Development and evaluation of a risk-based air quality index for Germany

We developed and evaluated a risk-based Air Quality Index (AQI) for Germany that harmonizes pollutant categories on risk equivalence, incorporates hourly concentrations, and improves health-relevant communication.

We identified pollutant-outcome pairs with causal or likely causal evidence, extracted effect estimates from systematic reviews and meta-analyses, and transformed daily mean effects into hourly effect estimates using German monitoring data. Equivalence coefficients were derived via Monte Carlo simulations and standardized to PM_2.5 as the reference pollutant. PM_2.5 thresholds were anchored in WHO Air Quality Guideline 2021 and the EU information threshold; risk-equivalent thresholds for PM₁₀, NO₂, O₃, and SO₂ were generated accordingly. An additional module was developed to account for concurrent elevation of multiple pollutants. The index was empirically evaluated using hourly data from German monitoring stations for 2019 and 2022, and the results were compared with the previous AQI. In sensitivity analyses, we assessed the effects of incorporating updated evidence and excluding asthma-related outcomes.

Application of the risk-based AQI resulted in a marked shift from "very good" to "good" and "moderate" categories in comparison with the previous AQI, particularly for PM_2.5 and O₃, reflecting stricter concentration thresholds. "Poor" classifications increased modestly, while "very poor" remained rare. The multipollutant episodes module affected less than 1% of hours.

Overall, the risk-based AQI represents a significant step toward health-based communication of air quality in Germany. By integrating scientific evidence into its design and providing tailored guidance, it enhances preventive public health protection. Its implementation may improve risk awareness and support behavioural changes that reduce exposure.