climate change continues. It is important to appropriately characterize the relationship between heatwave and health outcomes. However, it is unclear whether a case-crossover design can be effectively used to assess the event- or episode-related health effects. This study examined the association between exposure to heatwaves and mortality and emergency hospital admissions (EHAs) from non-external causes in Brisbane, Australia, using both case-crossover and time series analyses approaches.

Methods

Poisson generalised additive model (GAM) and time-stratified case-crossover analyses were used to assess the short-term impact of heatwaves on mortality and EHAs. Heatwaves exhibited a significant impact on mortality and EHAs after adjusting for air pollution, day of the week, and season.

Results

For time-stratified case-crossover analysis, odds ratios of mortality and EHAs during heatwaves were 1.62 (95% confidence interval (CI): 1.36–1.94) and 1.22 (95% CI: 1.14–1.30) at lag 1, respectively. Time series GAM models gave similar results. Relative risks of mortality and EHAs ranged from 1.72 (95% CI: 1.40–2.11) to 1.81 (95% CI: 1.56–2.10) and from 1.14 (95% CI: 1.06–1.23) to 1.28 (95% CI: 1.21–1.36) at lag 1, respectively. The risk estimates gradually attenuated after the lag of one day for both case-crossover and time series analyses.

Conclusions

The risk estimates from both case-crossover and time series models were consistent and comparable. This finding may have implications for future research on the assessment of event- or episode-related (e.g., heatwave) health effects.

Susceptibility to air pollution effects on mortality in Seoul, Korea: a case-crossover analysis of individual-level effect modifiers

Air pollution's mortality effects may differ by subpopulation; however, few studies have investigated this issue in Asia. We investigated susceptibility to air pollutants on total, cardiovascular, and respiratory mortality in Seoul, Korea for the period 2000-2007. We applied time-stratified case-crossover analysis, which allows direct modeling of interaction terms, to estimate susceptibility based on sex, age, education, marital status, and occupation. An interquartile range increase in pollution was associated with odds ratios of 0.94 (95% confidence interval, 0.25-1.62), 2.27 (1.03-3.53), 1.94 (0.80-3.09), and 2.21 (1.00-3.43) for total mortality and 1.95 (0.64-3.27), 4.82 (2.18-7.54), 3.64 (1.46-5.87), and 4.32 (1.77-6.92) for cardiovascular mortality for PM(10), nitrogen dioxide (NO(2)), sulfur dioxide (SO(2)), and carbon monoxide (CO), respectively. Ozone effect estimates were positive, but not statistically significant. Results indicate that some populations are more susceptible than others. For total or cardiovascular mortality, associations were higher for males, those 65-74 years, and those with no education or manual occupation for some pollutants. For example, the odds ratio for SO(2) and cardiovascular mortality was 1.19 (1.03-1.37) times higher for those with manual occupations than professional occupations. Our findings provide evidence that some populations are more susceptible to the effects of air pollution than others, which has implications for public policy and risk assessment for susceptible subpopulations.

Heat waves and cause-specific mortality at all ages

BACKGROUND

Mortality has been shown to increase with extremely hot ambient temperatures. Details on the specific cause of mortality can be useful for improving preventive policies. Infants are often identified as a population that is vulnerable to extreme heat conditions; however, information on heat and infant mortality is scarce, with no studies reporting on cause-specific mortality.

METHODS

The study includes all deaths in the Catalonia region of Spain during the warm seasons of 1983-2006 (503,389 deaths). We used the case-crossover design to evaluate the association between the occurrence of extremely hot days (days with maximum temperature above the 95th percentile) and mortality. Total mortality and infant mortality were stratified into 66 and 8 causes of death, respectively.

RESULTS

Three consecutive hot days increased total daily mortality by 19%. We calculated that 1.6% of all deaths were attributable to heat. About 40% of attributable deaths did not occur during heat-wave periods. The causes of death that were increased included cardiovascular and respiratory diseases, mental and nervous system disorders, infectious and digestive system diseases, diabetes, and some external causes such as suicide. In infants, the effect of heat was observed on the same day and was detected only for conditions originating in the perinatal period (relative risk = 1.53 [95% confidence interval = 1.16-2.02]). Within the perinatal causes, cardiovascular, respiratory, digestive system, and hemorrhagic and hematologic disorders were the causes of death with stronger effects.

CONCLUSIONS

Heat contributes to an increase in mortality from several causes. In infants, the first week of life is the most critical window of vulnerability.

Short-term exposure to air pollution and incidence of stroke and acute myocardial infarction in a Japanese population

BACKGROUND

Exposure to high levels of air pollution can increase the risk of cardiovascular events. However, there is no clear information in Japan on the effect of pollution on the incidence of stroke and acute myocardial infarction (AMI). Therefore, we investigated the effects of air pollution on the incidence of stroke and AMI in a setting where pollutant levels are rather low.

METHODS

Data were obtained from the Takashima Stroke and AMI Registry, which covers a population of approximately 55,000 in Takashima County in central Japan. We applied a time-stratified, bidirectional, case-crossover design to estimate the effects of air pollutants, which included suspended particulate matter (SPM), sulfur dioxide (SO(2)), nitrogen dioxide (NO(2)) and photochemical oxidants (Ox). We used the distributed lag model to estimate the effect of pollutant exposure 0-3 days before the day of event onset and controlled for meteorological covariates in all of the models.

RESULTS

There were 2,038 first-ever strokes (1,083 men, 955 women) and 429 first-ever AMI cases (281 men, 148 women) during 1988-2004. The mean pollutant levels were as follows: SPM 26.9 μg/m(3); SO(2) 3.9 ppb; NO(2) 16.0 ppb, and Ox 28.4 ppb. In single-pollutant and two-pollutant models, SO(2) was associated with the risk of cerebral hemorrhage. Other stroke subtypes and AMI were not associated with air pollutant levels.

CONCLUSIONS

We observed an association between SO(2) and hemorrhagic stroke; however, we found inconclusive evidence for a short-term effect of air pollution on the incidence of other stroke types and AMI.

Effects of diurnal temperature range on cardiovascular and respiratory hospital admissions in Korea

The effects of heat and cold waves have been studied as risk factors for cardiovascular and respiratory diseases. However, few studies have examined the effect of diurnal temperature changes on health. We hypothesized that the diurnal temperature range (DTR) may affect the rate of hospital admissions for cardiovascular- and respiratory-related diseases, and therefore investigated the risk of hospital admissions of cardiovascular (stroke, myocardial infarction, ischemic heart disease, cardiac failure, cardiac disease, and arrhythmia) and respiratory (asthma, chronic obstructive pulmonary disease, and pneumonia) diseases attributable to DTR in four metropolitan areas in Korea during 2003-2006. The area-combined effects of DTR on some cardiovascular and respiratory diseases were significantly increased by an increment of DTR. In particular, the effects on cardiac failure and asthma were significant with the percentage change of hospital admissions per 1 °C increment of DTR at 3.0% (95% CI, 1.4-4.6) and 1.1% (95% CI, 0.1-2.0), respectively, among 9 diseases. For those 75 years and older, the DTR effect on asthma admissions was greater than in those aged under 75 years. These results support the hypothesis of a positive association between DTR and cardiovascular and respiratory hospital admission.

Air pollution and hospital admissions for myocardial infarction: are there potentially sensitive groups?

Recent studies showed that air pollution is a risk factor for hospitalization for myocardial infarction (MI). However, there is limited evidence to suggest which subpopulations are at higher risk for MI arising from air pollution. This study was undertaken to examine the modifying effects of specific secondary cardiovascular diagnosis (including hypertension, diabetes, congestive heart failure, and arrhythmias) on the relationship between hospital admissions for MI and exposure to ambient air pollutants. Hospital admissions for MI and ambient air pollution data for Taipei were obtained for the period 1999-2009. The relative risk of hospital admissions for MI was estimated using a case-crossover approach. None of the secondary diagnosis examined showed significant evidence of effect modification. It would appear that the correlation between air pollutant exposure and MI occurrence is not affected by predisposing factors present in other cardiovascular diseases.

The effects of particulate matter sources on daily mortality: a case-crossover study of Barcelona, Spain

BACKGROUND

Dozens of studies link acute exposure to particulate matter (PM) air pollution with premature mortality and morbidity, but questions remain about which species and sources in the vast PM mixture are responsible for the observed health effects. Although a few studies exist on the effects of species and sources in U.S. cities, European cities-which have a higher proportion of diesel engines and denser urban populations-have not been well characterized. Information on the effects of specific sources could aid in targeting pollution control and in articulating the biological mechanisms of PM.

OBJECTIVES

Our study examined the effects of various PM sources on daily mortality for 2003 through 2007 in Barcelona, a densely populated city in the northeast corner of Spain.

METHODS

Source apportionment for PM ≤ 2.5 μm and ≤ 10 µm in aerodynamic diameter (PM2.5 and PM10) using positive matrix factorization identified eight different factors. Case-crossover regression analysis was used to estimate the effects of each factor.

RESULTS

Several sources of PM2.5, including vehicle exhaust, fuel oil combustion, secondary nitrate/organics, minerals, secondary sulfate/organics, and road dust, had statistically significant associations (p < 0.05) with all-cause and cardiovascular mortality. Also, in some cases relative risks for a respective interquartile range increase in concentration were higher for specific sources than for total PM2.5 mass.

CONCLUSIONS

These results along with those from our multisource models suggest that traffic, sulfate from shipping and long-range transport, and construction dust are important contributors to the adverse health effects linked to PM.

Quantifying the health impacts of future changes in temperature in California

BACKGROUND

Several epidemiological studies demonstrate associations between high summer temperatures and increased mortality. However, the quantitative implications of projected future increases in temperature have not been well characterized.

OBJECTIVE

This study quantifies the effects of projected future temperatures on both mortality and morbidity in California, including the potential effects of mitigation.

DATA AND METHODS

We first estimated the association between temperature and mortality for populations close to weather stations throughout the state. These dose-response estimates for mortality were then combined with local measures of current and projected changes in population, and projected changes in temperature, using a baseline of average temperatures from 1961 to 1990, for the years 2025 and 2050. The latter were based on two greenhouse gas emissions scenarios (A2 and B1) developed for the Intergovernmental Panel on Climate Change. In addition, we assessed the impacts of future adaptation through use of air conditioners. Several sensitivity analyses were conducted to determine the likely range of estimates.

RESULTS

These analyses indicate that for the high emissions scenario, the central estimate of annual premature mortality ranges from 2100 to 4300 for the year 2025 and from 6700 to 11,300 for 2050. The highest estimates are from the models that use age-specific dose-response functions, while the low estimates are from the models that adjust for ozone. Estimates using the low emissions scenario are roughly half of these estimates. Mitigation based on our estimates of the effects of 10% and 20% increase in air conditioner use would generate reductions of 16% and 33% in the years 2025 and 2050, respectively.

CONCLUSION

Our estimates suggest significant public health impacts associated with future projected increases in temperature.

Saharan dust and associations between particulate matter and daily mortality in Rome, Italy

BACKGROUND

Outbreaks of Saharan-Sahel dust over Euro-Mediterranean areas frequently induce exceedances of the Europen Union's 24-hr standard of 50 μg/m3 for particulate matter (PM) with aerodynamic diameter ≤ than 10 μm (PM10).

OBJECTIVES

We evaluated the effect of Saharan dust on the association between different PM fractions and daily mortality in Rome, Italy.

METHODS

In a study of 80,423 adult residents who died in Rome between 2001 and 2004, we performed a time-series analysis to explore the effects of PM2.5, PM2.5-10, and PM10 on natural, cardiac, cerebrovascular, and respiratory mortality. We defined Saharan dust days by combining light detection and ranging (LIDAR) observations and analyses from operational models. We tested a Saharan dust-PM interaction term to evaluate the hypothesis that the effects of PM, especially coarse PM (PM2.5-10), on mortality would be enhanced on dust days.

RESULTS

Interquartile range increases in PM2.5-10 (10.8 μg/m3) and PM10 (19.8 μg/m3) were associated with increased mortality due to natural, cardiac, cerebrovascular, and respiratory causes, with estimated effects ranging from 2.64% to 12.65% [95% confidence interval (CI), 1.18-25.42%] for the association between PM2.5-10 and respiratory mortality (0- to 5-day lag). Associations of PM2.5-10 with cardiac mortality were stronger on Saharan dust days (9.73%; 95% CI, 4.25-15.49%) than on dust-free days (0.86%; 95% CI, -2.47% to 4.31%; p = 0.005). Saharan dust days also modified associations between PM10 and cardiac mortality (9.55% increase; 95% CI, 3.81-15.61%; vs. dust-free days: 2.09%; 95% CI, -0.76% to 5.02%; p = 0.02).

CONCLUSIONS

We found evidence of effects of PM2.5-10 and PM10 on natural and cause-specific mortality, with stronger estimated effects on cardiac mortality during Saharan dust outbreaks. Toxicological and biological effects of particles from desert sources need to be further investigated and taken into account in air quality standards.

A time-stratified case-crossover study of fine particulate matter air pollution and mortality in Guangzhou, China

OBJECTIVES

Few studies in China investigate health impact of fine particulate matter (PM(2.5)) due to lack of monitoring data and the findings are controversial. The aim of this study is to examine the short-association between PM(2.5) and daily mortality in Guangzhou, the economic center of south China.

METHODS

In Guangzhou, we measured daily PM(2.5) concentrations between 2007 and 2008 and conducted a time-stratified case-crossover analysis to explore the association between PM(2.5) and daily mortality, and examine potential effect modifiers including age, sex, and education.

RESULTS

The averaged PM(2.5) concentration in 2007-2008 was 70.1 μg/m(3) in Guangzhou, which was approximately seven times higher than the WHO Air Quality Guidelines for PM(2.5) (annual average: 10 μg/m(3)). Regression analysis showed that ambient PM(2.5) was associated with mortality from all causes and cardiorespiratory diseases. An increase of 10 μg/m(3) in 2-day moving average (lag01) concentration of PM(2.5) corresponds to 0.90% [95% confidence interval (CI): 0.55, 1.26%] increase of total mortality, 1.22% (95% CI: 0.63, 1.68%) increase of cardiovascular mortality, and 0.97% (95% CI: 0.16, 1.79%) increase of respiratory mortality. The associations were stronger in the elderly (aged 65 years or more), in females, and in those with low education level, but the differences were statistically insignificant. After adjustment for nitrogen dioxide (NO(2)), however, the effects of PM(2.5) decreased and became statistically insignificant.

CONCLUSIONS

Our findings provided new information for the adverse health effects of PM(2.5) in China, and may have some implications for environmental policy making and standard setting in Guangzhou.

Apparent temperature and cause-specific mortality in Copenhagen, Denmark: a case-crossover analysis

Temperature, a key climate change indicator, is expected to increase substantially in the Northern Hemisphere, with potentially grave implications for human health. This study is the first to investigate the association between the daily 3-hour maximum apparent temperature (Tapp_max), and respiratory, cardiovascular and cerebrovascular mortality in Copenhagen (1999–2006) using a case-crossover design. Susceptibility was investigated for age, sex, socio-economic status and place of death. For an inter-quartile range (7 °C) increase in Tapp_max, an inverse association was found with cardiovascular mortality (−7% 95% CI −13%; −1%) and none with respiratory and cerebrovascular mortality. In the cold period all associations were inverse, although insignificant.

Short-Term Effects of Nitrogen Dioxide on Mortality and Susceptibility Factors in 10 Italian Cities: The EpiAir Study

BACKGROUND

Several studies have shown an association between nitrogen dioxide (NO2) and mortality. In Italy, the EpiAir multicentric study, "Air Pollution and Health: Epidemiological Surveillance and Primary Prevention," investigated short-term health effects of air pollution, including NO2.

OBJECTIVES

To study the individual susceptibility, we evaluated the association between NO2 and cause-specific mortality, investigating individual sociodemographic features and chronic/acute medical conditions as potential effect modifiers.

METHODS

We considered 276,205 natural deaths of persons > 35 years of age, resident in 10 Italian cities, and deceased between 2001 and 2005. We chose a time-stratified case-crossover analysis to evaluate the short-term effects of NO2 on natural, cardiac, cerebrovascular, and respiratory mortality. For each subject, we collected information on sociodemographic features and hospital admissions in the previous 2 years. Fixed monitors provided daily concentrations of NO2, particulate matter ≤ 10 μm in aerodynamic diameter (PM10) and ozone (O3).

RESULTS

We found statistically significant associations with a 10-μg/m3 increase of NO2 for natural mortality [2.09% for lag 0-5; 95% confidence interval (CI), 0.96-3.24], for cardiac mortality (2.63% for lag 0-5; 95% CI, 1.53-3.75), and for respiratory mortality (3.48% for lag 1-5; 95% CI, 0.75-6.29). These associations were independent from those of PM10 and O3. Stronger associations were estimated for subjects with at least one hospital admission in the 2 previous years and for subjects with three or more specific chronic conditions. Some cardiovascular conditions (i.e., ischemic heart disease, pulmonary circulation impairment, heart conduction disorders, heart failure) and diabetes appeared to confer a strong susceptibility to air pollution.

CONCLUSIONS

Our results suggest significant and likely independent effects of NO2 on natural, cardiac, and respiratory mortality, particularly among subjects with specific cardiovascular preexisting chronic conditions and diabetes.

Fine particulate air pollution (PM2.5) and the risk of acute ischemic stroke

BACKGROUND

Short-term changes in levels of fine ambient particulate matter (PM2.5) may increase the risk of acute ischemic stroke; however, results from prior studies have been inconsistent. We examined this hypothesis using data from a multicenter prospective stroke registry.

METHODS

We analyzed data from 9202 patients hospitalized with acute ischemic stroke, having a documented date and time of stroke onset, and residing within 50 km of a PM2.5 monitor in 8 cities in Ontario, Canada. We evaluated the risk of ischemic stroke onset associated with PM2.5 in each city using a time-stratified case-crossover design, matching on day of week and time of day. We then combined these city-specific estimates using random-effects meta-analysis techniques. We examined whether the effects of PM2.5 differed across strata defined by patient characteristics and ischemic stroke etiology.

RESULTS

Overall, PM2.5 was associated with a -0.7% change in ischemic stroke risk per 10-μg/m increase in PM2.5 (95% confidence interval = -6.3% to 5.1%). These overall negative results were robust to a number of sensitivity analyses. Among patients with diabetes mellitus, PM2.5 was associated with an 11% increase in ischemic stroke risk (1% to 22%). The association between PM2.5 and ischemic stroke risk varied according to stroke etiology, with the strongest associations observed for strokes due to large-artery atherosclerosis and small-vessel occlusion.

CONCLUSIONS

These results do not support the hypothesis that short-term increases in PM2.5 levels are associated with ischemic stroke risk overall. However, specific patient subgroups may be at increased risk of particulate-related ischemic strokes.

Apparent temperature and cause-specific emergency hospital admissions in Greater Copenhagen, Denmark

One of the key climate change factors, temperature, has potentially grave implications for human health. We report the first attempt to investigate the association between the daily 3-hour maximum apparent temperature (Tapp_max) and respiratory (RD), cardiovascular (CVD), and cerebrovascular (CBD) emergency hospital admissions in Copenhagen, controlling for air pollution. The study period covered 1 January 2002−31 December 2006, stratified in warm and cold periods. A case-crossover design was applied. Susceptibility (effect modification) by age, sex, and socio-economic status was investigated. For an IQR (8°C) increase in the 5-day cumulative average of Tapp_max, a 7% (95% CI: 1%, 13%) increase in the RD admission rate was observed in the warm period whereas an inverse association was found with CVD (−8%, 95% CI: −13%, −4%), and none with CBD. There was no association between the 5-day cumulative average of Tapp_max during the cold period and any of the cause-specific admissions, except in some susceptible groups: a negative association for RD in the oldest age group and a positive association for CVD in men and the second highest SES group. In conclusion, an increase in Tapp_max is associated with a slight increase in RD and decrease in CVD admissions during the warmer months.

Short-term association between ambient air pollution and risk of hospitalization for acute myocardial infarction: results of the cardiovascular risk and air pollution in Tuscany (RISCAT) study

Air pollutant levels have been widely associated with increased hospitalizations and mortality from cardiovascular disease. In this study, the authors focused on pollutant levels and triggering of acute myocardial infarction (AMI). Data on AMI hospitalizations, air quality, and meteorologic conditions were collected in 6 urban areas of Tuscany (central Italy) during 2002-2005. Levels of particulate matter with an aerodynamic diameter ≤10 μm (PM(10)) (range of 4-year mean values, 28.15-40.68 μg/m(3)), nitrogen dioxide (range, 28.52-39.72 μg/m(3)), and carbon monoxide (range, 0.86-1.28 mg/m(3)) were considered, and increases of 10 μg/m(3) (0.1 mg/m(3) for carbon monoxide) were analyzed. A time-stratified case-crossover approach was applied. Area-specific conditional regression models were fitted, adjusting for time-dependent variables. Stratified analyses and analyses in bipollutant models were performed. Pooled estimates were derived from random-effects meta-analyses. Among 11,450 AMI hospitalizations, the meta-analytical odds ratio at lag(2) (2-day lag) was 1.013 (95% confidence interval (CI): 1.000, 1.026) for PM(10), 1.022 (95% CI: 1.004, 1.041) for nitrogen dioxide, and 1.007 (95% CI: 1.002, 1.013) for carbon monoxide. More susceptible subgroups were elderly persons (age ≥75 years), females, and older patients with hypertension and chronic obstructive pulmonary disease. This study adds to evidence for a short-term association between air pollutants and AMI onset, also evident at low pollutant levels, suggesting a need to focus on more vulnerable subjects.

Does air pollution trigger infant mortality in Western Europe? A case-crossover study

BACKGROUND

Numerous studies show associations between fine particulate air pollutants [particulate matter with an aerodynamic diameter ≤ 10 μm (PM₁₀)] and mortality in adults.

OBJECTIVES

We investigated short-term effects of elevated PM₁₀ levels on infant mortality in Flanders, Belgium, and studied whether the European Union (EU) limit value protects infants from the air pollution trigger.

METHODS

In a case-crossover analysis, we estimated the risk of dying from nontraumatic causes before 1 year of age in relation to outdoor PM₁₀ concentrations on the day of death. We matched control days on temperature to exclude confounding by variations in daily temperature.

RESULTS

During the study period (1998-2006), PM₁₀ concentration averaged 31.9 ± 13.8 μg/m³. In the entire study population (n = 2,382), the risk of death increased by 4% [95% confidence interval (CI), 0-8%; p = 0.045] for a 10-μg/m³ increase in daily mean PM₁₀. However, this association was significant only for late neonates (2-4 weeks of age; n = 372), in whom the risk of death increased by 11% (95% CI, 1-22%; p = 0.028) per 10-μg/m³ increase in PM₁₀. In this age class, infants were 1.74 (95% CI, 1.18-2.58; p = 0.006) times more likely to die on days with a mean PM₁₀ above the EU limit value of 50 μg/m3 than on days below this cutoff.

CONCLUSIONS

Even in an affluent region in Western Europe, where infant mortality is low, days with higher PM air pollution are associated with an increased risk of infant mortality. Assuming causality, the current EU limit value for PM₁₀, which may be exceeded on 35 days/year, does not prevent PM₁₀ from triggering mortality in late neonates.

Potential for bias in case-crossover studies with shared exposures analyzed using SAS

The case-crossover method is an efficient study design for evaluating associations between transient exposures and the onset of acute events. In one common implementation of this design, odds ratios are estimated using conditional logistic or stratified Cox proportional hazards models, with data stratified on each individual event. In environmental epidemiology, where aggregate time-series data are often used, combining strata with identical exposure histories may be computationally convenient. However, when the SAS software package (SAS Institute Inc., Cary, North Carolina) is used for analysis, users can obtain biased results if care is not taken to properly account for multiple cases observed at the same time. The authors show that fitting a stratified Cox model with the "Breslow" option for handling tied failure times (i.e., ties = Breslow) provides unbiased health-effects estimates in case-crossover studies with shared exposures. The authors' simulations showed that using conditional logistic regression-or equivalently a stratified Cox model with the "ties = discrete" option-in this setting leads to health-effect estimates which can be biased away from the null hypothesis of no association by 22%-39%, even for small simulated relative risks. All methods tested by the authors yielded unbiased results under a simulated scenario with a relative risk of 1.0. This potential bias does not arise in R (R Foundation for Statistical Computing, Vienna, Austria) or Stata (Stata Corporation, College Station, Texas).

Evaluation of coseasonality of influenza and invasive pneumococcal disease: results from prospective surveillance

BACKGROUND

The wintertime co-occurrence of peaks in influenza and invasive pneumococcal disease (IPD) is well documented, but how and whether wintertime peaks caused by these two pathogens are causally related is still uncertain. We aimed to investigate the relationship between influenza infection and IPD in Ontario, Canada, using several complementary methodological tools.

METHODS AND FINDINGS

We evaluated a total number of 38,501 positive influenza tests in Central Ontario and 6,191 episodes of IPD in the Toronto/Peel area, Ontario, Canada, between 1 January 1995 and 3 October 2009, reported through population-based surveillance. We assessed the relationship between the seasonal wave forms for influenza and IPD using fast Fourier transforms in order to examine the relationship between these two pathogens over yearly timescales. We also used three complementary statistical methods (time-series methods, negative binomial regression, and case-crossover methods) to evaluate the short-term effect of influenza dynamics on pneumococcal risk. Annual periodicity with wintertime peaks could be demonstrated for IPD, whereas periodicity for influenza was less regular. As for long-term effects, phase and amplitude terms of pneumococcal and influenza seasonal sine waves were not correlated and meta-analysis confirmed significant heterogeneity of influenza, but not pneumococcal phase terms. In contrast, influenza was shown to Granger-cause pneumococcal disease. A short-term association between IPD and influenza could be demonstrated for 1-week lags in both case-crossover (odds ratio [95% confidence interval] for one case of IPD per 100 influenza cases  = 1.10 [1.02-1.18]) and negative binomial regression analysis (incidence rate ratio [95% confidence interval] for one case of IPD per 100 influenza cases  = 1.09 [1.05-1.14]).

CONCLUSIONS

Our data support the hypothesis that influenza influences bacterial disease incidence by enhancing short-term risk of invasion in colonized individuals. The absence of correlation between seasonal waveforms, on the other hand, suggests that bacterial disease transmission is affected to a lesser extent. Please see later in the article for the Editors' Summary.

Urban air pollution and emergency room admissions for respiratory symptoms: a case-crossover study in Palermo, Italy

BACKGROUND

Air pollution from vehicular traffic has been associated with respiratory diseases. In Palermo, the largest metropolitan area in Sicily, urban air pollution is mainly addressed to traffic-related pollution because of lack of industrial settlements, and the presence of a temperate climate that contribute to the limited use of domestic heating plants. This study aimed to investigate the association between traffic-related air pollution and emergency room admissions for acute respiratory symptoms.

METHODS

From January 2004 through December 2007, air pollutant concentrations and emergency room visits were collected for a case-crossover study conducted in Palermo, Sicily. Risk estimates of short-term exposures to particulate matter and gaseous ambient pollutants including carbon monoxide, nitrogen dioxide, and sulfur dioxide were calculated by using a conditional logistic regression analysis.

RESULTS

Emergency departments provided data on 48,519 visits for respiratory symptoms. Adjusted case-crossover analyses revealed stronger effects in the warm season for the most part of the pollutants considered, with a positive association for PM10 (odds ratio = 1.039, 95% confidence interval: 1.020 - 1.059), SO2 (OR = 1.068, 95% CI: 1.014 - 1.126), nitrogen dioxide (NO2: OR = 1.043, 95% CI: 1.021 - 1.065), and CO (OR = 1.128, 95% CI: 1.074 - 1.184), especially among females (according to an increase of 10 μg/m3 in PM10, NO2, SO2, and 1 mg/m3 in CO exposure). A positive association was observed either in warm or in cold season only for PM10.

CONCLUSIONS

Our findings suggest that, in our setting, exposure to ambient levels of air pollution is an important determinant of emergency room (ER) visits for acute respiratory symptoms, particularly during the warm season. ER admittance may be considered a good proxy to evaluate the adverse effects of air pollution on respiratory health.

Respiratory virus infection and risk of invasive meningococcal disease in central Ontario, Canada

Background

In temperate climates, invasive meningococcal disease (IMD) incidence tends to coincide with or closely follow peak incidence of influenza virus infection; at a seasonal level, increased influenza activity frequently correlates with increased seasonal risk of IMD.

Methods

We evaluated 240 cases of IMD reported in central Ontario, Canada, from 2000 to 2006. Associations between environmental and virological (influenza A, influenza B and respiratory syncytial virus (RSV)) exposures and IMD incidence were evaluated using negative binomial regression models controlling for seasonal oscillation. Acute effects of weekly respiratory virus activity on IMD risk were evaluated using a matched-period case-crossover design with random directionality of control selection. Effects were estimated using conditional logistic regression.

Results

Multivariable negative binomial regression identified elevated IMD risk with increasing influenza A activity (per 100 case increase, incidence rate ratio = 1.18, 95% confidence interval (CI): 1.06, 1.31). In case-crossover models, increasing weekly influenza A activity was associated with an acute increase in the risk of IMD (per 100 case increase, odds ratio (OR)  = 2.03, 95% CI: 1.28 to 3.23). Increasing weekly RSV activity was associated with increased risk of IMD after adjusting for RSV activity in the previous 3 weeks (per 100 case increase, OR = 4.31, 95% CI: 1.14, 16.32). No change in disease risk was seen with increasing influenza B activity.

Conclusions

We have identified an acute effect of influenza A and RSV activity on IMD risk. If confirmed, these finding suggest that influenza vaccination may have the indirect benefit of reducing IMD risk.

Invited commentary: application of case-crossover methods to investigate triggers of preterm birth

Properties of the case-crossover design have appeal for investigation of acute triggers of preterm birth. Measured and unmeasured time-invariant risk factors are controlled by design, such that maternal race, socioeconomic status, and other personal factors will not confound the exposure-preterm birth association. In this issue of the Journal, Basu et al. (Am J Epidemiol. 2010;172(10):1108-1117) apply the case-crossover approach to assess the short-term relation between ambient apparent temperature and preterm birth. Novel application of the design to preterm birth, a "fatal" event exhibiting dramatic within-subject changes in risk, merits a review of the assumptions underlying the design. Implications of the referent time periods selected and the potential for confounding by seasonal patterns of conception are discussed in this commentary. The provocative associations observed by Basu et al. between high ambient apparent temperature and preterm birth should stimulate follow-up analyses and could ultimately have important public health implications. Future research can also help delineate the relative strengths and weaknesses of different temporal analytic strategies for investigating short-term associations between various exposures and preterm birth.

Ambient air pollution and daily mortality in Anshan, China: a time-stratified case-crossover analysis

Few case-crossover studies were conducted in China to investigate the acute health effects of air pollution. We conducted a time-stratified case-crossover analysis to examine the association between air pollution and daily mortality in Anshan, a heavily-polluted industrial city in northeastern China. Daily mortality, air pollution, and weather data in 2004-2006 in Anshan were collected. Time-stratified case-crossover approach was used to estimate the effect of air pollutants (PM(10), SO(2), NO(2) and CO) on total and cardiopulmonary mortality. Controls were selected as matched days of the week in the same month. Potential effect modifiers, such as gender and age, were also examined. We found significant associations between air pollution and daily mortality from cardiovascular diseases in Anshan. A 10μg/m(3) elevation of 2-day moving average (lag 01) concentration in PM(10), SO(2), NO(2) and CO corresponded to 0.67% (95% CI: 0.29%, 1.04%), 0.38% (95% CI: -0.06%, 0.83%), 2.11% (95% CI: 0.22%, 4.00%) and 0.04% (95% CI: 0.01%, 0.07%) increase of cardiovascular mortality. The associations for total and respiratory mortality were generally positive but statistically insignificant. The air pollution health effects were significantly modified by age, but not by gender. Conclusively, our study showed that short-term exposure to air pollution was associated with increased cardiovascular mortality in Anshan. These findings may have implications for local environmental and social policies.

High ambient temperature and the risk of preterm delivery

With temperatures expected to increase because of climate change, it is essential to study the health outcomes of elevated temperature in vulnerable populations, such as expectant mothers. In this study, the authors estimated the association between heat and humidity, as measured by apparent temperature, and preterm delivery. They conducted a case-crossover analysis of almost 60,000 births spanning 16 counties in California that occurred from 1999 to 2006 between May and September. The authors identified cases of preterm birth from a state registry of births, which were combined with meteorologic and air pollution monitoring data based on residential zip code. High ambient temperature was significantly associated with preterm birth for all mothers, regardless of maternal racial/ethnic group, maternal age, maternal education, or sex of the infant. Results indicated that an 8.6% increase (95% confidence interval: 6.0, 11.3) in preterm delivery was associated with a 10°F (5.6°C) increase in weekly average (lag06) apparent temperature. Greater associations were observed for younger mothers, blacks, and Asians. These associations were independent of air pollutants. Given the significant associations for apparent temperature and preterm delivery found in this study, more large-scale studies of temperature and preterm delivery are warranted.

The effects of temperature and use of air conditioning on hospitalizations

Several investigators have documented the effect of temperature on mortality, although fewer have studied its impact on morbidity. In addition, little is known about the effectiveness of mitigation strategies such as use of air conditioners (ACs). The authors investigated the association between temperature and hospital admissions in California from 1999 to 2005. They also determined whether AC ownership and usage, assessed at the zip-code level, mitigated this association. Because of the unique spatial pattern of income and climate in California, confounding of AC effects by other local factors is less likely. The authors included only persons who had a temperature monitor within 25 km of their residential zip code. Using a time-stratified case-crossover approach, the authors observed a significantly increased risk of hospitalization for multiple diseases, including cardiovascular disease, ischemic heart disease, ischemic stroke, respiratory disease, pneumonia, dehydration, heat stroke, diabetes, and acute renal failure, with a 10°F increase in same-day apparent temperature. They also found that ownership and usage of ACs significantly reduced the effects of temperature on these health outcomes, after controlling for potential confounding by family income and other socioeconomic factors. These results demonstrate important effects of temperature on public health and the potential for mitigation.

Triggering of transmural infarctions, but not nontransmural infarctions, by ambient fine particles

BACKGROUND

Previous studies have reported increased risk of myocardial infarction (MI) after increases in ambient particulate matter (PM) air pollution concentrations in the hours and days before MI onset.

OBJECTIVES

We hypothesized that acute increases in fine PM with aerodynamic diameter < or = 2.5 microm (PM(2.5)) may be associated with increased risk of MI and that chronic obstructive pulmonary disease (COPD) and diabetes may increase susceptibility to PM(2.5). We also explored whether both transmural and nontransmural infarctions were acutely associated with ambient PM(2.5) concentrations.

METHODS

We studied all hospital admissions from 2004 through 2006 for first acute MI of adult residents of New Jersey who lived within 10 km of a PM(2.5) monitoring site (n = 5,864), as well as ambient measurements of PM(2.5), nitrogen dioxide, sulfur dioxide, carbon monoxide, and ozone.

RESULTS

Using a time-stratified case-crossover design and conditional logistic regression showed that each interquartile-range increase in PM(2.5) concentration (10.8 microg/m3) in the 24 hr before arriving at the emergency department for MI was not associated with MI overall but was associated with an increased relative risk of a transmural infarction. We found no association between the same increase in PM(2.5) and nontransmural infarction. Further, subjects with COPD appeared to be particularly susceptible, but those with diabetes were not.

CONCLUSIONS

This PM-transmural infarction association is consistent with earlier studies of PM and MI. The lack of association with nontransmural infarction suggests that future studies that investigate the triggering of MI by ambient PM(2.5) concentrations should be stratified by infarction type.

Case-crossover analysis of air pollution health effects: a systematic review of methodology and application

BACKGROUND

Case-crossover is one of the most used designs for analyzing the health-related effects of air pollution. Nevertheless, no one has reviewed its application and methodology in this context.

OBJECTIVE

We conducted a systematic review of case-crossover (CCO) designs used to study the relationship between air pollution and morbidity and mortality, from the standpoint of methodology and application.

DATA SOURCES AND EXTRACTION

A search was made of the MEDLINE and EMBASE databases.Reports were classified as methodologic or applied. From the latter, the following information was extracted: author, study location, year, type of population (general or patients), dependent variable(s), independent variable(s), type of CCO design, and whether effect modification was analyzed for variables at the individual level.

DATA SYNTHESIS

The review covered 105 reports that fulfilled the inclusion criteria. Of these, 24 addressed methodological aspects, and the remainder involved the design's application. In the methodological reports, the designs that yielded the best results in simulation were symmetric bidirectional CCO and time-stratified CCO. Furthermore, we observed an increase across time in the use of certain CCO designs, mainly symmetric bidirectional and time-stratified CCO. The dependent variables most frequently analyzed were those relating to hospital morbidity; the pollutants most often studied were those linked to particulate matter. Among the CCO-application reports, 13.6% studied effect modification for variables at the individual level.

CONCLUSIONS

The use of CCO designs has undergone considerable growth; the most widely used designs were those that yielded better results in simulation studies: symmetric bidirectional and time-stratified CCO. However, the advantages of CCO as a method of analysis of variables at the individual level are put to little use.

Impact of fine and ultrafine particles on emergency hospital admissions for cardiac and respiratory diseases

BACKGROUND

Little is known about the short-term effects of ultrafine particles.

METHODS

We evaluated the effect of particulate matter with an aerodynamic diameter <or=10 microm (PM10), <or=2.5 microm (PM2.5), and ultrafine particles on emergency hospital admissions in Rome 2001-2005. We studied residents aged >or=35 years hospitalized for acute coronary syndrome, heart failure, lower respiratory tract infections, and chronic obstructive pulmonary disease (COPD). Information was available for factors indicating vulnerability, such as age and previous admissions for COPD. Particulate matter data were collected daily at one central fixed monitor. A case-crossover analysis was performed using a time-stratified approach. We estimated percent increases in risk per 14 microg/m PM10, per 10 microg/m PM2.5, and per 9392 particles/mL.

RESULTS

An immediate impact (lag 0) of PM2.5 on hospitalizations for acute coronary syndrome (2.3% [95% confidence interval = 0.5% to 4.2%]) and heart failure (2.4% [0.3% to 4.5%]) was found, whereas the effect on lower respiratory tract infections (2.8% [0.5% to 5.2%]) was delayed (lag 2). Particle number concentration showed an association only with admissions for heart failure (lag 0-5; 2.4% [0.2% to 4.7%]) and COPD (lag 0; 1.6% [0.0% to 3.2%]). The effects were generally stronger in the elderly and during winter. There was no clear effect modification with previous COPD.

CONCLUSIONS

We found sizeable acute health effects of fine and ultrafine particles. Although differential reliability in exposure assessment, in particular of ultrafine particles, precludes a firm conclusion, the study indicates that particulate matter of different sizes tends to have diverse outcomes, with dissimilar latency between exposure and health response.

Air pollution and emergency room visits for cardiac arrhythmia in a subtropical city: Taipei, Taiwan

This study was undertaken to determine whether there was an association between air pollutant levels and emergency room (ER) visits for cardiac arrhythmia in Taipei, Taiwan. ER visits for cardiac arrhythmia and ambient air pollution data for Taipei were obtained for the period 2000-2006. The relative risk of ER visits was estimated using a case-crossover approach, controlling for weather variables, day of the week, seasonality, and long-term time trends. In the single-pollutant model, on warm days (> or =23 degrees C), statistically significant positive associations were found for all pollutants except SO(2). On cool days (<23 degrees C), all pollutants were also significantly associated with the number of ER visits for cardiac arrhythmia, except SO(2). For the two-pollutant model, results for O(3) and NO(2) remained statistically significant on both warm and cool days. This study provides evidence that higher levels of ambient air pollutants increase the risk of ER visits for cardiac arrhythmia.

Air pollution and hospital admissions for pneumonia: are there potentially sensitive groups?

Recent studies have shown that air pollution is a risk factor for hospitalization for pneumonia. However, there is limited evidence to suggest what subpopulations are at greater risk from air pollution. This study was undertaken to examine the modifying effect of specific secondary diagnosis (including hypertension, diabetes, asthma, and upper respiratory infections) on the relationship between hospital admissions for pneumonia and ambient air pollutants. Hospital admissions for pneumonia and ambient air pollution data for Kaohsiung were obtained for the period 1996-2006. The relative risk of hospital admission was estimated using a case-crossover approach. We found that an interquartile range (IQR) increase in PM(10), NO(2), and CO on warm days was associated with a 28% (95% confidence interval [CI], 11-48%), 40% (95% CI, 20-63%), and 31% (95% CI, 19-45%), respectively, increase in admissions for pneumonia with upper respiratory infections (URI), but only 23% (95% CI, 18-28%), 30% (95% CI, 25-36%), and 24% (95% CI, 21-28%), respectively, increase for persons without URI. An IQR increase in PM(10), SO(2), and CO on cool days was associated with a 70% (95% CI, 50-92%), 30% (95% CI, 15-47%), and 64% (95% CI, 47-83%), respectively, increase in admissions in persons with URI, and an increase of 64% (95% CI, 57-70%), 18% (95% CI, 14-22%), and 55% (95% CI, 50-60%), respectively, in admissions in persons without URI. Our findings provide evidence that patients with comorbid URI may increase the risk of hospital admissions for pneumonia, in relation to air pollutant levels.

An introduction to epidemiologic and statistical methods useful in environmental epidemiology

Many developments in the design and analysis of environmental epidemiology have been made in air pollution studies. In the analysis of the short-term effects of particulate matter on daily mortality, Poisson regression models with flexible smoothing methods have been developed for the analysis of time-series data. Another option for such studies is the use of case–crossover designs, and there have been extensive discussions on the selection of control periods. In the Study on Respiratory Disease and Automobile Exhaust project conducted by the Japanese Ministry of the Environment, we adopted a new 2-stage case–control design that is efficient when both exposure and disease are rare. Based on our experience in conducting air pollution epidemiologic studies, we review 2-stage case–control designs, case–crossover designs, generalized linear models, generalized additive models, and generalized estimating equations, all of which are useful approaches in environmental epidemiology.

Short-term associations between ambient air pollutants and pediatric asthma emergency department visits

RATIONALE

Certain outdoor air pollutants cause asthma exacerbations in children. To advance understanding of these relationships, further characterization of the dose-response and pollutant lag effects are needed, as are investigations of pollutant species beyond the commonly measured criteria pollutants.

OBJECTIVES

Investigate short-term associations between ambient air pollutant concentrations and emergency department visits for pediatric asthma.

METHODS

Daily counts of emergency department visits for asthma or wheeze among children aged 5 to 17 years were collected from 41 Metropolitan Atlanta hospitals during 1993-2004 (n = 91,386 visits). Ambient concentrations of gaseous pollutants and speciated particulate matter were available from stationary monitors during this time period. Rate ratios for the warm season (May to October) and cold season (November to April) were estimated using Poisson generalized linear models in the framework of a case-crossover analysis.

MEASUREMENTS AND MAIN RESULTS

Both ozone and primary pollutants from traffic sources were associated with emergency department visits for asthma or wheeze; evidence for independent effects of ozone and primary pollutants from traffic sources were observed in multipollutant models. These associations tended to be of the highest magnitude for concentrations on the day of the emergency department visit and were present at relatively low ambient concentrations.

CONCLUSIONS

Even at relatively low ambient concentrations, ozone and primary pollutants from traffic sources independently contributed to the burden of emergency department visits for pediatric asthma.

Air pollution exposure--a trigger for myocardial infarction?

The association between ambient air pollution exposure and hospitalization for cardiovascular events has been reported in several studies with conflicting results. A case-crossover design was used to investigate the effects of air pollution in 660 first-time myocardial infarction cases in Stockholm in 1993–1994, interviewed shortly after diagnosis using a standard protocol. Air pollution data came from central urban background monitors. No associations were observed between the risk for onset of myocardial infarction and two-hour or 24-hour air pollution exposure. No evidence of susceptible subgroups was found. This study provides no support that moderately elevated air pollution levels trigger first-time myocardial infarction.

Modifiers of short-term effects of ozone on mortality in eastern Massachusetts--a case-crossover analysis at individual level

BACKGROUND

Substantial epidemiological studies demonstrate associations between exposure to ambient ozone and mortality. A few studies simply examine the modification of this ozone effect by individual characteristics and socioeconomic status, but socioeconomic status was usually coded at the city level.

METHODS

This study used a case-crossover design to examine whether impacts of ozone on mortality were modified by socioeconomic status coded at the tract or characteristics at an individual level in eastern Massachusetts, US for a period May-September, 1995-2002, with a total of 157,197 non-accident deaths aging 35 years or older. We used moving averages of maximal 8-hour concentrations of ozone monitored at 8 stationary stations as personal exposure.

RESULTS

A 10 ppb increase in the four-day moving average of maximal 8-hour ozone was associated with 1.68% (95% confidence interval (CI): 0.51%, 2.87%), 1.96% (95% CI: -1.83%, 5.90%), 8.28% (95% CI: 0.66%, 16.48%), 0.44% (95% CI: -1.45%, 2.37%), -0.83% (95% CI: -2.94%, 1.32%), -1.09% (95% CI: -4.27%, 2.19%) and 6.5% (95% CI: 1.74%, 11.49%) changes in all natural deaths, respiratory disorders, diabetes, cardiovascular diseases, heart diseases, acute myocardial infarction and stroke, respectively. We did not find any evidence that the associations were significantly modified by socioeconomic status or individual characteristics although small differences of estimates across subpopulations were demonstrated.

CONCLUSIONS

Exposure to ozone was associated with specific cause mortality in Eastern Massachusetts during May-September, 1995-2002. There was no evidence that effects of ozone on mortality were significantly modified by socioeconomic status and individual characteristics.

Seasonal effect of PM(10) concentrations on mortality and morbidity in Seoul, Korea: a temperature-matched case-crossover analysis

BACKGROUND

Explorations of interactions between air pollution and seasonal changes have represented one approach in examining the consequences of global warming. However, only a few studies have focused on evaluating the effects of seasonal air pollution using data on both morbidity and mortality in Asia.

METHOD

We examined the associations between PM(10) concentrations and mortality and hospital admissions in Seoul, Korea for the periods 2000-2006 and 2001-2006. We employed a temperature-matched case-crossover design, where reference periods matched case days in regard to temperature (same rounded to degrees celsius ( degrees C)), month, and year.

RESULTS

A total of 238,826 deaths were identified, along with 98,570 and 93,553 inpatient admissions for cardiovascular and respiratory diseases, respectively. We found that the association with PM(10) and mortality/morbidity increased during the summer. During the study period, 10microg/m(3) increase in PM(10) was associated with the increase in mortality by 0.28% (95% confidence interval: 0.12, 0.44), 0.51% (0.19, 0.83), and 0.59% (-0.08, 1.26) for non-accidental, cardiovascular, and respiratory causes. 10microg/m(3) increase in PM(10) was also associated with increase in hospitalization from cardiovascular and respiratory causes by 0.77% (0.53, 1.01) and 1.19% (0.94, 1.44). In the summer, the increase in mortality and hospitalization was 0.57% (0.20, 0.93), 0.64% (-0.10, 1.38), 0.50% (-1.02, 2.05), 1.52% (0.89, 2.16), and 1.55% (0.87, 2.22).

CONCLUSIONS

This study provides evidence that the effect of PM(10) on mortality and morbidity varies with season and increases during the summer season.

Air pollution and hospital admissions for myocardial infarction in a subtropical city: Taipei, Taiwan

This study was undertaken to determine whether there was a correlation between air pollutant levels and hospital admissions for myocardial infarction (MI) in Taipei, Taiwan. Hospital admissions for MI and ambient air pollution data for Taipei were obtained for the period 1996-2006. The relative risk of hospital admissions was estimated using a case-crossover approach, controlling for weather variables, day of the week, seasonality, and long-term time trends. In the single-pollutant models, on warm days (>23 degrees C) statistically significant positive associations were found for all pollutants except sulfur dioxide (SO(2)). On cool days (<23 degrees C), all pollutants were significantly associated with increased MI admissions except SO(2). For the two-pollutant model, ozone (O(3)) and nitrogen dioxide (NO(2)) were significant in combination with each of the other four pollutants both on warm and cool days for higher admissions for MI. This study provides evidence that higher levels of ambient air pollutants increase the risk of hospital admissions for MI.

Let the sun shine in: effects of ultraviolet radiation on invasive pneumococcal disease risk in Philadelphia, Pennsylvania

Background

Streptococcus pneumoniae is a common cause of community acquired pneumonia and bacteremia. Excess wintertime mortality related to pneumonia has been noted for over a century, but the seasonality of invasive pneumococcal disease (IPD) has been described relatively recently and is poorly understood. Improved understanding of environmental influence on disease seasonality has taken on new urgency due to global climate change.

Methods

We evaluated 602 cases of IPD reported in Philadelphia County, Pennsylvania, from 2002 to 2007. Poisson regression models incorporating seasonal smoothers were used to identify associations between weekly weather patterns and case counts. Associations between acute (day-to-day) environmental fluctuations and IPD occurrence were evaluated using a case-crossover approach. Effect modification across age and sex strata was explored, and meta-regression models were created using stratum-specific estimates for effect.

Results

IPD incidence was greatest in the wintertime, and spectral decomposition revealed a peak at 51.0 weeks, consistent with annual periodicity. After adjustment for seasonality, yearly increases in reporting, and temperature, weekly incidence was found to be associated with clear-sky UV index (IRR per unit increase in index: 0.70 [95% CI 0.54-0.91]). The effect of UV index was highest among young strata and decreased with age. At shorter time scales, only an association with increases in ambient sulphur oxides was linked to disease risk (OR for highest tertile of exposure 0.75, 95% CI 0.60 to 0.93).

Conclusion

We confirmed the wintertime predominance of IPD in a major urban center. The major predictor of IPD in Philadelphia is extended periods of low UV radiation, which may explain observed wintertime seasonality. The mechanism of action of diminished light exposure on disease occurrence may be due to direct effects on pathogen survival or host immune function via altered 1,25-(OH)₂-vitamin-D metabolism. These findings may suggest less diminution in future IPD risk with climate change than would be expected if wintertime seasonality was driven by temperature.

Summer temperature-related mortality: effect modification by previous winter mortality

BACKGROUND

Several studies have described seasonal patterns of mortality, with rates higher in winter and lower in summer. Few researchers, however, have analyzed how the mortality rate in winter may influence the temperature-mortality association in the following summer. In the present paper, we addressed the question of whether the association between summer temperature and mortality among the elderly is modified by the previous winter mortality rate.

METHODS

We selected all deaths in Rome during 1987-2005 among persons 65 years old or older. We collected data on daily mean temperature and humidity. We estimated the effect of summer apparent temperature on mortality by using a time-series approach, and tested the effect modification based on the mortality rate during the preceding winter.

RESULTS

The effect of summer apparent temperature on mortality was stronger in years characterized by low mortality in the previous winter (relative risk for days at 30 degrees C vs. days at 20 degrees C = 1.73 [95% confidence interval = 1.50-2.01]), as contrasted with years with medium (1.32 [1.25-1.41]) or high winter mortality (1.34 [1.17-1.55]). The percentages of attributable risks for summer heat were 28%, 18%, and 18% for years characterized by low, medium, or high winter mortality rates respectively.

CONCLUSIONS

Low-mortality winters may inflate the pool of the elderly susceptible population at risk for dying from high temperature the following summer.

The effect of temperature on hospital admissions in nine California counties

OBJECTIVES

This study examined the association between mean daily apparent temperature and hospital admissions for several diseases in nine California counties from May to September, 1999 to 2005.

METHODS

We conducted a time-stratified case-crossover study limited to cases with residential zip codes located within 10 km of a temperature monitor. County-specific estimates were combined, using a random effects meta-analysis. The analyses also considered the effects of ozone and particulate matter (PM(2.5)).

RESULTS

We found that a 10 degrees F increase in mean apparent temperature was associated with a 3.5% [95% confidence interval (CI) 1.5-5.6] increase in ischemic stroke and increases in several other disease-specific outcomes including all respiratory diseases (2.0%, 95% CI 0.7-3.2), pneumonia (3.7%, 95% CI 1.7-3.7), dehydration (10.8%, 95% CI 8.3-13.6), diabetes (3.1%, 95% CI 0.4-5.9), and acute renal failure (7.4%, 95% CI 4.0-10.9). There was little evidence that the temperature effects we found were due to confounding by either PM(2.5) or ozone.

CONCLUSION

Our results indicate that increases in ambient temperature have important public health impacts on morbidity.