Abstract

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.