Association between low-concentration PM

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.