When safe Isn’t safe: Ischemic heart disease mortality below PM2.5 standards in urban and suburban Bangkok under tropical temperature extremes

While PM 2.5 is a well-established cardiovascular hazard, research from tropical, rapidly urbanizing regions remains scarce, especially in suburban zones characterized by unregulated and diverse emission sources. Thailand's 24-hour PM 2.5 standard may underestimate the short-term ischemic heart disease (IHD) mortality risks, particularly under thermal extremes. This study assessed the acute association between PM 2.5 and IHD mortality in urban and suburban Bangkok and evaluated how cold and heat extremes modify this association across demographic and clinical subgroups. We performed a time-series analysis on daily IHD mortality from 2014 to 2022, using a generalized linear model (GLM) across lags 0-7. Models were adjusted for meteorological (temperature, humidity), co-pollutants (NO 2 , SO 2 ), day of week, public holidays, and COVID-19 lockdown effects. Analyses were stratified by geographic area, sex, age, IHD subtype, temperature extremes, and PM 2.5 thresholds per Thai (≤37.5 µg/m³) and WHO (≤15 µg/m³) standards. PM 2.5 was significantly associated with IHD mortality in urban (lag 0-2) and suburban (lag 0-1) settings. Suburban areas showed steeper exposure-response slopes. The strongest associations occurred for acute myocardial infarction, particularly in men and elderly. Elevated risks were evident even at PM 2.5 levels below Thai (RR=1.05; 95%CI:1.01-1.09) and WHO (RR=1.08; 95%CI:1.03-1.14) thresholds in suburban. Cold exposure significantly amplified IHD risk in urban elderly (RR=1.27; 95%CI:1.05-1.53), while extreme heat increased risk in suburban men (RR=1.15; 95%CI:1.05-1.26). Findings support progressively aligning the Thai standard with WHO guidelines through phased reduction targets. Findings challenge the current regulatory PM 2.5 standard, emphasizing urgent policy revisions and climate-sensitive health protections in tropical developing cities.