Update of Air Quality Health Index (AQHI) and harmonization of health protection and climate mitigation

Hong Kong has used a risk-based Air Quality Health Index (AQHI) system for nearly a decade, which aggregates hospitalization risks associated with short-term exposure to pollutants. Serving as an air pollution risk communication tool, it keeps the public informed about the current air quality, anchoring around the World Health Organization's (WHO) 2005 Air Quality Guidelines (AQGs). Given the WHO's recent update to the AQGs in 2021, it is pertinent to examine how Hong Kong's air quality risk communication might be enhanced using these updated guidelines.The AQHI currently focuses only on short-term health risks, potentially leading to an underestimation of cumulative impacts. To address this, we propose enhancing the AQHI to include both long-term and short-term health risks. Short-term risks are estimated based on excess hospitalization risks from various pollutants using local data, while long-term risks are estimated using beta coefficients from the WHO due to limited local data. We further suggest using the WHO's Interim Targets (ITs) as thresholds for health risk categories, eliminating arbitrary cut-points.Promoting both public health protection and climate mitigation is important but challenging, particularly with the surge in ozone levels due to enhanced NOx controls in VOC-limited regions like Hong Kong and Guangdong. Nevertheless, despite the rise in ozone, combined health risks in these areas have decreased due to reductions in NO2 and PM concentrations. Shifting to an AQHI-like total health risk assessment emphasizes the harmonization of health protection and climate mitigation goals. This health-focused framework posits that rigorous NOx controls can effectively serve both policy objectives in tandem.