RESEARCH REPORT

Stronger associations between daily mortality and fine particulate air pollution in summer than in winter: evidence from a heavily polluted region in western Europe

T S Nawrot¹, R Torfs², F Fierens³, S De Henauw⁴, P H Hoet¹, G Van Kersschaever⁵, G De Backer⁴ and B Nemery¹

¹ Occupational and Environmental Medicine, School of Public Health, KULeuven, Leuven, Belgium
² VITO, Flemish Institute for Technological Research, Mol, Belgium
³ Interregional Cell for the Environment, Brussels, Belgium
⁴ Department of Public Health, Ghent University, Ghent, Belgium
⁵ Department of General Practice, School of Public Health, KULeuven, Leuven, Belgium

Correspondence to:
Correspondence to:
Dr B Nemery
Occupational and Environmental Medicine and Pneumology, Unit of Lung Toxicology, Herestraat 49 (O&N 706), KU Leuven, B-3000 Leuven, Belgium; ben.nemery{at}med.kuleuven.ac.be

Background: Numerous studies have shown a strong association between daily mortality and small particulate with a diameter of <10 µm (PM₁₀) air pollution, but the effects of season have not always been well characterised.

Aim: To study the shape of the association between short-term mortality and PM₁₀ across seasons and quintiles of outdoor temperature.

Design, setting and participants: Daily data on mortality (n = 354 357), outdoor temperature and PM₁₀ in Flanders, Belgium, from January 1997 to December 2003, were analysed across warm versus cold periods of the year (April–September v October–March), with seasons and quintiles of outdoor temperature as possible effect modifiers.

Results: There was a significant (p<0.001) interaction between PM₁₀ and period of the year in relation to mortality. To allow for non-linearity, daily mean PM₁₀ concentrations were categorised into quartiles. Season-specific PM₁₀ quartiles showed a strong and steep linear association between mortality and PM₁₀ in summer and a less linear association in spring and autumn, whereas in winter the association was less strong and mortality was only increased in the highest PM₁₀ quartile. The effect sizes expressed as the percentage increase in mortality on days in the highest season-specific PM₁₀ quartile versus the lowest season-specific PM₁₀ quartile were 7.8% (95% CI 6.1 to 9.6) in summer, 6.3% (4.7 to 7.8) in spring, 2.2% (0.58 to 3.8) in autumn and 1.4% (0.06 to 2.9) in winter. An analysis by quintiles of temperature confirmed these effect sizes.

Conclusion: The short-term effect of particulate air pollution on mortality strongly depends on outdoor temperature, even in a temperate climate.

Abbreviations: ICD, International Classification of Diseases

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