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

Diurnal temperature range is a risk factor for coronary heart disease death

Background

Although the relation between day-to-day temperature change and coronary heart disease (CHD) mortality is well established, it is unknown whether temperature variation within 1 day, ie, diurnal temperature range (DTR), is an independent risk factor for acute CHD death.

Methods

We used time-series and case-crossover approaches to assess the relation between DTR and daily CHD mortality between 2001 and 2004 in Shanghai, China. Specifically, we used exposures averaged over periods varying from 1 to 5 days to assess the effects of DTR on CHD mortality. We estimated the percent increase in the number of daily deaths related to CHD that were associated with DTR, after adjustment for daily meteorologic conditions (temperature and relative humidity) and levels of outdoor air pollutants.

Results

Both time-series and case-crossover analyses showed that DTR was significantly associated with the number of daily deaths related to CHD. A 1 °C increase in 2-day lagged DTR corresponded to a 2.46% (95% CI, 1.76% to 3.16%) increase in CHD mortality on time-series analysis, a 3.21% (95% CI, 2.23% to 4.19%) increase on unidirectional case-crossover analysis, and a 2.13% (95% CI, 1.04% to 3.22%) increase on bidirectional case-crossover analysis.

Conclusions

Our findings suggest that DTR is an independent risk factor for acute CHD death.

Why "winter" vomiting disease? Seasonality, hydrology, and Norovirus epidemiology in Toronto, Canada

Norovirus is a common cause of gastroenteritis, and is thought to be the causative agent in 68-90% of all gastroenteritis outbreaks. The seasonality of disease occurrence is sufficiently stereotyped to result in this disease being dubbed "winter vomiting disease." The genesis of this seasonality has been obscure. We sought to identify environmental factors associated with Norovirus outbreaks in Toronto, Canada. We evaluated 253 outbreaks of gastroenteritis linked to Norovirus between November 2005 and March 2008. Poisson regression models were constructed to evaluate associations between average environmental exposures and case counts. A case-crossover approach was used to evaluate associations between acute changes in environment and outbreak risk. Case-crossover analysis indicated an association between low Lake Ontario temperature (<or=4 degrees C) (hazard ratio [HR], 5.61 [95% CI, 2.81-11.12]) and high flow (>2.5 m(3)/s) in the Don River (HR, 3.17 [95% CI, 2.30-4.36]), 1-7 days prior to case occurrence. For both exposure variables, the highest hazard ratios were found 24-48 h prior to case onset. Regression models provided further support for these patterns. The association between local watershed conditions and Norovirus outbreak risk suggest a source-water reservoir for this pathogen. We hypothesize that the reservoir may be maintained through the discharge of wastewater containing virus particles; wintertime seasonality may be explained by enhanced viral persistence at low temperatures.

Environmental determinants of campylobacteriosis risk in Philadelphia from 1994 to 2007

Campylobacter species infections are a common cause of acute gastroenteritis, and may uncommonly be complicated by renal, neurological, and rheumatologic sequelae. Although excess summertime campylobacteriosis has been observed, environmental mechanisms driving disease seasonality are poorly understood. We sought to evaluate the relationship between environmental factors and campylobacteriosis risk in a major North American metropolitan area. We evaluated 1532 cases of campylobacteriosis reported in Philadelphia between 1994 and 2007. We constructed Poisson regression models with oscillatory smoothers, and also used case-crossover design, to evaluate the associations between environmental exposures and disease risk on weekly and daily time scales. Both methods control for confounding by seasonally oscillating environmental factors. Incidence was greatest in June and July, with annual periodicity. Weekly incidence was associated with increasing relative humidity, (incidence rate ratio (IRR) per % 1.017, 95% CI 1.008-1.025), temperature (IRR per degrees C 1.041, 95% CI 1.011-1.072), and decreasing Delaware River temperature during the same week (IRR per degrees C 0.922, 95% CI 0.883-0.962), and at 4-week lags (IRR per degrees C 0.953, 95% CI 0.919-0.990). No acute associations were identified in case-crossover analyses. Our findings affirm the summertime seasonality of campylobacteriosis in Philadelphia, and the link between warm, humid weather and disease risk. However, the link between low river temperatures and enhanced campylobacteriosis risk in humans described here is novel, consistent with known links between watershed temperature and Campylobacter survival, and implicates local watersheds as epidemiologically important reservoirs for foodborne pathogens.

Modifying effect of age on the association between ambient ozone and nighttime primary care visits due to asthma attack

Background

We examined the association between short-term exposure to outdoor air pollution and nighttime primary care visits due to asthma attack. We also investigated the modifying effects of age on this association.

Methods

A case–crossover study was conducted at a primary care clinic in metropolitan Tokyo. The subjects were 308 children aged 0–14 years and 95 adolescents and adults aged 15–64 years. All subjects made visits to the clinic for an asthma attack at between 7 PM and 12 AM. Data on hourly concentrations of particulate matter with a 50% cut-off aerodynamic diameter ≤2.5 µm (PM_2.5), ozone, and nitrogen dioxide (NO₂) were obtained. A conditional logistic regression model was used to estimate odds ratios (ORs) of primary care visits per unit increment of each air pollutant.

Results

Among children, the ORs in warmer months per 10 ppb increment of the 24-hour mean concentration of ozone were 1.16 (95% confidential interval [CI], 1.00–1.33) adjusted for temperature, and 1.29 (95% CI, 1.08–1.55) adjusted for PM_2.5, NO₂, and temperature. With respect to modification of the association by age, the ORs for 24-hour mean concentration of ozone—after adjustment for PM_2.5, NO₂ and temperature in warmer months—in children aged 0–1 years, 2–5 years, and 6–14 years were 1.06 (95% CI, 0.63–1.78), 1.37 (95% CI, 1.05–1.71), and 1.25 (95% CI, 0.87–1.82), respectively. There was no association between ozone and primary care visits among adults.

Conclusions

An association was found between ozone and nighttime primary care visits for asthma attack in warmer months; the association was greater among preschool children.

[Case-crossover design: Basic essentials and applications]

Case-crossover analysis is an observational epidemiological design that was proposed by Maclure in 1991 to assess whether a given intermittent or unusual exposure may have triggered an immediate short-term, acute event. The present article outlines the basics of case-crossover designs, as well as their applications and limitations. The case-crossover design is based on exclusively selecting case subjects. To calculate relative risk, exposure during the period of time prior to the event (case period) is compared against the same subject's exposure during one or more control periods. This method is only appropriate when the exposures are transient in time and have acute short-term effects. For exposures in which there is no trend, a unidirectional approach is the most frequent and consists of selecting one or more control periods prior to the case period. When the exposure displays a time trend (e.g., air pollution), a unidirectional approach will yield biased estimates, and therefore bidirectional case-crossover designs are used, which select control time intervals preceding and subsequent to that of the event. The case-crossover design is being increasingly used across a wide range of fields, including factors triggering traffic, occupational and domestic accidents and acute myocardial infarction, and those involved in air pollution and health and pharmacoepidemiology, among others. Insofar as data-analysis is concerned, case-crossover designs can generally be regarded as matched case-control studies and consequently conditional logistic regression can be applied. Lastly, this study analyzes practical examples of distinct applications of the case-crossover design.

Environmental exposures and invasive meningococcal disease: an evaluation of effects on varying time scales

Invasive meningococcal disease (IMD) is an important cause of meningitis and bacteremia worldwide. Seasonal variation in IMD incidence has long been recognized, but mechanisms responsible for this phenomenon remain poorly understood. The authors sought to evaluate the effect of environmental factors on IMD risk in Philadelphia, Pennsylvania, a major urban center. Associations between monthly weather patterns and IMD incidence were evaluated using multivariable Poisson regression models controlling for seasonal oscillation. Short-term weather effects were identified using a case-crossover approach. Both study designs control for seasonal factors that might otherwise confound the relation between environment and IMD. Incidence displayed significant wintertime seasonality (for oscillation, P < 0.001), and Poisson regression identified elevated monthly risk with increasing relative humidity (per 1% increase, incidence rate ratio = 1.04, 95% confidence interval: 1.004, 1.08). Case-crossover methods identified an inverse relation between ultraviolet B radiation index 1–4 days prior to onset and disease risk (odds ratio = 0.54, 95% confidence interval: 0.34, 0.85). Extended periods of high humidity and acute changes in ambient ultraviolet B radiation predict IMD occurrence in Philadelphia. The latter effect may be due to decreased pathogen survival or virulence and may explain the wintertime seasonality of IMD in temperate regions of North America.

Case-crossover analysis of air pollution and cardiorespiratory hospitalizations: using routinely collected health and environmental data for tracking: science and data

BACKGROUND

From the early 1900s until its closure in February 1998, a Steel coke oven in Pittsburgh, Pennsylvania, was a key source of air pollution. A case-crossover study was performed to assess the associations between daily air pollution and cardiorespiratory (International Classification of Disease Ninth Revision [ICD-9]: 390-519) hospitalizations before and after plant closure and to evaluate how closure influenced these associations.

METHODS

Air pollution data, climatic data, and cardiorespiratory hospitalizations among residents ages 65 years and older were obtained for the period of 1996 through 2000 for the study area. Data were analyzed using a case-crossover design and conditional logistical regression. Two distinct referent-sampling approaches were compared.

RESULTS

Significant associations were observed between the fourth quartile in PM10 and cardiorespiratory hospitalizations (odds ratio [OR]: 1.12; 95% CI: 1.02-1.23) and cardiovascular hospitalizations only (ICD-9: 390-459) (OR: 1.13; 95% CI: 1.01-1.26) before the plant closure. After closure of the plant, PM10 was not significantly associated with cardiorespiratory or cardiovascular disease hospitalizations. Moreover, the referent sampling approaches did not greatly alter the estimations in the case-crossover analysis.

CONCLUSIONS

Existing secondary data are an economical source to assess the impact of point source pollution on the environmental landscape. The findings suggest that closure of the steel coke plant was associated with a reduction risk of the cardiovascular hospitalizations.

Tracking associations between ambient ozone and asthma-related emergency department visits using case-crossover analysis

Traditional environmental public health surveillance consists of separately measuring hazards, exposures, and health outcomes. The Environmental Public Health Tracking (EPHT) Network seeks to accrue additional information by linking hazard or exposure data to health outcomes data. A natural progression is to consider tracking the "link" itself, that is, to track the association between an environmental hazard and a health outcome. The Maine EPHT Program conducted a case-crossover analysis to measure associations between daily estimated ambient ozone and particulate matter (PM2.5) and asthma-related emergency department (ED) visits for 2000-2003. We found an overall association of 7 percent (95% confidence interval, 4-11) excess asthma-related ED visits per 10-ppb increase in ozone averaged over 4 days (lag 0-3). The association was positive in the first 3 years and negative in the last. The excess risk was concentrated among females aged 15 to 34 and males younger than 15. The methodology for tracking associations between ambient air quality and acute morbidity is not generalizable to most other EPHT topic areas, but there are ample reasons to pursue this activity. The analysis can potentially help evaluate the effectiveness of regulatory and intervention programs, as well as inform us about trends, sensitive subpopulations, and changing potency of air constituents.

Air pollution and hospital admissions for myocardial infarction in a tropical city: Kaohsiung, Taiwan

This study was undertaken to determine whether there was an association between air pollutant levels and increased hospital admissions for myocardial infarction (MI) in Kaohsiung, Taiwan. Hospital admissions for MI and ambient air pollution data for Kaohsiung 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 (>25 degrees C), statistically significant positive associations were found in all pollutants except particulate matter (PM(10)) and sulfur dioxide (SO(2)). On cool days (<25 degrees C), all pollutants were significantly associated with MI admissions except for ozone (O(3)). For the two-pollutant model, O(3) and carbon monoxide (CO) were significant in combination with each of the other four pollutants on warm days. On cool days, nitrogen dioxide (NO(2)) remained statistically significant in all the two-pollutant models. This study provides evidence that higher levels of ambient air pollutants increase the risk of higher frequency of hospital admissions for MI.

Maternal smoking during pregnancy and child outcomes: real or spurious effect?

Maternal smoking during pregnancy (MSDP) is a major public health concern with clearly established consequences to both mother and newborn (e.g., low birth weight, altered cardiorespiratory responses). MSDP has also been associated with higher rates of a variety of poor cognitive and behavioral outcomes in children, including attention deficit hyperactivity disorder (ADHD), conduct disorder, impaired learning and memory, and cognitive dysfunction. However, the evidence suggesting causal effects of MSDP for these outcomes is muddied in the existing literature due to the frequent inability to separate prenatal exposure effects from other confounding environmental and genetic factors. Carefully designed studies using genetically sensitive strategies can build on current evidence and begin to elucidate the likely complex factors contributing to associations between MSDP and child outcomes.

Air pollution and emergency room visits for arrhythmias: are there potentially sensitive groups?

Recent studies showed that air pollution is a risk factor for hospitalization for arrhythmias. However, there is limited evidence to suggest which subpopulations are at higher risk for arrhythmia development due to increased air pollutant exposure. This study was undertaken to examine the modifying effect of specific secondary diagnosis (including hypertension, diabetes, and congestive heart failure) on the relationship between frequency of emergency room (ER) visits for arrhythmias and ambient air pollutants concentrations. ER visits for arrhythmias 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. Data showed an increased risk of ER visits for arrhythmias in relation to increased O(3) levels among individuals with a secondary diagnosis of hypertension and congestive heart failure.

Going with the flow: legionellosis risk in Toronto, Canada is strongly associated with local watershed hydrology

Legionella species are increasingly recognized as a cause of both healthcare- and community-acquired pneumonia (so-called "Legionnaire's disease"). These pathogens are ubiquitous in the environment, but environmental factors in the occurrence of sporadic legionellosis remain poorly understood. We analyzed all legionellosis cases identified in the Greater Toronto Area of Ontario from 1978 to 2006, and evaluated seasonal and environmental patterns in legionellosis case occurrence by using both negative binomial models and case-crossover analysis. A total of 837 cases were reported during the study period. After adjusting for seasonal effects, changes in the local watershed, rather than weather, were the strongest contributors to legionellosis risk. A 3.6-fold increase (95% confidence interval (CI), 2.4-5.3) in odds of disease was identified with decreasing watershed levels approximately 4 weeks before case-occurrence. We also found a 33% increase (95% CI, 8-64%) in odds of disease with decreasing lake temperature during the same period and a 34% increase (95% CI, 14-57%) with increasing humidity 5 weeks before case-occurrence. We conclude that local watershed ecology influences the risk of legionellosis, notwithstanding the availability of advanced water treatment capacity in Toronto. Enhancement of risk might occur through direct contamination of water sources or via introduction of micronutrients or commensal organisms into residential and hospital water supplies. These observations suggest testable hypotheses for future empiric studies.

Rapid effects of air pollution on ventricular arrhythmias

AIMS

Air pollution has been associated with ventricular arrhythmias in patients with implantable cardioverter defibrillators (ICDs) for exposure periods of 24-48 h. Only two studies have investigated exposure periods <24 h. We aimed to explore such effects during the 2 and 24 preceding hours as well as in relation to distance from the place of the event to the air pollution monitor.

METHODS AND RESULTS

We used a case-crossover design to investigate the effects of particulate matter <10 microm in diameter (PM10) and nitrogen dioxide (NO2) in 211 patients with ICD devices in Gothenburg and Stockholm, Sweden. Events interpreted as ventricular arrhythmias were downloaded from the ICDs, and air pollution data were collected from urban background monitors. We found an association between 2 h moving averages of PM10 and ventricular arrhythmia [odds ratio (OR) 1.31, 95% confidence interval (CI) 1.00-1.72], whereas the OR for 24 h moving averages was 1.24 (95% CI 0.87-1.76). Corresponding ORs for events occurring closest to the air pollution monitor were 1.76 (95% CI 1.18-2.61) and 1.74 (95% CI 1.07-2.84), respectively. Events occurring in Gothenburg showed stronger associations than in Stockholm.

CONCLUSION

Moderate increases in air pollution appear to be associated with ventricular arrhythmias in ICD patients already after 2 h, although future studies including larger numbers of events are required to confirm these findings. Representative geographical exposure classification seems important in studies of these effects.

Air pollution and hospital admissions for congestive heart failure: are there potentially sensitive groups?

Recent studies have shown that air pollution is a risk factor for hospitalization for congestive heart failure (CHF). 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, dysrhythmia, and chronic obstructive pulmonary disease) on the relationship between hospital admissions for CHF and ambient air pollutants. Hospital admissions for CHF and ambient air pollution data for Taipei were obtained for the period from 1996 to 2005. The relative risk of hospital admission was estimated using a case-crossover approach. None of the secondary diagnosis we examined (hypertension, diabetes, dysrhythmia, and chronic obstructive pulmonary disease (COPD)) showed much evidence of effect modification.

A Case-Crossover Analysis of Particulate Air Pollution and Cardiac Arrhythmia in Patients with Implantable Cardioverter Defibrillators

We investigated the relationship between air pollution and incidence of cardiac arrhythmia in a study of patients with implantable cardioverter defibrillators (ICDs). Thirty-four patients (ages 15-85 yr, 80% male) with ICDs residing in the Vancouver, Canada, area were included in the analyses, representing all patients attending the 2 ICD clinics in the study region who had recorded at least 1 ICD discharge during the 14 February to 31 December 2000 study period. Air pollutant (PM(2.5), PM(10), SO(4)(2-), elemental carbon [EC], organic carbon [OC], O(3), SO(2), NO(2), and CO) concentrations on days for which ICD discharges were observed ("case days") were compared to concentrations on control days in case-crossover analyses. Control days were selected symmetrically, 7 days before and after each case day. ICD discharges occurring within 72 h of 1 another were grouped and considered as 1 discharge event. Temperature, relative humidity, barometric pressure, rainfall, and wind speed were included simultaneously as covariates. Sensitivity analyses examined the effect of grouping ICD discharges, of including meteorological variables, and of excluding discharges that were considered inappropriate by a cardiologist. As in previous studies, mean concentrations and interquartile ranges of air pollutants in Vancouver were low (e.g., PM(2.5) mean = 8.2 microg/m(3)). Although in general there were no statistically significant results, there were trends that might indicate associations between pollutants and ICD discharges. Odds ratios (OR) were consistently higher in summer than in winter (e.g., lag 0 per interquartile range increase in EC: 1.09 [0.86-1.37] vs. 0.61 [0.31-1.18]) and, in general, the highest ORs were observed for same-day effects. The one major exception was the observation of high ORs for ozone in winter (e.g., lag 1: 2.27 [0.67-7.66]). While an OR of 1.55 (0.51-4.70) was observed in summer at lag 0 for PM(10), no indications of positive associations were observed for PM(2.5) or SO(4)(2-). For indicators of local combustion-source pollution, EC, OC, CO, and SO(2), ORs were elevated at all lags (0-3 days) in summer. In summary, this study provides little evidence that specific components of PM affect risk of cardiac arrhythmias, although power limited the ability of the study to detect small effects.

Air Pollution and Hospital Admissions for Congestive Heart Failure in a Tropical City: Kaohsiung, Taiwan

This study was undertaken to determine whether there was an association between air pollutant levels and hospital admissions for congestive heart failure (CHF) in Kaohsiung, Taiwan. Hospital admissions for CHF and ambient air pollution data for Kaohsiung were obtained for the period 1996-2004. The relative risk of hospital admission 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 (> 25 degrees C) statistically significant positive associations were found in all pollutants except sulfur dioxide (SO(2)). On cool days (< 25 degrees C), all pollutants were significantly associated with CHF admissions. For the two-pollutant model, CO and O(3) were significant in combination with each of the other four pollutants on warm days. On cool days, NO(2) remained statistically significant in all the two-pollutant models. This study provides evidence that higher levels of ambient air pollutants increase the risk of hospital admissions for CHF and that the effects of air pollutants on hospital admissions for CHF were temperature dependent.

A multicounty analysis identifying the populations vulnerable to mortality associated with high ambient temperature in California

The association between ambient temperature and mortality has been established worldwide, including the authors' prior study in California. Here, they examined cause-specific mortality, age, race/ethnicity, gender, and educational level to identify subgroups vulnerable to high ambient temperature. They obtained data on nine California counties from May through September of 1999-2003 from the National Climatic Data Center (countywide weather) and the California Department of Health Services (individual mortality). Using a time-stratified case-crossover approach, they obtained county-specific estimates of mortality, which were combined in meta-analyses. A total of 231,676 nonaccidental deaths were included. Each 10 degrees F (approximately 4.7 degrees C) increase in mean daily apparent temperature corresponded to a 2.6% (95% confidence interval (CI): 1.3, 3.9) increase for cardiovascular mortality, with the most significant risk found for ischemic heart disease. Elevated risks were also found for persons at least 65 years of age (2.2%, 95% CI: 0.04, 4.0), infants 1 year of age or less (4.9%, 95% CI: -1.8, 11.6), and the Black racial/ethnic group (4.9%, 95% CI: 2.0, 7.9). No differences were found by gender or educational level. To prevent the mortality associated with ambient temperature, persons with cardiovascular disease, the elderly, infants, and Blacks among others should be targeted.

Temperature and mortality in nine US cities

BACKGROUND

Extreme temperatures have been associated with increased mortality worldwide. The extent to which air pollutants may confound or modify this association remains unclear.

METHODS

We examined the association between mean apparent temperature and total mortality in 9 cities across the United States during the warm season (May to September) from 1999 to 2002. We applied case-crossover and time-series analyses, adjusting for day of the week and season in time-series analysis. City-specific estimates were then combined using a meta-analysis. A total of 213,438 deaths for all causes occurred in these cities during the study period.

RESULTS

We found that mortality increased with apparent temperature. A 5.5 degrees C (10 degrees F) increase in apparent temperature was associated with an increase in mortality of 1.8% (95% confidence interval = 1.09% to 2.5%) when using case-crossover analysis and with an increase of 2.7% (2.0% to 3.5%) using the time-series analysis.

CONCLUSIONS

This study provides evidence of increased mortality due to elevated apparent temperature exposure, with no confounding or effect modification due to air pollution.

Particulate matter and daily mortality: a case-crossover analysis of individual effect modifiers

BACKGROUND

Several time-series studies have established the relationship between particulate matter (PM10) and mortality. We adopted a case-crossover design to evaluate whether individual socio-demographic characteristics and chronic or acute medical conditions modify the PM10-mortality association.

METHODS

We selected all natural deaths (321,024 subjects) occurring among adult (aged 35+ years) residents of 9 Italian cities between 1997 and 2004. We had access to individual information on socio-demographic variables, location of death, and chronic conditions (hospital admissions in the preceding 2-year period). For in-hospital deaths, we collected information on treatment wards at time of death and acute medical conditions. In a case-crossover analysis we adjusted for time, population changes, and meteorological conditions.

RESULTS

PM10 was associated with mortality among subjects age 65 years and older (0.75% increase per 10 microg/m3 [95% confidence interval = 0.42% to 1.09%]), with a more pronounced effect among people age 85 and older. A weaker effect was found among the most affluent people. The effect was present for both out-of-hospital and in-hospital deaths, especially among those treated in general medicine and other less specialized wards. PM10 effects were stronger among people with diabetes (1.03% [0.28% to 1.79%]) and chronic obstructive pulmonary disease (0.84% [0.17% to 1.52%]). The acute conditions with the largest effect estimates were acute impairment of pulmonary circulation (4.56% [0.75% to 8.51%]) and heart failure (1.67% [0.30% to 3.04%]).

CONCLUSIONS

Several factors, including advanced age, type of hospital ward, and chronic and acute health conditions, modify the PM10-related risk of death. Altered pulmonary circulation and heart failure are important effect modifiers, suggesting that cardiac decompensation is a possible mechanism of the fatal PM10 effect.

Does temperature modify the association between air pollution and mortality? A multicity case-crossover analysis in Italy

Adverse health effects of particulate matter <10 microm in aerodynamic diameter (PM(10)) and high temperatures are well known, but the extent of their interaction on mortality is less clear. This paper describes effect modification of temperature in the PM(10)-mortality association and tests the hypothesis that higher PM(10) effects in summer are due to enhanced exposure to particles. All deaths of residents of nine Italian cities between 1997 and 2004 were selected. The case-crossover approach was adopted to estimate the effect of PM(10) on mortality by season and temperature level. Three strata of temperature corresponding to low, medium, and high "ventilation" were identified, and the interaction between PM(10) and temperature within each stratum was examined. Season and temperature levels strongly modified the PM(10)-mortality association: for a 10-microg/m(3) variation in PM(10), a 2.54% increase in risk of death in summer (95% confidence interval: 1.31, 3.78) compared with 0.20% (95% confidence interval: -0.08, 0.49) in winter. Analysis of the interaction between PM(10) and temperature within temperature strata resulted in positive but, in most cases, nonstatistically significant coefficients. The authors found much higher PM(10) effects on mortality during warmer days. The hypothesis that such an effect is attributable to enhanced exposure to particles in summer could not be rejected.

Is there adaptation in the ozone mortality relationship: a multi-city case-crossover analysis

BACKGROUND

Ozone has been associated with daily mortality, mainly in the summer period. Despite the ample literature on adaptation of inflammatory and pulmonary responses to ozone, and the link, in cohort studies, between lung function and mortality risk there has been little done to date to examine the question of adaptation in the acute mortality risk associated with ambient ozone.

METHODS

We applied a case-crossover design in 48 US cities to examine the ozone effect by season, by month and by age groups, particularly focusing on whether there was an adaptation effect.

RESULTS

We found that the same day ozone effect was highest in summer with a 0.5% (95% CI: 0.38, 0.62) increase in total mortality for 10 ppb increase in 8-hr ozone, whilst the effect decrease to null in autumn and winter. We found higher effects in the months May- July with a 0.46% (95% CI: 0.24, 0.68) increase in total mortality for 10 ppb increase in ozone in June, and a 0.65% (95% CI: 0.47, 0.82) increase in mortality during July. The effect decreased in August and became null in September. We found similar effects from the age group 51-60 up to age 80 and a lower effect in 80 years and older.

CONCLUSION

The mortality effects of ozone appear diminished later in the ozone season, reaching the null effect previously reported in winter by September. More work should address this issue and examine the biological mechanism of adaptation.

Airport and city-centre temperatures in the evaluation of the association between heat and mortality

A variety of ambient exposure indicators have been used to evaluate the impact of high temperature on mortality and in the identification of susceptible population sub-groups, but no study has evaluated how airport and city centre temperatures differ in their association with mortality during summer. This study considers the differences in temperatures measured at the airport and in the city centre of three Italian cities (Milan, Rome and Turin) and investigates the impact of these measures on daily mortality. The case-crossover design was applied to evaluate the association between daily mean apparent temperature (MAT) and daily total mortality. The analysis was conducted for the entire population and for subgroups defined by demographic characteristics, socioeconomic status and chronic comorbidity (based on hospitalisation during the preceding 2 years). The percentage risk of dying, with 95% confidence intervals (95% CI), on a day with MAT at the 95th percentile with respect to the 25th percentile of the June-September daily distribution was estimated. Airport and city-centre temperature distributions, which vary among cities and between stations, have a heterogeneous impact on mortality. Milan was the city with the greatest differences in mean MAT between airport and city stations, and the overall risk of dying was greater when airport MAT (+47% increase, 95%CI 38-57) was considered in comparison to city MAT (+37% increase, 95%CI 30-45). In Rome and Turin, the results were very similar for both apparent temperature measures. In all cities, the elderly, women and subjects with previous psychiatric conditions, depression, heart and circulation disorders and cerebrovascular disease were at higher risk of dying during hot days, and the degree of effect modification was similar using airport or city-centre MAT. Studies on the impact of meteorological variables on mortality, or other health indicators, need to account for the possible differences between airport and city centre meteorological variables in order to give more accurate estimates of health effects.

The effect of ambient air pollution during early pregnancy on fetal ultrasonic measurements during mid-pregnancy

BACKGROUND

Over the past decade there has been mounting evidence that ambient air pollution during pregnancy influences fetal growth.

OBJECTIVES

This study was designed to examine possible associations between fetal ultrasonic measurements collected from 15,623 scans (13-26 weeks gestation) and ambient air pollution during early pregnancy.

METHODS

We calculated mothers' average monthly exposures over the first 4 months of pregnancy for the following pollutants: particulate matter < 10 microm aerodynamic diameter (PM10), ozone, nitrogen dioxide, and sulfur dioxide. We examined associations with fetal femur length (FL), biparietal diameter (BPD), head circumference (HC), and abdominal circumference (AC). Final analyses included scans from only those women within 2 km of an air pollution monitoring site. We controlled for long-term trend, season, temperature, gestation, mother's age, socioeconomic status, and fetal sex.

RESULTS

A reduction in fetal AC was associated with O3 during days 31-60 [-1.42 mm; 95% confidence interval (CI), -2.74 to -0.09], SO2 during days 61-90 (-1.67 mm; 95% CI, -2.94 to -0.40), and PM10 during days 91-120 (-0.78 mm; 95% CI, -1.49 to -0.08). Other results showed a reduction in BPD (-0.68 mm; 95% CI, -1.09 to -0.27) associated with SO2 during days 0-30, a reduction in HC (-1.02 mm; 95% CI, -1.78 to -0.26) associated with PM10 during days 91-120, and a reduction in FL associated with PM10 during days 0-30 (-0.28 mm; 95% CI, -0.48 to -0.08) and 91-120 (-0.23; 95% CI, -0.42 to -0.04).

CONCLUSION

We found strong effects of ambient air pollution on ultrasound measures. Future research, including more individually detailed data, is needed to confirm our results.

Characterizing temperature and mortality in nine California counties

BACKGROUND

Elevated temperature has been associated with increased mortality. Few epidemiologic studies, however, have considered air pollutants as potential confounders or effect modifiers. None has focused on California, where the climate is generally mild and pollution levels tend to be high-an ideal setting to examine the independent effect of temperature from air pollution.

METHODS

We examined the association between mean daily apparent temperature and nonaccidental mortality in 9 counties throughout California from May to September 1999-2003. Data were obtained from the National Climatic Data Center (temperature and relative humidity), the California Department of Health Services (mortality), and the California Air Resources Board (particulate matter, ozone, carbon monoxide, and nitrogen dioxide). We conducted a time-stratified case-crossover study, with a time-series analysis as a sensitivity analysis, adjusting for day of the week using both methods and adjusting for time trend in the time-series analysis. We first obtained county-specific estimates and then combined them using meta-analytic methods.

RESULTS

A total of 248,019 deaths were included. Each 10 degrees (Fahrenheit) increase in same-day mean apparent temperature corresponded to a 2.3% increase in mortality (95% confidence interval = 1.0%-3.6%) in the case-crossover analysis for all 9 counties combined, with nearly identical results produced from the time-series analysis. No air pollutant examined was found to be a significant confounder or effect modifier.

CONCLUSIONS

Even without extremes in apparent temperature, we observed an association between temperature and mortality in California that was independent of air pollution.

[Contribution of case-crossover design to the analysis of short-term health effects of air pollution: reanalysis of air pollution and health data]

BACKGROUND

During the last decades, numerous studies have shown significant links between short-term exposure to air pollution and health. Time series design have been widely used in order to study these associations. In recent years, the case-crossover design has been applied to the analysis of acute effects of environmental exposures, especially air pollution. The aims of this paper are to describe the case-crossover design and to compare this approach with time series design to assess the association between air pollution and health.

METHODS

In the case-crossover approach, a case-control study is conducted whereby each person who had a health event (case period) is matched with himself/herself on a nearby time period where he/she did not have the event (control period). Review of the literature shows that the referent selection strategies can be associated to a bias in the estimation of the health effect of air pollution. In comparison with time series design, the case-crossover design is easier to conduct, and individual factors can be taken into account. Nevertheless, it is not possible to take into account the overdispersion in the health indicator with this approach.

RESULTS AND CONCLUSION

In conclusion, we suggest to use time series analysis with population data and case-crossover design with individual data.

Air pollution and hospital admissions for congestive heart failure in a subtropical city: Taipei, Taiwan

This study was undertaken to determine whether there was an association between air pollutant levels and hospital admissions for congestive heart failure (CHF) in Taipei, Taiwan. Hospital admissions for CHF and ambient air pollution data for Taipei were obtained for the period from 1996 to 2004. The relative risk of hospital admission 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, the number of CHF admissions was significantly associated with the environmental presence of the contaminants particulate matter (PM(10)), nitrogen dioxide (NO(2)), carbon monoxide (CO), and ozone (O(3)) on warm days (>20 degrees C). However, statistically significant positive effects on increased CHF admissions on cool days (<20 degrees C) was observed only for CO levels. For the two-pollutant model, NO(2) and O(3) were significant in combination with each of the other four pollutants on warm days for enhanced CHF admissions. This study provides evidence that higher levels of ambient air pollutants increase the risk of hospital admissions for CHF.

Air pollution and ED visits for asthma in Australian children: a case-crossover analysis

OBJECTIVE

We aimed to determine the effects of ambient air pollutants on emergency department (ED) visits for asthma in children.

METHODS

We obtained routinely collected ED visit data for asthma (ICD9 493) and air pollution (PM(10), PM(2.5), O(3), NO(2), CO and SO(2)) and meteorological data for metropolitan Sydney for 1997-2001. We used the time stratified case-crossover design and conditional logistic regression to model the association between air pollutants and ED visits for four age-groups (1-4, 5-9, 10-14 and 1-14 years). Estimated relative risks for asthma ED visits were calculated for an exposure corresponding to the inter-quartile range in pollutant level. We included same day average temperature, same day relative humidity, daily temperature range, school holidays and public holidays in all models.

RESULTS

Associations between ambient air pollutants and ED visits for asthma in children were most consistent for all six air pollutants in the 1-4 years age-group, for particulates and CO in the 5-9 years age-group and for CO in the 10-14 years age-group. The greatest effects were most consistently observed for lag 0 and effects were greater in the warm months for particulates, O(3) and NO(2). In two pollutant models, effect sizes were generally smaller compared to those derived from single pollutant models.

CONCLUSION

We observed the effects of ambient air pollutants on ED attendances for asthma in a city where the ambient concentrations of air pollutants are relatively low.

Air pollution and hospital admissions for pneumonia in a tropical city: Kaohsiung, Taiwan

This study was undertaken to determine whether there was an association between air pollutant levels and hospital admissions for pneumonia in Kaohsiung, Taiwan. Hospital admissions for pneumonia and ambient air pollution data for Kaohsiung were obtained for the period of 1996-2004. The relative risk of hospital admission 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 (= 25 degrees C) statistically significant positive associations were found for all pollutants. On cool days (< 25 degrees C), all pollutants were also significantly associated with number of pneumonia admissions. For the two-pollutant model, O3 and CO were significant in combination with each of the other four pollutants on warm days. On cool days, PM10 and NO2 remained statistically significant in all the two-pollutant models. This study provides evidence that higher levels of ambient air pollutants increase the risk of hospital admissions for pneumonia. The effects of air pollutants on hospital admissions for pneumonia were temperature dependent.

Air pollution and hospital admissions for asthma in a tropical city: Kaohsiung, Taiwan

This study was undertaken to determine whether there is an association between air pollutants levels and hospital admissions for asthma in Kaohsiung, Taiwan. Hospital admissions for asthma and ambient air pollution data for Kaohsiung were obtained for the period from 1996 through 2003. The relative risk of hospital admission 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 (> or = 25 degrees C) statistically significant positive associations were found in all pollutants except sulfur dioxide (SO2). On cool days (< or = 25 degrees C) all pollutants were significantly associated with asthma admissions For the two pollutant models, CO and O3 were significant in combination with each of the other four pollutants on warm days. On cool days NO2 remained statistically significant in all the two-pollutant models. This study provides evidence that higher levels of ambient pollutants increase the risk of hospital admissions for asthma.

Air pollution and hospital admissions for chronic obstructive pulmonary disease in a subtropical city: Taipei, Taiwan

This study was undertaken to determine whether there was an association between air pollutant levels and hospital admissions for chronic obstructive pulmonary disease (COPD) in Taipei, Taiwan. Hospital admissions for COPD and ambient air pollution data for Taipei were obtained for the period 1996-2003. The relative risk of hospital admission 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 = 20 degrees C) statistically significant positive associations were found in all pollutants except sulphur dioxide (SO2). However, statistically significant effects for COPD admissions on cool days (< 20 degrees C) were observed only for SO2 levels. For the two-pollutant model, NO2 and O3 were significant in combination with each of the other four pollutants on warm days. This study provides evidence that higher levels of ambient air pollutants increases the risk of hospital admissions for COPD in Taiwan.

Temperature, temperature extremes, and mortality: a study of acclimatisation and effect modification in 50 US cities

OBJECTIVES

The authors examined the increase in mortality associated with hot and cold temperature in different locations, the determinants of the variability in effect estimates, and its implications for adaptation.

METHODS

The authors conducted a case-crossover study in 50 US cities. They used daily mortality and weather data for 6 513 330 deaths occurring during 1989-2000. Exposure was assessed using two approaches. First, the authors determined exposure to extreme temperatures using city-specific indicator variables based on the local temperature distribution. Secondly, they used piecewise linear variables to assess exposure to temperature on a continuous scale above/below a threshold. Effects of hot and cold temperature were examined in season-specific models. In a meta-analysis of the city-specific results, the authors examined several city characteristics as effect modifiers.

RESULTS

Mortality increases associated with both extreme cold (2-day cumulative increase 1.59% (95% CI 0.56 to 2.63)) and extreme heat (5.74% (95% CI 3.38 to 8.15)) were found, the former being especially marked for myocardial infarction and cardiac arrest deaths. The increase in mortality was less marked at less extreme temperatures. The effect of extreme cold (defined as a percentile) was homogeneous across cities with different climates, suggesting that only the unusualness of the cold temperature (and not its absolute value) had a substantial impact on mortality (that is, acclimatisation to cold). Conversely, heat effects were quite heterogeneous, with the largest effects observed in cities with milder summers, less air conditioning and higher population density. Adjustment for ozone led to similar results, but some residual confounding could be present due to other uncontrolled pollutants.

CONCLUSIONS

The authors confirmed in a large sample of cities that both cold and hot temperatures increase mortality risk. These findings suggest that increases in heat-related mortality due to global warming are unlikely to be compensated for by decreases in cold-related mortality and that population acclimatisation to heat is still incomplete.

Mortality risk associated with short-term exposure to traffic particles and sulfates

BACKGROUND

Many studies have shown that airborne particles are associated with increased risk of death, but attention has more recently focused on the differential toxicity of particles from different sources. Geographic information system (GIS) approaches have recently been used to improve exposure assessment, particularly for traffic particles, but only for long-term exposure.

OBJECTIVES

We analyzed approximately 100,000 deaths from all, cardiovascular, and respiratory causes for the years 1995-2002 using a case-crossover analysis.

METHODS

Estimates of exposure to traffic particles were geocoded to the address of each decedent on the day before death and control days, with these estimates derived from a GIS-based exposure model incorporating deterministic covariates, such as traffic density and meteorologic factors, and a smooth function of latitude and longitude.

RESULTS

We estimate that an IQR increase in traffic particle exposure on the day before death is associated with a 2.3% increase [95% confidence interval (CI), 1.2 to 3.4%] in all-cause mortality risk. Stroke deaths were particularly elevated (4.4%; 95% CI, -0.2 to 9.3%), as were diabetes deaths (5.7%; 95% CI, -1.7 to 13.7%). Sulfate particles are spatially homogeneous, and using a central monitor, we found that an IQR increase in sulfate levels on the day before death is associated with a 1.1% (95% CI, 0.1 to 2.0%) increase in all-cause mortality risk.

CONCLUSIONS

Both traffic and powerplant particles are associated with increased deaths in Boston, with larger effects for traffic particles.

Air pollution and postneonatal mortality in a tropical city: Kaohsiung, Taiwan

With growing evidence of the association between daily mortality and air pollution in adults, it is important to investigate whether infants are also susceptible to the adverse health effects of ambient air pollutants. The purpose of this study is to examine the relationship between air pollution and postneonatal mortality in Kaohsiung, Taiwan, a large industrial city with a tropical climate, during the period 1994-2000, using a case-crossover analysis. Case-crossover analysis provides an alternative to Poisson time-series regression for studying the short-term adverse health effects of air pollution. The air pollutants examined included particulate matter (PM(10)), sulfur dioxide (SO(2)), ozone (O(3)), nitrogen dioxide (NO(2)), and carbon monoxide (CO). The risk of postneonatal deaths was estimated to increase by 4.0% per 67 microg/m(3) (the interquartile range in daily ambient concentration of PM(10)) for PM(10), 1.8% per 17.84 ppb for NO(2), 5.1% per 0.31 ppm for CO, and 4.6% per 19.20 ppb for O(3). Although positive, none of these associations achieved statistical significance. The established link between air pollution levels and infant mortality may not be as strong in cities with tropical climates, although other factors such as differences in pollutant mix or the underlying health of the postneonates may explain the lack of a strong association in this study. Further studies of this type in cities with varying climates and cultures are needed.

Air pollution and hospital admissions for chronic obstructive pulmonary disease in a tropical city: Kaohsiung, Taiwan

This study was undertaken to determine whether there is an association between air pollutants levels and hospital admissions for chronic obstructive pulmonary disease (COPD) in Kaohsiung, Taiwan. Hospital admissions for COPD and ambient air pollution data for Kaohsiung were obtained for the period from 1996 to 2003. The odds ratio of hospital admission 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 (> or =25 degrees C) statistically significant positive associations were found in all pollutants except sulphur dioxide (SO2). On cool days (< 25 degrees C), all pollutants were significantly associated with COPD admissions. For the two-pollutant models, CO and O3 were significant in combination with each of the other four pollutants on warm days. On cool days, NO2 remained statistically significant in all the two-pollutant models. This study provides evidence that higher levels of ambient pollutants increase the risk of hospital admissions for COPD.

Socioeconomic status, particulate air pollution, and daily mortality: differential exposure or differential susceptibility

BACKGROUND

Short-term increases in particulate air pollution are linked with increased daily mortality and morbidity. Socioeconomic status (SES) is a determinant of overall health. We investigated whether social class is an effect modifier of the PM(10) (particulate matter with diameter <10 micron)-daily mortality association, and possible mechanisms for this effect modification.

METHODS

Area-based traffic emissions, income, and SES were available for each resident in Rome. All natural deaths (83,253 subjects) occurring in Rome among city residents (aged 35+ years) during the period 1998-2001 were identified. For each deceased individual, all the previous hospitalizations within 2 years before death were available via a record linkage procedure. PM(10) daily data were available from two urban monitoring sites. A case-crossover analysis was utilized in which control days were selected according to the time stratified approach (same day of the week during the same month). Conditional logistic regression was used.

RESULTS

Due to the social class distribution in the city, exposure to traffic emissions was higher among those with higher area-based income and SES. Meanwhile, people of lower social class had suffered to a larger extent from chronic diseases before death than more affluent residents, especially diabetes mellitus, hypertension, heart failure, and chronic obstructive pulmonary diseases. Overall, PM(10) (lag 0-1) was strongly associated with mortality (1.1% increase, 95%CI = 0.7-1.6%, per 10 microg/m(3)). The effect was more pronounced among persons with lower income and SES (1.9% and 1.4% per 10 microg/m(3), respectively) compared to those in the upper income and SES levels (0.0% and 0.1%, respectively).

CONCLUSIONS

The results confirm previous suggestions of a stronger effect of particulate air pollution among people in low social class. Given the uneven geographical distributions of social deprivation and traffic emissions in Rome, the most likely explanation is a differential burden of chronic health conditions conferring a greater susceptibility to less advantaged people.

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

This study was undertaken to determine whether there is an association between exposure to air pollutants levels and number of hospital admissions for asthma in Taipei, Taiwan. Hospital admissions for asthma and ambient air pollution data for Taipei were obtained for the period from 1996 through 2003. The relative risk of hospital admission for asthma was estimated using a case-crossover approach, controlling for weather variables, day of the week, seasonality, and long-term time trends. In the single-air-pollutant model, on warm days (> or = 25 degrees C) statistically significant positive associations were found for SO2, NO2, and CO levels with an increase in asthmatic admissions. On cool days (< 25 degrees C), all air pollutants were significantly associated with elevated asthma admissions except SO2. For the two-air-pollutant model, CO significantly increases hospital admissions for asthma in combination with each of the other four pollutants on warm days. On cool days, NO2 and O3 significantly elevated asthma rates in all the two-air-pollutant models. This study provides evidence that higher levels of ambient air pollutant concentrations increase the risk of hospital admissions for asthma.

Seasonality and coronary heart disease deaths in United States firefighters

United States firefighters have a high on-duty fatality rate, and coronary heart disease is the leading cause. Seasonality affects the incidence of cardiovascular events in the general population, but its effects on firefighters are unknown. This study statistically examined the seasonal and annual variation of all on-duty coronary heart disease deaths among US firefighters between 1994 and 2004 using the chi-square distribution and Poisson regression model of the monthly fatality counts. It also examined the effect of ambient temperature (apparent as well as wind chill temperature) on coronary heart disease fatalities during the study span using a time-stratified, case-crossover study design. When grouped by season, we observed the distribution of the 449 coronary heart disease fatalities to show a relative peak in winter (32%) and relative nadir in spring (21%). This pattern was significantly different (p=0.005) from the expected distribution under the null hypothesis of season having no effect. The pattern persisted in additional analyses, stratifying the deaths by the type of duty in which the firefighters were engaged at the time of their deaths. In the Poisson regression model of the monthly fatality counts, the overall goodness-of-fit between the actual and predicted case counts was excellent (chi(4)(2)=16.63; p=0.002). Two distinct peaks were detected: one in January-February and the other in August-September. Overall temperature was not associated with increased risk of on-duty death. After allowing for different effects of temperature in mild/hot versus cold periods, a 1 degrees C increase was not protective in cold weather; nor did it increase the risk of death in warmer weather. The findings of this study reveal statistical evidence for excess coronary heart disease deaths among firefighters during winter; however, the temporal pattern of coronary heart disease deaths was not linked to temperature variation. The seasonal pattern was also found to be independent of duty-related risks.

Correlation between air pollution and postneonatal mortality in a subtropical city: Taipei, Taiwan

With growing evidence of the association between daily mortality and air pollution exposure in adults, it is important to investigate whether infants are also susceptible. The purpose of this study was to examine the relationship between air pollution exposure and postneonatal, defined as infant of more than 27 d and less than 1 yr old, mortality in Taipei, Taiwan's largest city, which has a subtropical climate, for the period 1994-2000, using a case-crossover analysis. This design is an alternative to Poisson time-series regression for studying the short-term adverse health effects of air pollution. The air pollutants examined included particulate matter (PM10), sulfur dioxide (SO2), ozone (O3), nitrogen dioxide (NO2), and carbon monoxide (CO). The risk of postneonatal deaths was estimated to increase by 3.1% for PM10, 4.1% for SO2, 1.7% for NO2, 3.8% for CO, and 0.1% for O3 for each interquartile range change, respectively. However, the associations were without statistical significance. The established link between air pollution levels and infant mortality may not be as strong in cities with subtropical climates, although other factors such as differences in pollutant component composition or the underlying health of the postneonates may explain the lack of a strong association in this study. Further studies of this type in cities with varying climates and cultures are needed.

A case-crossover study of fine particulate matter air pollution and onset of congestive heart failure symptom exacerbation leading to hospitalization

Persons with congestive heart failure may be susceptible to ambient air pollution. The authors evaluated the association between exposure to particulate matter with an aerodynamic diameter of <2.5 microm (PM2.5) and onset of symptom exacerbation leading to hospital admission in Baltimore, Maryland. They used a case-crossover design for 135 case events occurring among 125 persons with prevalent congestive heart failure who were admitted to a single hospital through the emergency department during 2002. The case period was assigned using three index times: 8-hour and 24-hour periods of symptom onset and date of hospital admission. Controlling for weather, the authors detected a modest relative increase in risk for cases defined by 8-hour symptom onset for an interquartile-range increase in PM2.5 at a 2-day lag (odds ratio=1.09, 95% confidence interval: 0.91, 1.30). A corresponding increase in risk was not observed when admission date was used to define the case period. A series of simulations based on study data indicated that the study had adequate statistical power to detect odds ratios of 1.2 or higher. Although overall findings were not statistically significant, the identification of case events defined by an 8-hour onset period may be more relevant than either a 24-hour onset period or the admission date for estimating harmful effects of air pollutant exposure on cardiovascular health.

Vulnerability to heat-related mortality: a multicity, population-based, case-crossover analysis

BACKGROUND

Although studies have documented increased mortality during heat waves, little information is available on the subgroups most susceptible to these effects. We evaluated the effects of summertime high temperature on daily mortality among population subgroups defined by demographic characteristics, socioeconomic status, and episodes of hospitalization for various conditions during the preceding 2 years.

METHODS

We studied a total of 205,019 residents of 4 Italian cities (Bologna, Milan, Rome, and Turin) age 35 or older who died during 1997-2003. The case-crossover design was applied to evaluate the association between mean apparent temperature (same and previous day) and all-cause mortality. Pooled odds ratios (ORs) and 95% confidence intervals (CIs) of dying at 30 degrees C (apparent temperature) relative to 20 degrees C were estimated accounting for time, population changes, and air pollution.

RESULTS

We found an overall OR of 1.34 (CI = 1.27-1.42) at 30 degrees C relative to 20 degrees C. The odds ratio increased with age and was higher among women (OR = 1.45; 1.37-1.52) and among widows and widowers (1.50; 1.33-1.69). Low area-based income modestly increased the effect. Among the preexisting medical conditions investigated, effect modification was detected for previous psychiatric disorders (1.69; 1.39-2.07), depression (1.72; 1.24-2.39), heart conduction disorders (1.77; 1.38-2.27), and circulatory disorders of the brain (1.47; 1.34-1.62). Temperature-related mortality was higher among people residing in nursing homes, and a large effect was also detected for hospitalized subjects.

CONCLUSIONS

Subsets of the population that are particularly vulnerable to high summer temperatures include the elderly, women, widows and widowers, those with selected medical conditions, and those staying in nursing homes and healthcare facilities.

Intracerebral haemorrhage associated with hourly concentration of ambient particulate matter: case-crossover analysis

AIMS

To examine the association of hourly time lagged concentration of ambient particulate matter and death due to stroke.

METHODS

Mortality data for five years (January 1990 to December 1994) were obtained from the Ministry of Health, Labour, and Welfare of Japan. Data were used only if the deceased was 65 years old or older at the time of death, if death was attributed to intracerebral haemorrhage or ischaemic stroke, and if the deceased lived in one of 13 major urban areas. Hourly mean concentrations of PM7, NO2, and photochemical oxidants were measured at monitoring stations in the 13 areas. Time stratified case-crossover analysis was used to examine the data for evidence of triggering stroke mortality.

RESULTS

The 1-hour mean concentration of PM7 measured about 2 hours before death was associated with the risk of death due to intracerebral haemorrhage from April to September (odds ratio = 2.40, 95% CI 1.48 to 3.89, for exposure to PM7 of more than 200 microg/m3 (threshold)). The higher risk was independent of the 24-hour mean concentration of PM7. PM7 was not associated with death due to ischaemic stroke.

CONCLUSIONS

Transiently high concentrations of PM7 are associated with death due to intracerebral haemorrhage. Air quality standards or guidelines for particulate matter should be based not only on 24-hour mean concentrations, but also on hourly data.

On the equivalence of case-crossover and time series methods in environmental epidemiology

The case-crossover design was introduced in epidemiology 15 years ago as a method for studying the effects of a risk factor on a health event using only cases. The idea is to compare a case's exposure immediately prior to or during the case-defining event with that same person's exposure at otherwise similar "reference" times. An alternative approach to the analysis of daily exposure and case-only data is time series analysis. Here, log-linear regression models express the expected total number of events on each day as a function of the exposure level and potential confounding variables. In time series analyses of air pollution, smooth functions of time and weather are the main confounders. Time series and case-crossover methods are often viewed as competing methods. In this paper, we show that case-crossover using conditional logistic regression is a special case of time series analysis when there is a common exposure such as in air pollution studies. This equivalence provides computational convenience for case-crossover analyses and a better understanding of time series models. Time series log-linear regression accounts for overdispersion of the Poisson variance, while case-crossover analyses typically do not. This equivalence also permits model checking for case-crossover data using standard log-linear model diagnostics.

A role of high impact weather events in waterborne disease outbreaks in Canada, 1975 - 2001

Recent outbreaks of Escherichia coli O157:H7, Campylobacter, and Cryptosporidium have heightened awareness of risks associated with contaminated water supply. The objectives of this research were to describe the incidence and distribution of waterborne disease outbreaks in Canada in relation to preceding weather conditions and to test the association between high impact weather events and waterborne disease outbreaks. We examined extreme rainfall and spring snowmelt in association with 92 Canadian waterborne disease outbreaks between 1975 and 2001, using case-crossover methodology. Explanatory variables including accumulated rainfall, air temperature, and peak stream flow were used to determine the relationship between high impact weather events and the occurrence of waterborne disease outbreaks. Total maximum degree-days above 0 degrees C and accumulated rainfall percentile were associated with outbreak risk. For each degree-day above 0 degrees C the relative odds of an outbreak increased by a factor of 1.007 (95% confidence interval [CI] = 1.002 - 1.012). Accumulated rainfall percentile was dichotomized at the 93rd percentile. For rainfall events greater than the 93rd percentile the relative odds of an outbreak increased by a factor of 2.283 (95% [CI] = 1.216 - 4.285). These results suggest that warmer temperatures and extreme rainfall are contributing factors to waterborne disease outbreaks in Canada. This could have implications for water management and public health initiatives.

The effect of ozone and PM10 on hospital admissions for pneumonia and chronic obstructive pulmonary disease: a national multicity study

A case-crossover study was conducted in 36 US cities to evaluate the effect of ozone and particulate matter with an aerodynamic diameter of < or =10 microm (PM10) on respiratory hospital admissions and to identify which city characteristics may explain the heterogeneity in risk estimates. Respiratory hospital admissions and air pollution data were obtained for 1986-1999. In a meta-analysis based on the city-specific regression models, several city characteristics were evaluated as effect modifiers. During the warm season, the 2-day cumulative effect of a 5-ppb increase in ozone was a 0.27% (95% confidence interval (CI): 0.08, 0.47) increase in chronic obstructive pulmonary disease admissions and a 0.41% (95% CI: 0.26, 0.57) increase in pneumonia admissions. Similarly, a 10-microg/m(3) increase in PM10 during the warm season resulted in a 1.47% (95% CI: 0.93, 2.01) increase in chronic obstructive pulmonary disease at lag 1 and a 0.84% (95% CI: 0.50, 1.19) increase in pneumonia at lag 0. Percentage of households with central air conditioning reduced the effect of air pollution, and variability of summer apparent temperature reduced the effect of ozone on chronic obstructive pulmonary disease. The study confirmed, in a large sample of cities, that exposure to ozone and PM10 is associated with respiratory hospital admissions and provided evidence that the effect of air pollution is modified by certain city characteristics.

The relationship between changes in daily air pollution and hospitalizations in Perth, Australia 1992-1998: a case-crossover study

A case-crossover study was undertaken to investigate the relationship between daily air pollutant concentrations and daily hospitalizations for selected disease categories in Perth, Western Australia. Daily measurements of particles (measured by nephelometry and PM2.5), photochemical oxidants (measured as ozone), nitrogen dioxide (NO2) and carbon monoxide (CO) concentrations were obtained from 1992 to 1998 via a metropolitan network of monitoring stations. Daily PM2.5 concentrations were estimated using monitored data, modelling and interpolation. Hospital morbidity data for respiratory, cardiovascular (CVD), gastrointestinal (GI) diseases, chronic obstructive pulmonary diseases (COPD) excluding asthma; pneumonia/influenza diseases; and asthma were obtained and categorized into all ages, less than 15 years and greater than 65 years. Gastrointestinal morbidity was used as a control disease. The data were analyzed using conditional logistic regression. The results showed a small number of significant associations for daily changes in particle concentrations, nitrogen dioxide and carbon monoxide for the respiratory diseases, CODP, pneumonia, asthma and CVD hospitalizations. Changes in ozone concentrations were not significantly associated with any disease outcomes. These data provide useful information on the potential health impacts of air pollution in an airshed with very low sulphur dioxide concentrations and lower nitrogen dioxide concentrations commonly found in many other cities.