Short-term effects of air pollutants on daily mortality in the Stockholm county - A spatiotemporal analysis

Short-term exposure to air pollutants has been extensively related to daily mortality, however most of the evidence comes from studies conducted in major cities, and little is known on the extent of the spatial heterogeneity in the effects within areas including both urban and non-urban settings. We aimed to investigate the short-term association of air pollutants with daily cause-specific mortality in the Stockholm county, and to test whether an association exists also outside the metropolitan area. We used a spatiotemporal random forest model to predict daily concentrations of fine and inhalable particulate matter (PM2.5 and PM10), nitrogen dioxide (NO2) and ozone (O-3) at 1-km spatial resolution over Sweden for 2005-2016. We collected data on daily mortality for each small area for market statistics (SAMS) of the Stockholm county, to which we matched daily exposures to air pollutants and air temperature. We applied a case-crossover design to investigate the short-term association between the four pollutants and mortality from non-accidental, cardiovascular and respiratory causes. We compared the associations in and out the Stockholm urban area, by SAMS population density and across the 26 municipalities of the county. We found weak effects of most air pollutants on cause-specific mortality in the full year analysis, with estimates much larger and significant only during the warmer months (April to September): non-accidental mortality increased by 4.58% (95% confidence interval - 95% CI: 0.89%, 8.41%) and by 2.21% (95% CI: 0.71%, 3.73%) per 10 mu g/m(3) increase in lag 0-1 PM2.5 and O-3, respectively. Associations were in general higher in the Stockholm city and in SAMS with high population density. When comparing the 26 municipalities, we didn't detect a significant heterogeneity in the short-term associations with air pollutants. In conclusion, we found a suggestion of a harmful role of air pollution also in non-urban areas, but the study was underpowered to draw firm conclusions. We consider this study as a pilot to investigate the spatial heterogeneity of the association between daily air pollution and mortality at the national level in Sweden.