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Mei Lin,¹ Yue Chen,¹ Richard T. Burnett,² Paul J. Villeneuve,¹ and Daniel Krewski³

¹Department of Epidemiology and Community Medicine, Faculty of Medicine, University of Ottawa; ²Environmental Health Directorate, Health Canada; ³McLaughlin Center for Population Health Risk Assessment, University of Ottawa, Ottawa, Ontario, Canada

Abstract

In this study, we used both case-crossover and time-series analyses to assess the associations between size-fractionated particulate matter and asthma hospitalization among children 6-12 years old living in Toronto between 1981 and 1993. Specifically, we used exposures averaged over periods varying from 1 to 7 days to assess the effects of particulate matter on asthma hospitalization. We calculated estimates of the relative risk of asthma hospitalization adjusted for daily weather conditions (maximum and minimum temperatures, and average relative humidity) for an incremental exposure corresponding to the interquartile range in particulate matter. Both bidirectional case-crossover and time-series analyses revealed that coarse particulate matter (PM_10-2.5) averaged over 5-6 days was significantly associated with asthma hospitalization in both males and females. The magnitude of this effect appeared to increase with increasing number of days of exposure averaging for most models, with the relative risk estimates stabilizing at about 6 days. Using a bidirectional case-crossover analysis, the estimated relative risks were 1.14 [95% confidence interval (CI) , 1.02, 1.28] for males and 1.18 (95% CI, 1.02, 1.36) for females, for an increment of 8.4 µg/m³ in 6-day averages of PM_10-2.5. The corresponding relative risk estimates were 1.10 and 1.18, respectively, when we used time-series analysis. The effect of PM_10-2.5 remained positive after adjustment for the effects of the gaseous pollutants carbon monoxide (CO) , nitrogen dioxide (NO₂) , sulfur dioxide (SO₂) , and ozone (O₃) . We did not find significant effects of fine particulate matter (PM_2.5) or of thoracic particulate matter (PM₁₀) on asthma hospitalizations using either of these two analytic approaches. For the most part, relative risk estimates from the unidirectional case-crossover analysis were more pronounced compared with both bidirectional case-crossover and time-series analyses. Key words: asthma hospitalization, case-crossover analysis, coarse particulate matter, risk assessment, time-series analysis. Environ Health Perspect 110:575-581 (2002) . [Online 15 April 2002]

http://ehpnet1.niehs.nih.gov/docs/2002/110p575-581lin/ abstract.html

Address correspondence to Y. Chen, Department of Epidemiology and Community Medicine, University of Ottawa, 451 Smyth Road, Ottawa, Ontario, Canada K1H 8M5. Telephone: (613) 562-800 ext. 8287. Fax: (613) 562-5465. E-mail: chen@zeus.med.uottawa.ca

We thank N.J. Birkett and Y. Shi for helpful technical advice.

This work was supported by a grant from the Toxic Substances Research Initiative (TSRI 29) and by the Natural Sciences and Engineering Research Council of Canada. Y.C. holds a Canadian Institutes of Health Research investigator award. D.K. is the NSERC/SSHRC/McLaughlin Chair in Population Health Risk Assessment at the University of Ottawa. The work was done at the Department of Epidemiology and Community Medicine, Faculty of Medicine, Ottawa, Canada.

Received 6 August 2001 ; accepted 13 November 2001.