The scientific community is currently very interested in the effect modification of air pollution by meteorological variables. The recent paper by Dr Nawrot and colleagues reports findings from a time-series epidemiology study on the effects of temperature on the association between relative mortality risk and PM₁₀ in Belgium.¹ Their 7-year data showed a stronger overall association and no threshold between PM₁₀ and mortality in summer, but a weaker association and a threshold of 30 μg/m³ in winter. Based on these findings they state that “the shape of the dose–response relationship curve between air pollution and mortality strongly depends on outdoor temperature”.

We would like to point out that, although a weaker dependency cannot be ruled out, at least a substantial fraction of the observation is most likely attributable to the well-known relationship between personal exposures and air pollution levels at fixed monitoring sites. This relationship is strongly modified by the amount of time spent indoors. Infiltration of outdoor air PM_2.5 particles indoors reduces exposure levels relative to ambient levels by 30–40% on average in Europe.² The associated strong seasonal pattern³ is caused by variation in the times for which windows …