Introduction Substantial epidemiological evidence suggests that both short- and long-term exposure to fine particulate matter (PM₂.₅) increases cardiovascular disease (CVD) risk. However, uncertainty persists regarding the cardiovascular effects of low-level PM₂.₅ exposure. This study aimed to clarify the association between PM₂.₅ and CVD morbidity in Haikou, China. Methods A time-series design with Distributed Lag Non-linear Models (DLNMs) was employed to assess the short-term associations between PM₂.₅ exposure and daily CVD-related emergency department (ED) visits across three major hospitals in Haikou (2018–2021), with stratified analyses by sex and age. Results Among 988,020 total ED visits, 69,099 (7.0%) were CVD-related. Our analyses revealed a distinctive three-phase, S-shaped nonlinear association between short-term PM₂.₅ exposure and emergency CVD visits, characterized by pronounced lag effects. Specifically, the risk of CVD exhibited an initial decline, followed by an increase and subsequent attenuation at higher concentrations. Discussion We propose that this complex pattern reflects a balance between adaptive hormetic responses at low exposures, toxic effects at moderate levels, and behavioral adaptations during high pollution episodes. These findings underscore that maintaining PM₂.₅ concentrations within a moderate range may yield greater public health benefits in low-pollution regions like Haikou, offering critical guidance for refining air quality standards and preventive interventions.
Zhang et al. (2025) studied this question.