Impacts of PM before and after COVID-19 outbreak on emergency mental disorders: A population-based quasi-experimental and case-crossover study.

The ongoing COVID-19 pandemic is a great challenge to mental health, but fine particulate matter (PM), an increasingly reported risk factor for mental disorders, has been greatly alleviated during the pandemic in many countries. It remains unknown whether COVID-19 outbreak can affect the association between PM exposure and the risk of mental disorders. This study aimed to investigate the associations of total and cause-specific mental disorders with PM exposure before and after the COVID-19 outbreak in China. Data on daily emergency department visits (EDVs) and hospitalizations of mental disorders from 2016 to 2021 were obtained from Anhui Mental Health Center for Hefei city. An interrupted time series analysis was used to quantify the impact of COVID-19 outbreak on EDVs and hospitalizations of mental disorders. A time-stratified case-crossover analysis was employed to evaluate the association of mental disorders with PM exposure before and after the COVID-19 outbreak, especially in the three months following the COVID-19 outbreak. After COVID-19 outbreak, there was an immediate and significant decrease in total mental disorders, including a reduction of 15% (95% CI: 3%-26%) in EDVs and 44% (95% CI: 36%-51%) in hospitalizations. PM exposure was associated with increased risk of EDVs and hospitalizations for total and cause-specific mental disorders (schizophrenia, schizotypal and delusional disorders; neurotic, stress-related, and somatoform disorders) before COVID-19 outbreak, but this PM-related risk elevation significantly decreased after COVID-19 outbreak, with greater risk reduction at the first month after the outbreak. However, young people (0-45 years) were still vulnerable to PM exposure after the COVID-19 outbreak. This study first reveals that the risk of PM-related emergency mental disorders decreased after the COVID-19 outbreak in China. The low concentration of PM might benefit mental health and greater efforts are required to mitigate air pollution in the post-COVID-19 era.