A stronger association of mental disorders with smaller particulate matter and a modifying effect of air temperature

Mental disorders (MDs) can be triggered by adverse weather conditions and particulate matter (PM) such as PM_2.5 and PM₁₀ (aerodynamic diameter ≤2.5 μm and ≤10 μm). However, there is a dearth of evidence on the role of smaller PM (e.g. PM₁, aerodynamic diameter ≤1 μm) and the potential modifying effects of weather conditions. We aimed to collect daily data on emergency department visits and hospitalisations for schizophrenia-, mood-, and stress-related disorders in a densely populated Chinese city (Hefei) between 2016 and 2019. A time-stratified case-crossover analysis was used to examine the short-term association of MDs with PM₁, PM_2.5, and PM₁₀. The potential modifying effects of air temperature conditions (cold and warm days) were also explored. The three size-fractioned PMs were all associated with an increased risk of MDs; however, the association differed between emergency department visit and hospitalisation. Specifically, PM₁ was primarily associated with an increased risk of emergency department visit, whereas PM_2.5 was primarily associated with an increased risk of hospitalisation, and PM₁₀ was associated with an increased risk of both emergency department visit and hospitalisation. The PM-MD association appeared to be greatest (although not significant) for PM₁ (odds ratio range: 1.014-1.055), followed by PM_2.5 (odds ratio range: 1.001-1.009) and PM₁₀ (odds ratio range: 1.001-1.006). Furthermore, the PM-MD association was observed on cold days; notably, the association between PM and schizophrenia-related disorders was significant on both cold and warm days. Our results suggest that the smaller the PM, the greater the risk of MDs, and that the PM-MD association could be determined by air temperature conditions.