Associations of Long-Term PM2.5 Exposure and Physical Activity Levels with Metabolic Syndrome and Health-Related Quality of Life: A Cross-Sectional Study in Chiang Mai, Thailand

Background/Objectives: Although the adverse metabolic effects of PM2.5 and the health benefits of physical activity are well-established, evidence on whether physical activity modifies the association between PM2.5 exposure and metabolic syndrome or health-related quality of life (HRQoL) remains limited. Methods: This observational analytical cross-sectional study examined the modifying effect of physical activity on the associations between long-term PM2.5 exposure and metabolic syndrome and HRQoL in Chiang Mai, Thailand, and to explore these associations across physical activity levels using stratified analyses. A total of 347 participants (209 from higher PM2.5 areas and 138 from lower PM2.5 areas) were recruited in Chiang Mai between March and May 2024. Metabolic syndrome was assessed using blood tests and anthropometric measurement, while HRQoL was evaluated using the Thai version of the SF-36 questionnaire. Multivariable logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs) for metabolic syndrome. HRQoL differences were analyzed using generalized linear models with robust standard errors. Interaction between PM2.5 exposure and physical activity was assessed to examine potential effect modification. All models were adjusted for age, sex, BMI, smoking status, and educational level, with additional stratified analyses across physical activity levels. Results: Higher long-term PM2.5 exposure was associated with lower odds of metabolic syndrome (OR = 0.34, 95% CI: 0.14–0.83) but was not associated with HRQoL. Physical activity was not independently associated with either outcome, and no interaction between PM2.5 exposure and physical activity was observed. In stratified analyses, the inverse association between PM2.5 exposure and metabolic syndrome was observed only among individuals with high physical activity, while significantly lower HRQoL scores were observed among those with moderate and high physical activity levels. Conclusions: Higher long-term PM2.5 exposure was associated with lower odds of metabolic syndrome and lower HRQoL. Physical activity was not independently associated with these outcomes, and no interaction between PM2.5 exposure and physical activity was observed. Stratified analyses suggested variation in these associations across physical activity levels.