Bidirectional modification effects of non-optimal temperature and short-term exposure to fine particulate matter on cardiovascular mortality and their different subtypes: a time-stratified case-crossover study in multiple cities

Both fine particulate matter (PM 2.5 ) and non-optimal temperatures are well-recognized environmental hazard factors for cardiovascular diseases (CVD). However, the bidirectional modifying effects of these two factors on CVD mortality remain systematically understudied, particularly across different CVD subtypes and population strata. To examine the bidirectional modifying effects of PM 2.5 short-term exposure and non-optimal temperatures on CVD and its subtype mortality risk, identify susceptible populations, and provide evidence for comprehensive environmental risk prevention strategies. Using 111,567 CVD deaths in Anhui Province, China, between 2019 and 2023, a time-stratified case-crossover design was carried out. To assess the independent and bidirectional modifying effects of PM 2.5 and non-optimal temperature, an analytical approach integrating conditional logistic regression with distributed lag non‑linear models was utilized. To evaluate the bidirectional modifying effects of PM 2.5 and temperature, stratification analysis was conducted based on the high, medium and low levels of PM 2.5 concentration and temperature respectively, with 25% and 75% as cut-off values. The effects of extreme high temperature (EHT) and extreme low temperature (ELT) were estimated with The Minimum Mortality Temperature (MMT) as a reference at different PM 2.5 exposure levels. Finally, two-sample Z-tests were performed to compare PM 2.5 -associated effects between temperature strata, with the low temperature tertile as the reference. Significant bidirectional modifying effects were observed between PM 2.5 and low temperature on CVD mortality. High PM 2.5 exposure amplified the ELT-related mortality risk, odds ratio (OR) rose from 1.03 (95%CI: 0.92, 1.14) to 1.90 (95%CI: 1.41, 2.58), while low temperature intensified the PM 2.5 -related risk OR rose from 1.02 (95%CI: 0.98, 1.07) to 1.19 (95%CI: 1.18, 1.21). For high temperature, only a unidirectional amplification by PM 2.5 was observed, with no evidence of reverse modification. These bidirectional effects were most pronounced in acute CVD subtypes, but not in chronic subtypes. The elderly and married people were identified as sensitive groups for the bidirectional modification effect. PM 2.5 and low temperature exhibit synergistic bidirectional modifying effects on CVD mortality, particularly for acute CVD events and among vulnerable subgroups. Persistent progress in air quality can serve as a synergistic tactic to lessen the cardiovascular health burdens associated with climate change.