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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 PM2.5 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 are kept open, e.g. in Helsinki windows are kept open for a median of 0.3 h/day in winter compared with 24 h/day in summer.4 Ventilation via open windows particularly affects infiltration of the coarse fraction of PM10.
Janssen et al. have shown that in the US an increase in the prevalence of air conditioning in cities results in a decrease in the slope and significance of the daily morbidity–ambient PM10 regression.5 They believe that this is due to the difference in the ambient PM10 penetration between the seasons and the effects of air conditioning versus no air conditioning.
Therefore, while the work by Nawrot and colleagues opens up an important discussion on effect modification by meteorological variables, the most urgent topic in this field is to quantify the effects on exposures. Only after these effects have been controlled can analysis of the effect modification by temperature and other variables be properly conducted.
The scientific community, especially epidemiologists, are very excited about the topic of effect modification, by temperature and other weather variables, of the health impact of air pollution, especially particulate matter. However, at least to a substantial degree, this modification is caused by seasonal variations in population behaviour, and not necessarily by any differences in the toxicity of the particles. Therefore, exposure modification should be studied and included in the interpretation of findings such as those reported by Nawrot et al.
The toxicity of different particles is of the utmost importance in exposure reduction policy development. Furthermore, estimates of the public health costs associated with air pollution are dependent on the dose–response functions estimated in epidemiological studies. If interpreted incorrectly, very wrong actions – or no action at all – may be taken at the expense of public health.