Can short-term exposure to air pollution trigger acute health effects in a region with low pollutant concentrations?

The daily air pollution limits set by the World Health Organization (WHO) are crucial in shaping air quality policies worldwide. This time-stratified case-crossover study investigates the effects of short-term exposure to outdoor air pollution on mortality, hospital admission and sick leaves in Baden-Württemberg, a region in Germany with consistently low pollution levels compared to other regions in Europe. Using a two-stage distributed lag non-linear model (DLNM), we examine the distribution of health risk in the region and estimate risks within a 7-day lag from the health event day (lag 0).The exposure-response functions reveal significant associations between all-cause mortality, hospital admissions, and sick leaves and PM2.5 and NO2 at concentrations lower than the daily WHO limits (and EU Air Quality Directive (AQD) 2030). The lag-response function highlights that the relative risks (RR) of acute hospital admissions, sick leaves and mortality, are significantly positive at different lags for PM2.5, whereas for NO2, the RR declines from the event day onwards.This spatio-temporal analysis, accounting for regional heterogeneity, underscores a pronounced exposure-lag-response relationship even in a low-pollution region, emphasizing the need for reassessing existing air quality standards.