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

A comparison of methods for the analysis of recurrent health outcome data with environmental covariates

Recurrent events such as repeated hospital admissions for the same health outcome occur frequently in environmental health studies. Dewanji and Moolgavkar proposed a flexible parametric model and a conditional likelihood analysis for recurrent events based on a Poisson process formulation. In this paper, we examine the statistical properties of the Dewanji-Moolgavkar (DM) estimator of the risk of an adverse health outcome associated with environmental exposures based on recurrent event data using computer simulation. We also compare the DM approach with both case-crossover analysis for multiple observations and time series analysis when there are no subject-specific covariates. When using a correctly specified model, the DM method produced better estimates with respect to relative mean square error when each subject had constant or curved baseline intensity functions than it did when baseline intensities were increasing or decreasing in a linear fashion. For under-specified models, the DM method outperformed case-crossover analysis for decreasing straight line intensity functions, was outperformed by case-crossover analysis for increasing straight line intensity functions, and was roughly equivalent to case-crossover analysis for constant and curved intensity functions. Case-crossover analysis produced superior risk estimates more frequently than the other two methods in the cases considered here, especially for linear representations of the baseline intensities.

Air pollution and hospital admissions for ischemic and hemorrhagic stroke among medicare beneficiaries

BACKGROUND AND PURPOSE

The association between short-term elevations in ambient air particles and increased cardiovascular morbidity and mortality is well documented. Ambient particles may similarly increase the risk of stroke.

METHODS

We evaluated the association between daily levels of respirable particulate matter (aerodynamic diameter < or =10 microm, PM10) and hospital admission for ischemic and hemorrhagic stroke among Medicare recipients (age > or =65 years) in 9 US cities using a 2-stage hierarchical model. In the first stage, we applied the time-stratified case-crossover design to estimate the effect of PM10 in each city. We used a 3-day unconstrained, distributed lag model to simultaneously estimate the effect of PM10 0 to 2 days before the admission day and controlled for meteorological covariates in all of the models. In the second stage, we used random-effects metaanalytic techniques to combine the city-specific effect estimates.

RESULTS

Ischemic (n=155,503) and hemorrhagic (19,314) stroke admissions were examined separately. For ischemic stroke, an interquartile range increase in PM10 was associated with a 1.03% (95% CI, 0.04% to 2.04%) increase in admissions on the same day only. Similar results were observed with CO, NO2, and SO2. For hemorrhagic stroke, no association was observed with any pollutant 0 to 2 days before admission.

CONCLUSIONS

These results suggest that elevations in ambient particles may transiently increase the risk of ischemic, but not hemorrhagic, stroke. Studies with more accurate assessment of timing of stroke onset are necessary to confirm or refute these findings.

Case???Crossover Analyses of Air Pollution Exposure Data

The case-crossover design has been widely used to study the association between short-term air pollution exposure and the risk of an acute adverse health event. The design uses cases only; for each individual case, exposure just before the event is compared with exposure at other control (or "referent") times. Time-invariant confounders are controlled by making within-subject comparisons. Even more important in the air pollution setting is that time-varying confounders can also be controlled by design by matching referents to the index time. The referent selection strategy is important for reasons in addition to control of confounding. The case-crossover design makes the implicit assumption that there is no trend in exposure across the referent times. In addition, the statistical method that is used-conditional logistic regression-is unbiased only with certain referent strategies. We review here the case-crossover literature in the air pollution context, focusing on key issues regarding referent selection. We conclude with a set of recommendations for choosing a referent strategy with air pollution exposure data. Specifically, we advocate the time-stratified approach to referent selection because it ensures unbiased conditional logistic regression estimates, avoids bias resulting from time trend in the exposure series, and can be tailored to match on specific time-varying confounders.

A call for reporting the relevant exposure term in air pollution case-crossover studies

The exposure term in the case-crossover design consists in the difference between the ambient concentration on the event day and the concentration(s) on some control day(s). So far, all air pollution case-crossover studies presented the distribution of the daily ambient pollutant concentrations but do not publish the distributional properties of the relevant exposure term--that is, the concentration difference. This article shows that this difference can be very small for a large fraction of event days, therefore, seriously limiting the statistical power to refute the null hypothesis. Publishing the distribution of the relevant differences will improve the interpretation and discussion of findings from case-crossover studies, particularly in cases with statistically non-significant associations.

The effect of particulate air pollution on emergency admissions for myocardial infarction: a multicity case-crossover analysis

Recently, attention has focused on whether particulate air pollution is a specific trigger of myocardial infarction (MI). The results of several studies of single locations assessing the effects of ambient particular matter on the risk of MI have been disparate. We used a multicity case-crossover study to examine risk of emergency hospitalization associated with fine particulate matter (PM) with aerodynamic diameter < 10 microm (PM10) for > 300,000 MIs during 1985-1999 among elderly residents of 21 U.S. cities. We used time-stratified controls matched on day of the week or on temperature to detect possible residual confounding by weather. Overall, we found a 0.65% [95% confidence interval (CI), 0.3-1.0%] increased risk of hospitalization for MI per 10 microg/m3 increase in ambient PM10 concentration. Matching on apparent temperature yielded a 0.64% increase in risk (95% CI, 0.1-1.2%). We found that the effect size for PM10 doubled for subjects with a previous admission for chronic obstructive pulmonary disease or a secondary diagnosis of pneumonia, although these differences did not achieve statistical significance. There was a weaker indication of a larger effect on males but no evidence of effect modification by age or the other diagnoses. We also found that the shape of the exposure-response relationship between MI hospitalizations and PM10 is almost linear, but with a steeper slope at levels of PM10 < 50 microg/m3. We conclude that increased concentrations of ambient PM10 are associated with increased risk of MI among the elderly.

Coarse particulate matter and hospitalization for respiratory infections in children younger than 15 years in Toronto: a case-crossover analysis

OBJECTIVES

The purpose of this study was to examine the association between ambient air pollution and hospitalization for respiratory infections among children who were younger than 15 years in Toronto during a 4-year period (1998-2001).

METHODS

Exposures averaged during periods that varied from 1 to 7 days were used to assess the effects of air pollutants, including thoracic particulate matter (PM10), fine (PM2.5) and coarse (PM10-2.5) particulate matter, carbon monoxide (CO), sulfur dioxide (SO2), nitrogen dioxide (NO2), and ozone (O3), on hospitalization for respiratory infections. A case-crossover design was used to calculate odds ratios for the hospitalization adjusted for daily weather conditions with an incremented exposure corresponding to the interquartile range in air pollution exposures.

RESULTS

When particulate matter and gaseous pollutants were mutually taken into account, the effect remained pronounced for PM10-2.5 in both boys and girls. The adjusted odds ratio for 6-day average exposure to PM10-2.5 with an increment of 6.5 microg/m3 was 1.15 (95% confidence interval: 1.02-1.30) for boys and 1.18 (95% confidence interval: 1.01-1.36) for girls. The effect also remained for PM10 in boys and for NO2 in girls. PM2.5, CO, SO2, and O3 showed no significant effects on hospitalization for respiratory infection in both genders when other pollutants were taken into consideration.

CONCLUSION

Our study suggested a detrimental effect of relatively low levels of ambient particulate matter and gaseous pollutants, especially coarse particulate matter and NO2, on hospitalization for respiratory infections in children.

A case-crossover analysis of out-of-hospital coronary deaths and air pollution in Rome, Italy

RATIONALE

Out-of-hospital coronary heart disease death is a major public health problem, but the association with air pollution is not well understood.

OBJECTIVES

We evaluated the association between daily ambient air pollution levels (particle number concentration [PNC]--a proxy for ultrafine particles [diameter < 0.1 microm], mass of particles with diameter less than 10 microm [PM10]; CO, NO2, and O3) and the occurrence of fatal, nonhospitalized coronary events.

METHODS

Subjects were 5,144 out-of-hospital fatalities (410-414, International Classification of Diseases-9; 1998-2000) who had been residents of Rome. Hospitalizations during the 3 yr before death were considered to identify comorbidities (e.g., diabetes, hypertension, heart failure, dysrhythmia, chronic obstructive pulmonary disease). Statistical analyses were performed using a case-crossover design.

MEASUREMENTS AND MAIN RESULTS

The association with out-of-hospital coronary deaths was statistically significant for PNC, PM10, and CO. Air pollution on the day of death had the strongest effect (e.g., 7.6% increase [95% confidence interval, 2.0-13.6%]) for an interquartile range of PNC, 27,790 particles/cm3. The 65-74- and 75+-yr age groups were at higher risk than the 35-64-yr age group, and there was a suggestion of effect modification for people with hypertension and chronic obstructive pulmonary disease.

CONCLUSIONS

Air pollutants originating from combustion processes, including ultrafine particles, are related to fatal, nonhospitalized coronary events. The effect is stronger among people over 65 years of age, but is not limited to a group with a specific comorbidity.

Particulate air pollution and the rate of hospitalization for congestive heart failure among medicare beneficiaries in Pittsburgh, Pennsylvania

The authors used a case-crossover approach to evaluate the association between ambient air pollution and the rate of hospitalization for congestive heart failure among Medicare recipients (aged > or =65 years) residing in Allegheny County (Pittsburgh area), Pennsylvania, during 1987-1999. They also explored effect modification by age, gender, and specific secondary diagnoses. During follow-up, 55,019 patients were admitted with a primary diagnosis of congestive heart failure. In single-pollutant models, particulate matter with an aerodynamic diameter of <10 microm (PM(10)), carbon monoxide, nitrogen dioxide, and sulfur dioxide-but not ozone-were positively and significantly associated with the rate of admission on the same day. The strongest associations were observed with carbon monoxide, nitrogen dioxide, and PM(10). The associations with carbon monoxide and nitrogen dioxide were the most robust in two-pollutant models, remaining statistically significant even after adjusting for other pollutants. Patients with a recent myocardial infarction were at greater risk of particulate-related admission; otherwise, there was no significant effect modification by age, gender, or other secondary diagnoses. These results suggest that short-term elevations in air pollution from traffic-related sources may trigger acute cardiac decompensation in heart failure patients and that those with certain comorbid conditions may be more susceptible to these effects.

Temperature and mortality among the elderly in the United States: a comparison of epidemiologic methods

BACKGROUND

Time-series analyses have been used for decades to investigate time-varying environmental exposures. Recently, the case-crossover design has been applied to assess acute effects of air pollution. Our objective was to compare time-series and case-crossover analyses using varying referent periods (ie, unidirectional, ambidirectional, and time-stratified).

METHODS

We examined the association between temperature and cardiorespiratory mortality among the elderly population in the 20 largest metropolitan areas of the United States. Risks were estimated by season and geographic region in 1992. We obtained weather data from the National Climatic Data Center and mortality data from the Division of Vital Statistics. Conditional logistic regression (case-crossover) and Poisson regression (time-series) were used to estimate the increased risk of cardiorespiratory mortality associated with a 10 degrees F increase in daily temperature, accounting for dew-point temperature and other potential confounding factors.

RESULTS

In the time-stratified case-crossover analysis, the strongest associations were found in the summer; in the Southwest, Southeast, Northwest, Northeast, and Midwest, the odds ratios were 1.15 (95% confidence interval=1.07-1.24), 1.10 (0.96-1.27), 1.08 (0.92-1.26), 1.08 (1.02-1.15), and 1.01 (0.92-1.11), respectively. Mostly null or negative associations were found in the winter, spring, and fall. The ambidirectional case-crossover and the time-series analyses produced quantitatively similar results to those from the time-stratified analysis. The unidirectional analysis produced conflicting results.

CONCLUSIONS

Inferences from studies of weather and mortality using the ambidirectional or time-stratified case-crossover approaches and the time-series analyses are comparable and provide consistent findings in this study.

Air pollution and hospital admissions for cardiovascular disease in Taipei, Taiwan

This study was undertaken to determine whether there is an association between air pollutants levels and hospital admissions for cardiovascular diseases (CVD) in Taipei, Taiwan. Hospital admissions for CVD and ambient air pollution data for Taipei were obtained for the period 1997-2001. 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. For the one-pollutant models, on warm days (>or=20 degrees C) statistically significant positive associations were found between levels of particulate matter <10-microm aerodynamic diameter (PM10), nitrogen dioxide (NO2), carbon monoxide (CO), and ozone (O3). On cool days (<20 degrees C), all pollutants except O3 and SO2 were significantly associated with CVD admissions. For the two-pollutant models, CO, NO2, and O3 were significant in combination with each of the other four pollutants on warm days. On cool days, PM10 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 CVD.

Smoking and the risk of oral clefts: exploring the impact of study designs

BACKGROUND

Maternal cigarette smoking is a suspected cause of oral clefts, although this association has not been firmly established. We used case-crossover, case-time-control, and bidirectional case-crossover designs to supplement findings from a case-control study of maternal smoking and oral clefts among offspring in a large birth registry.

METHODS

Data are from the Swedish Medical Birth Registry. From 1983 through 1997 there were 678 recorded cases of cleft palate and 1175 cases of cleft lip with or without palate. Maternal smoking status was ascertained in early pregnancy. Controls for the case-control study were a random sample of infants born without a cleft; controls for the case-crossover designs were nonmalformed infants born to case mothers.

RESULTS

Cleft palate was positively associated with maternal smoking in all study designs, whereas cleft lip with or without cleft palate was associated with smoking only in the case-control design. In the case-control design, the odds ratios for cleft palate were 1.2 (95% confidence interval = 1.0-1.5) for women who smoked 1 to 9 cigarettes per day and 1.4 (1.1-1.8) for women who smoked 10+ cigarettes per day. In the case-time-control analysis, the odds ratio for cleft palate with maternal smoking was 3.2 (1.3-7.4) and in the bidirectional case-crossover design, the odds ratio was 2.2 (1.1-4.1).

CONCLUSIONS

An association between smoking and cleft palate was supported by all designs, whereas that between smoking and cleft lip with or without cleft palate was not. Case-only designs are a viable option in birth registries and may yield more information than a case-control design alone.

How Sensitive Is the Association between Ozone and Daily Deaths to Control for Temperature?

RATIONALE

Air pollution has been associated with changes in daily mortality.

OBJECTIVES

Generally, studies use Poisson regression, with complicated modeling strategies, to control for season and weather, raising concerns that the results may be sensitive to these modeling protocols. For studies of ozone, weather control is a particular problem because high ozone days are generally quite hot.

METHODS

The case-crossover approach converts this problem into a case-control study, where the control for each person is the same person on a day near in time, when he or she did not die. This method controls for season and individual risk factors by matching. One can also choose the control day to have the same temperature as the event day.

MEASUREMENTS

I have applied this approach to a study of more than 1 million deaths in 14 U.S. cities.

MAIN RESULTS

I found that, with matching on temperature, a 10-ppb increase in maximum hourly ozone concentrations was associated with a 0.23% (95% confidence interval [CI] 0.01%, 0.44%) increase in the risk of dying. This finding was indistinguishable from the risk when only matching on season and controlling for temperature with regression splines (0.19%; 95% CI 03%, 0.35%). Control for suspended particulate matter with an aerodynamic diameter of 10 mum or less (PM(10)) did not change this risk. However, the association was restricted to the warm months (0.37% increase; 95% CI 0.11%, 0.62%), with no effect in the cold months.

CONCLUSIONS

The association between ozone and mortality risk is unlikely to be caused by confounding by temperature.

Air pollution and child respiratory health: a case-crossover study in Australia and New Zealand

RATIONALE

The strength of the association between outdoor air pollution and hospital admissions in children has not yet been well defined.

OBJECTIVES

To estimate the impact of outdoor air pollution on respiratory morbidity in children after controlling for the confounding effects of weather, season, and other pollutants.

METHODS

The study used data on respiratory hospital admissions in children (three age groups: < 1, 1-4, and 5-14 years) for five cities in Australia and two in New Zealand. Time series of daily numbers of hospital admissions were analyzed using the case-crossover method; the results from cities were combined using a random-effects meta-analysis.

MEASUREMENTS AND MAIN RESULTS

Significant increases across the cities were observed for hospital admissions in children for pneumonia and acute bronchitis (0, 1-4 years), respiratory disease (0, 1-4, 5-14 years), and asthma (5-14 years). These increases were found for particulate matter with a diameter less than 2.5 microm (PM2.5) and less than 10 microm (PM10), nephelometry, NO2, and SO2. The largest association found was a 6.0% increase in asthma admissions (5-14 years) in relation to a 5.1-ppb increase in 24-hour NO2.

CONCLUSIONS

This study found strong and consistent associations between outdoor air pollution and short-term increases in childhood hospital admissions. A number of different pollutants showed significant associations, and these were distinct from any temperature (warm or cool) effects.

Ozone Air Pollution Is Associated With Acute Myocardial Infarction

BACKGROUND

Despite the diversity of the studied health outcomes, types and levels of pollution, and various environmental settings, there is substantial evidence for a positive link between urban air pollution and cardiovascular diseases. The objective of this study was to test the associations between air pollutants and the occurrence of acute myocardial infarction (AMI).

METHODS AND RESULTS

Pollutant concentrations (SO2, NO2, and O3) were measured hourly as part of the automated air quality network. Since 1985, an AMI registry (the Toulouse MONICA Project) has been collecting data in the southwest of France. All cases of AMI and sudden and probable cardiac deaths are recorded for subjects 35 to 64 years of age. We studied the short-term exposure effect of pollution on the risk of AMI (from January 1, 1997, to June 30, 1999) using a case-crossover design method. We performed a conditional logistic regression analysis to calculate relative risks (RRs) and their 95% CIs. After adjustment for temperature, relative humidity, and influenza epidemics, the RRs (for an increase of 5 microg/m3 of O3 concentration) for AMI occurrence were significant for the current-day and 1-day-lag measurements (RR, 1.05; 95% CI, 1.01 to 1.08; P=0.009; and RR, 1.05; 95% CI, 1.01 to 1.09; P=0.007, respectively). Subjects 55 to 64 years of age with no personal history of ischemic heart disease were the most susceptible to develop an AMI (RR, 1.14; 95% CI, 1.06 to 1.23). NO2 and SO2 exposures were not significantly associated with the occurrence of AMI.

CONCLUSIONS

Observational data confirm that short-term O3 exposure within a period of 1 to 2 days is related to acute coronary events in middle-aged adults without heart disease, whereas NO2 and SO2 are not.

Relation Between Short-Term Fine-Particulate Matter Exposure and Onset of Myocardial Infarction

BACKGROUND

Epidemiologic studies have reported increases in the incidence of cardiovascular morbidity and myocardial infarction (MI) associated with increases in short-term and daily levels of fine-particulate matter air pollution, suggesting a role for particulate matter in triggering an MI.

METHODS

We studied the association between onset time of MI and preceding hourly measures of fine-particulate matter using a case-crossover study of 5793 confirmed cases of acute MI. We linked data from a community-wide database on acute MI from 1988-1994 in King County, Washington, with central site air pollution monitoring data on fine-particulate matter determined by nephelometry. We compared air pollution exposure levels averaged 1 hour, 2 hours, 4 hours, and 24 hours before MI onset to a set of time-stratified referent exposures from the same day of the week in the month of the case event.

RESULTS

: The estimated relative risk for a 10-microg/m increase in fine-particulate matter the hour before MI onset was 1.01 (95% CI=0.98-1.05). Analyses of pollutant levels at the other time points demonstrated a similar lack of association. No increased risk was found in all cases with preexisting cardiac disease (odds ratio = 1.05; 0.95-1.16). Stratification by known cardiovascular risk factors (hypertension, diabetes, and smoking status) also did not modify the relation between fine-particulate matter and MI onset.

CONCLUSION

Although a very small effect cannot be excluded, there was no consistent association between ambient levels of fine-particulate matter and risk of MI onset.

Relationship between air pollution and daily mortality in a subtropical city: Taipei, Taiwan

Air pollution has been associated with daily mortality in numerous studies over the past decade. However most of these studies were conducted in the United States and Europe with relatively few done in Asia. In the current study, the association between ambient air pollution and daily mortality in Taipei, Taiwan's largest city which has a subtropical climate was undertaken, for the period 1994-1998 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 (PM(10)), sulfur dioxide (SO(2)), ozone (O(3)), nitrogen dioxide (NO(2)), and carbon monoxide (CO). The largest observed effect, which was without statistical significance, was seen for NO(2) and CO levels on deaths due to respiratory diseases (ORs=1.013 and 1.014, respectively). The well established link between air pollution levels and daily mortality may not be as strong in cities in subtropical areas, although other factors such as differences in pollutant mix or the underlying health of the population 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.

Relationship between ambient air pollution and hospital admissions for cardiovascular diseases in kaohsiung, taiwan

This study was undertaken to determine whether there is an association between air pollutants levels and increased number of hospital admissions for cardiovascular diseases (CVD) in Kaohsiung, Taiwan. Hospital admissions records for CVD and ambient air contaminant data collected from monitoring station in Kaohsiung were obtained for the period 1997-2000. The relative risk of hospital admission for CVD 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 =25 degrees C) statistically significant positive associations were found between levels of particular matter of < 10 microm aerodynamic diameter (PM10), nitrogen dioxide (NO2), carbon monoxide (CO), and ozone (O3). On cool days (< 25 degrees C), all pollutants except O3 were significantly associated with increased CVD admissions. For the two-pollutant model, CO and O3 were both significant in combination with each of the other four contaminants on warm days. PM10 and NO2 remained significantly associated with elevated CVD admissions on warm days. On cool days, CO and NO2 remained statistically significant in all the two-pollutant models. This study provides evidence that higher levels of ambient contaminants, particularly CO, increase the risk of increased hospital admissions for CVD.

Evidence for an association between air pollution and daily stroke admissions in Kaohsiung, Taiwan

BACKGROUND AND PURPOSE

Many studies have reported increases in daily cardiovascular mortality and hospital admissions associated with increases in levels of air pollutants. However, little is known about the relationship between hospital admissions for stroke and air pollution. This study was undertaken to determine whether there is an association between air pollution and hospital admissions for stroke in Kaohsiung, Taiwan.

METHODS

Data on a total of 23 179 stroke admissions were obtained for the period 1997 through 2000. The relative risk of hospital admissions was estimated with a case-crossover approach.

RESULTS

In the single-pollutant models, on warm days (> or =20 degrees C), significant positive associations were found between levels of PM10, NO2, SO2, CO, and O3 and both primary intracerebral hemorrhage and ischemic stroke admissions. On cool days (<20 degrees C), only CO levels and ischemic stroke admissions were significantly associated. For the 2-pollutant models, PM10 and NO2 remained consistently and significantly associated with admissions for both types of stroke on warm days. We observed estimated relative risks of 1.54 (95% confidence interval [95%], 1.31 to 1.81) and 1.56 (95% CI, 1.32 to 1.84) for primary intracerebral hemorrhage for each interquartile range increase in PM10 and NO2. The values for ischemic stroke were 1.46 (95% CI, 1.32 to 1.61) and 1.55 (95% CI, 1.40 to 1.71), respectively. The effects of CO, SO2, and O3 were mostly nonsignificant when either NO2 or PM10 was controlled for.

CONCLUSIONS

This study provides an association between exposure to air pollution and hospital admissions for stroke.

Air Pollution and Myocardial Infarction in Rome

BACKGROUND

Daily air pollution is associated with increased hospital admissions for cardiovascular diseases, but there are few observations on the link with acute myocardial infarction. To evaluate the relation between various urban air pollutants (total suspended particulate, SO2, CO, NO2) and hospital admissions for acute myocardial infarction in Rome, Italy, we performed a case-crossover analysis and studied whether individual characteristics act as effect modifiers.

METHODS

We studied 6531 subjects residing in Rome and hospitalized for a first episode of acute myocardial infarction (International Classification of Diseases, 9th edition: 410) from January 1995 to June 1997. The following individual information was available: sex, age, date of hospitalization, coexisting illnesses (hypertension, 25%; diabetes, 15%), and cardiac severity (conduction disorders, 6%; cardiac dysrhythmias, 20%; heart failure, 11%). Daily air pollution data were taken from 5 city monitors. We used a time-stratified case-crossover design; control days were the same day of the week as the myocardial infarction occurred, in other weeks of the month.

RESULTS

Positive associations were found for total suspended particulate, NO2 and CO. The strongest and most consistent effect was found for total suspended particulate. The odds ratio (OR) associated with 10 micro g/m3 of total suspended particulate over the 0- to 2-day lag was 1.028 (95% confidence interval [CI] = 1.005-1.052). The association with total suspended particulate tended to be stronger among people older than 74 years of age (OR = 1.046; CI = 1.005-1.089), in the warm period of the year (OR = 1.046; CI = 1.008-1.087), and among subjects who had heart conduction disorders (OR = 1.080; CI = 0.987-1.181).

CONCLUSIONS

The results suggest that air pollution increases the risk of myocardial infarction, especially during the warm season. There was a tendency for a stronger effect among the elderly and people with heart conduction disturbances.

Relationship between air pollution and daily mortality in a tropical city: Kaohsiung, Taiwan

Air pollution has been associated with daily mortality in numerous studies over the past decade. However, most of these studies were conducted in the United States and Europe, with relatively few done in Asia. In this study, the association between ambient air pollution and daily mortality in Kaohsiung, Taiwan, a large industrial city with a tropical climate, was investigated 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). No significant effects were found between PM10 and SO2 exposure levels and respiratory-related mortality. The well-established link between air pollution levels and daily mortality may not be as strong in cities in tropical areas, although other factors such as differences in pollutant mixtures or underlying health of the population 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 Analysis of Air Pollution and Daily Mortality in Shanghai

We used a case-crossover approach to assess the association between air pollution and daily mortality in Shanghai from June 2000 to December 2001. By design, this method can avoid some common concerns about the time-series approach, which was most frequently used to assess the short-term effects of air pollution. Different control periods (unidirectional and bi-directional control samplings) were used for the analysis. With a bi-directional six control sampling approach, the results from a conditional logistic regression model controlling for weather conditions showed that each 10 microg/m3 increase over a 48-h moving average of PM10, SO2 and NO2 corresponds to 1.003 (95%CI 1.001-1.005), 1.016 (95%CI 1.011-1.021), and 1.020 (95%CI 1.012-1.027) relative risk of non-accident mortality, respectively. The association between air pollution and mortality for chronic obstructive pulmonary diseases (COPD) and cardiovascular causes was found to be closer than that for all causes. The results confirmed the deleterious role of the current air pollution level on human health in Shanghai, and provided information on the applicability of case-crossover design in studying the acute health effects of air pollution.

Effect of short-term exposure to gaseous pollution on asthma hospitalisation in children: a bi-directional case-crossover analysis

STUDY OBJECTIVE

Assess associations between short-term exposure to gaseous pollutants and asthma hospitalisation among boys and girls 6 to12 years of age.

DESIGN

A bi-directional case-crossover analysis was used. Conditional logistic regression models were fitted to the data for boys and girls separately. Exposures averaged over periods ranging from one to seven days were used to assess the effects of gaseous pollutants on asthma hospitalisation. Estimated relative risks for asthma hospitalisation were calculated for an incremental exposure corresponding to the interquartile range in pollutant levels, adjusted for daily weather conditions and concomitant exposure to particulate matter.

SETTING

Toronto, Ontario, Canada.

PARTICIPANTS

A total of 7319 asthma hospitalisations for children 6 to 12 years of age (4629 for boys and 2690 for girls) in Toronto between 1981 and 1993.

MAIN RESULTS

A significant acute effect of carbon monoxide on asthma hospitalisation was found in boys, and sulphur dioxide showed significant effects of prolonged exposure in girls. Nitrogen dioxide was positively associated with asthma admissions in both sexes. The lag time for certain gaseous pollutant effects seemed to be shorter in boys (around two to three days for carbon monoxide and nitrogen dioxide), as compared with girls (about six to seven days for sulphur dioxide and nitrogen dioxide). The effects of gaseous pollutants on asthma hospitalisation remained after adjustment of particulate matter. The data showed no association between ozone and asthma hospitalisation in children.

CONCLUSIONS

The study showed positive relations between gaseous pollutants (carbon monoxide, sulphur dioxide, and nitrogen dioxide) at comparatively low levels and asthma hospitalisation in children, using bi-directional case-crossover analyses. Though, the effects of certain specific gaseous pollutants were found to vary in boys and girls.

Effect of nitrogen dioxide and ozone on the risk of dying in patients with severe asthma

BACKGROUND

A study was performed to assess the acute association between air pollution, pollen and spores, and mortality in a population based cohort of subjects with asthma recruited from emergency room admissions for an asthma exacerbation using a case crossover design.

METHODS

Patients in Barcelona aged over 14 years who died during the period 1985-95 who had visited the emergency department of one of the four largest hospitals in the city for asthma during 1985-9 were included in the study (a total of 467 men and 611 women). Deaths were identified by record linkage of the cohort individuals with the Catalonia mortality registry. Causes of death were based on the underlying cause on the death certificate. Air pollution, pollen and spore levels were measured at the city monitoring stations which provide an average for the entire city.

RESULTS

Nitrogen dioxide was associated with mortality for all causes of death (adjusted odds ratio (OR) for an increase of the interquartile range = 1.50, 95% confidence interval (CI) 1.09 to 2.64) in asthmatic patients with more than one emergency room admission for asthma. The association was particularly strong for respiratory causes (OR 1.63, 95% CI 0.93 to 2.86). Ozone also increased the risk of death in asthmatic patients (OR 1.90, 95% CI 1.09 to 3.30) during spring and summer. The association with particles, pollen, and spores was not significant, and no interactions between air pollutants and pollen and spores were found.

CONCLUSION

Nitrogen dioxide and ozone may exacerbate severe asthma and even cause death among asthmatic subjects.

Lung cancer, cardiopulmonary mortality, and long-term exposure to fine particulate air pollution

CONTEXT

Associations have been found between day-to-day particulate air pollution and increased risk of various adverse health outcomes, including cardiopulmonary mortality. However, studies of health effects of long-term particulate air pollution have been less conclusive.

OBJECTIVE

To assess the relationship between long-term exposure to fine particulate air pollution and all-cause, lung cancer, and cardiopulmonary mortality.

DESIGN, SETTING, AND PARTICIPANTS

Vital status and cause of death data were collected by the American Cancer Society as part of the Cancer Prevention II study, an ongoing prospective mortality study, which enrolled approximately 1.2 million adults in 1982. Participants completed a questionnaire detailing individual risk factor data (age, sex, race, weight, height, smoking history, education, marital status, diet, alcohol consumption, and occupational exposures). The risk factor data for approximately 500 000 adults were linked with air pollution data for metropolitan areas throughout the United States and combined with vital status and cause of death data through December 31, 1998.

MAIN OUTCOME MEASURE

All-cause, lung cancer, and cardiopulmonary mortality.

RESULTS

Fine particulate and sulfur oxide--related pollution were associated with all-cause, lung cancer, and cardiopulmonary mortality. Each 10-microg/m(3) elevation in fine particulate air pollution was associated with approximately a 4%, 6%, and 8% increased risk of all-cause, cardiopulmonary, and lung cancer mortality, respectively. Measures of coarse particle fraction and total suspended particles were not consistently associated with mortality.

CONCLUSION

Long-term exposure to combustion-related fine particulate air pollution is an important environmental risk factor for cardiopulmonary and lung cancer mortality.

Selection bias and confounding in case-crossover analyses of environmental time-series data

The case-crossover study design is a popular analytic tool for estimating the effects of triggers of acute outcomes by environmental exposures. Although this approach controls for time-invariant confounders by design, it may allow for selection bias and confounding by time-varying factors. We conducted a simulation study of the sensitivity of the symmetric bidirectional case-crossover design to time-varying patterns in exposure and outcome. We identified the effects of selection bias and confounding on symmetric bidirectional case-crossover results and offer strategies to eliminate or reduce these biases. Selection bias results when exposure in the reference periods is not identically representative of exposure in the hazard periods, even when the distribution of exposure is stationary. This bias can be estimated and removed. Selection bias also occurs when the distribution of exposure is nonstationary, but the adjusted symmetric bidirectional case-crossover methodology substantially controls for this. Confounding results from a common temporal pattern in the exposure and the outcome time series, but can also be the result of patterns in exposure and outcome that, although asymptotically uncorrelated, are correlated at finite series lengths. All three biases are reduced by choosing shorter referent-spacing lengths. This effect is illustrated using data on air pollution and daily deaths in Chicago.

Correcting for the effects of location and atmospheric conditions on air pollution exposures in a case-crossover study

A limitation of most air pollution health effects studies is that they rely on monitoring data averaged over one or more ambient monitors to represent daily air pollution exposures for individuals. Such data analyses therefore implicitly require the assumption of a homogeneous spatial distribution for particulate matter (PM). This assumption may be suspected in the Pacific Northwest because of its hilly topography and local variations in wood burning. To examine the bias from substituting regional PM (i.e., the average of three ambient monitor measurements) for individual PM exposure, we conducted an exposure substudy to identify the influence of location factors, specifically urban versus suburban classification and topographic features ("upstream" versus "downstream"), on local ambient measurements. Using nephelometer measurements collected over 1 year in four locations, we developed regression models to predict local PM as a function of regional PM, atmospheric stagnation, temperature, and location. We found a significant interaction between atmospheric stagnation and topography, with the most upstream site having reduced PM levels on high stagnation days after controlling for regional PM. We also found a significant interaction with temperature at one downstream site thought to be heavily exposed to wood smoke in the winter. These results are consistent with the physics of surface radiation inversions. The interactions reordered the index versus referent exposures in a case-crossover analysis of out-of-hospital primary cardiac arrest for subjects living in specific locations, but did not meaningfully change the associations with PM from the analysis using regional PM as the exposure. The lack of change in these results may be due to limitations in the data used to correct the exposure estimates or to the absence of a PM effect among persons without prior heart disease who experienced a primary cardiac arrest.