Fine airborne particles: when alarming levels are the standard

Objectives To quantify the contribution of each individual month to the annual mortality burden attributable to particulate matter (PM) in 2015 in Milan, Italy, after authorities and media considered December 2015 as an outlying month carrying an exceptional population exposure to PM. Study design We used routinely available daily time series of air pollution and mortality to perform an assessment of the impact of PM exposure on population health. Methods By combining daily death counts with daily PM levels, as well as the yearly average of the number of deaths with the yearly average of PM concentrations, impact estimates were calculated in terms of deaths attributable (AD) to levels of PM10 and PM2.5 exceeding the daily or the annual European Union (EU) exposure limits. Results On a monthly basis, the estimated AD for exceeding the daily EU limits for more than 35 days were 18.4 (PM10) and 33.2 (PM2.5) between January and March, and 20.0 and 31.9 between October and December, respectively. On an annual basis, the EU limit for PM10 was almost met and, therefore, the estimated impact in terms of AD was practically null. Conclusions Impact results should be interpreted in the light of the skewness of the daily PM concentration distribution. The number of days above the limits is more important than the average annual concentration in determining the number of attributable deaths. The impact of PM on mortality is substantial during the whole winter season irrespective of its annual average concentration. Our estimates further stress the need for a revision of the current European air quality standards.