Long-term exposure to low-concentration sulfur dioxide and mental disorders in middle-aged and older urban adults

The World Health Organization loosened the air quality guideline for daily sulfur dioxide (SO₂) concentrations from 20 μg/m³ to 40 μg/m³. However, the guideline for SO₂ concentrations in 2021 raised public concerns since there was no sufficient evidence that low-concentration SO₂ exposure is harmless to the population's health, including mental health. We analyzed the associations between low-concentration SO₂ exposure and incidence risks of total and cause-specific mental disorders, including depressive disorder, anxiety disorder, bipolar disorder, and schizophrenia spectrum disorder. 245,820 urban participants with low-concentration SO₂ exposure (<8 μg/m³) at baseline were involved in the analyses from the UK Biobank. SO₂ exposure (2006-2022) was estimated using high-resolution annual mean concentration maps from the Department for Environment, Food and Rural Affairs. Mental disorders and corresponding symptoms were identified using healthcare records and an online questionnaire, respectively. Associations were examined using both time-independent (2006-2010) and time-dependent (from 2006 to 2022) Cox regression models and logistic regression models with full adjustments for potential confounders. Stratification analyses were further conducted to identify vulnerable populations. Long-term exposure to low-concentration SO₂ (per 1.36 μg/m³) was associated with increased risks of mental disorders, depressive disorder, and anxiety disorder with hazard ratios of 1.02 (95% confidence interval [CI]: 1.00, 1.03), 1.11 (95% CI: 1.07, 1.16), and 1.10 (95% CI: 1.06, 1.14) in the time-independent model, respectively. Associations were stronger for younger individuals. Additionally, the low-concentration SO₂ exposure was linked to several psychiatric symptoms, such as trouble concentrating and restlessness, with odds ratios of 1.07 (95% CI: 1.04, 1.10) and 1.11 (95% CI: 1.07, 1.14), respectively. This study demonstrated significant associations of long-term exposure to low-concentration SO₂ with mental disorders, highlighting the need for stricter regulations for SO₂ to better protect public health and improve air quality in urban areas, in support of the Sustainable Development Goals.