  The opportunity to assess short term impact of air pollution relies on the
causal interpretation of the exposure-outcome association, but up to now few
studies explicitly faced this issue within a causal inference framework. In
this paper, we reformulated the problem of assessing the short term impact of
air pollution on health using the potential outcome approach to causal
inference. We focused on the impact of high daily levels of PM10 on mortality
within two days from the exposure in the metropolitan area of Milan (Italy),
during the period 2003-2006. After defining the number of attributable deaths
in terms of difference between potential outcomes, we used the estimated
propensity score to match each high exposure-day with a day with similar
background characteristics but lower PM10 level. Then, we estimated the impact
by comparing mortality between matched days. We found that during the study
period daily exposures larger than 40 microgram per cubic meter were
responsible of 1079 deaths (116; 2042). The impact was more evident among the
elderly than in the younger classes of age. The propensity score matching
turned out to be an appealing method to assess historical impacts in this
field.
