Short-term exposure to PM_2.5 and outpatient department visits for mental and behavioural disorders in Thailand: a case-crossover study

Fine particulate matter (PM_2.5) has been linked to a range of adverse health outcomes, including emerging evidence of its impact on mental and behavioural disorders (MBDs). However, evidence from Thailand remains limited. This study aimed to investigate the association between short-term exposure to PM_2.5 and outpatient department (OPD) visits for MBDs across Thailand from January 2017 to December 2022. A two-stage statistical approach was employed. In the first stage, we applied a time-stratified case-crossover design with conditional quasi-Poisson regression to estimate province-specific associations between PM_2.5 and OPD visits for MBDs. In the second stage, a random-effect meta-analysis was used to pool the estimates at regional and national levels. The results revealed that the relative risk (RR) of OPD visits for MBDs nationally per a 10 µg/m³ increase in PM_2.5 was 1.006 (95% confidence interval (CI): 1.002, 1.010) at lag 0–1 day. Regionally, the Central region showed the strongest association at lag 0–2 day, with an RR of 1.013 (95% CI: 1.005, 1.021) per each 10 µg/m³ increase in PM_2.5. Subgroup analyses indicated significantly elevated risks among males, children aged 0–15 years, and adults aged 16–59 years. These findings provide evidence linking short-term PM_2.5 exposure to increased mental health service use in Thailand and underscore the importance of incorporating mental health outcomes into environmental health risk assessments. The results support the development of targeted policies to mitigate PM_2.5 exposure and protect vulnerable populations from its mental health impacts.