Integrated analysis of PM2.5 chemical characteristics and multi-pollutant health risks in Central Taiwan: Implications for optimizing the air quality health index (AQHI) during the COVID-19 pandemic

Conventional Air Quality Index (AQI) systems often underrepresent multi-pollutant health impacts. The Air Quality Health Index (AQHI), which integrates daily concentrations of PM_2.5, O₃, and NO₂ to estimate population-level health risks, addresses this limitation by accounting for multi-pollutant exposure effects. This study investigated the chemical characteristics of PM_2.5 in Taichung, Taiwan, from 2020 to 2022, focusing on polycyclic aromatic hydrocarbons (PAHs) and water-soluble ions (WSIs) across industrial and traffic areas. During pandemic restrictions, remote work and online learning increased electricity demand, coinciding with elevated total water-soluble ion concentrations, largely driven by increases in non-sea-salt sulfate (nss-SO₄^2-; 11.8 ± 4.12 μg/m³) in the industrial area. In contrast, PAH concentrations declined during lockdowns but exhibited a pronounced rebound during the warm season in the post-pandemic period, reaching approximately two to three times the levels observed during the pandemic stage. Source apportionment analyses indicated that traffic-related emissions, particularly diesel exhaust, were the dominant contributors to ambient PAHs. Health-risk analysis based on AQHI and outpatient records (2016-2022; stratified by pre-, during-, and post-pandemic stages) revealed that high-risk AQHI levels (≥7) were associated with increased respiratory and cardiovascular morbidity. Lag-effect analysis showed acute and persistent risks from PM_2.5 and NO₂, whereas O₃ exhibited delayed inverse statistical associations. The findings demonstrate that AQHI is a more sensitive short-term, health-relevant metric than AQI under pandemic-altered activity patterns. This integrated approach offers empirical evidence supporting Taiwan's adoption of AQHI for risk communication and for protecting vulnerable populations.