The Effect of Particulate Air Pollution on Emergency Admissions for Myocardial Infarction: A Multicity Case-Crossover Analysis

Article excerpt

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 [micro]m (P[M.sub.10]) 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 [micro]g/[m.sup.3] increase in ambient P[M.sub.10] 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 P[M.sub.10] 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 P[M.sub.10] is almost linear, but with a steeper slope at levels of P[M.sub.10] < 50 [micro]g/[m.sup.3]. We conclude that increased concentrations of ambient P[M.sub.10] are associated with increased risk of MI among the elderly. Key words: air pollution, cardiovascular diseases, case-crossover, myocardial infarction, particulate matter. doi:10.1289/ehp.7550 available via http://dx.doi.org/ [Online 16 March 2005]

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Ambient particulate matter (PM) air pollution has been repeatedly observed to be associated with increased risk of hospital admissions and deaths attributed to cardiovascular causes in studies conducted throughout the industrialized world (Anderson et al. 2003; Braga et al. 2001; Dockery 2001; Hoek et al. 2001; Katsouyanni et al. 1996; Pope et al. 2004a; Samet et al. 2000; Zanobetti et al. 2000a).

Similar relationships have been reported in locations reflecting a wide range of PM and of gaseous copollutant concentrations (Goldberg et al. 2001; Koken et al. 2003; Linnet al. 2000; Sunyer et al. 2003; Zmirou et al. 1998). Other studies have shown that these associations are not confounded by secular time trends, seasonal patterns, influenza epidemics (Braga et al. 2000), or weather (Samet et al. 1998; Schwartz 1999, 2000). In addition, a large study of essentially every U.S. city reported that airborne particles were the only air pollutant that showed an independent effect on daily deaths, and that those gaseous air pollutants did not confound the association between PM and daily deaths (Samet et al. 2000).

Although the association of airborne particles with cardiovascular events is clear, the mechanisms behind these associations are not fully understood. To further understanding of the mechanisms behind these observations, it is important to examine associations with more specific end points that may suggest specific pathways.

Recently, attention has focused on whether PM air pollution is a specific trigger of myocardial infarction (MI) [International Classification of Diseases, Revision 9 (ICD-9), code 410 (World Health Organization 1977)]. Peters et al. (2001a) conducted a case-crossover study of 772 patients presenting to Boston area hospitals with strictly defined MI and reported that elevated concentrations of ambient PM [fine PM with aerodynamic diameter < 2.5 [micro]m (P[M.sub.2.5]) and < 10 [micro]m (P[M.sub.10])] were strongly associated with higher risks of MI onset in the 2-hr period, and in the 24-hr period, preceding the event.

D'Ippoliti et al. (2003) analyzed hospital admissions for MI in Rome with a case-crossover analysis and found a strong association with total suspended PM. …