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 PM_2.5 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 PM_2.5, 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.