Air pollution, physical activity and mental health: longitudinal evidence from Belgium

Background and objectives

Depression and anxiety are leading contributors to the burden of disease in Belgium. Physical activity is a cost-effective intervention for mental health, yet whether ambient air pollution modifies its benefits remains largely unknown. Beyond mediation, air pollution may also interact with physical activity: increased ventilation during exercise in polluted environments raises inhaled pollutant doses, potentially undermining mental health benefits. This study examines whether residential PM2.5 modifies the effect of leisure-time and travel-related physical activity on anxiety, depression, and life satisfaction in a longitudinal cohort in Belgium.

Methods

BELCOHORT+ recruited Belgian adults aged 18–69; after exclusion criteria, the longitudinal analytic sample comprised 1004 participants. Physical activity was measured with the EHIS-PAQ at baseline (May 2024); mental health outcomes (GAD-7, PHQ-9, single-item life satisfaction) at baseline and follow-up (February 2025). Annual residential PM2.5 was derived from the Belgian Environmental Agency. Two Target Trial Emulations were conducted using Augmented Inverse Probability Weighting, with PM2.5 effect modification tested across three exposure strata (mean ± 1 SD).

Results

The median residential PM2.5 was 9.61 µg/m³; only 2.1% of participants met WHO guidelines. Travel-related physical activity (≥90 min/week) was associated with reduced anxiety (mean difference: −0.38; 95% CI: −0.72 to − 0.04) but not depression or life satisfaction. Leisure-time physical activity showed no associations. PM2.5 did not meaningfully modify effects across any domain or outcome.

Conclusions

Travel-related physical activity reduced anxiety independently of residential air pollution levels. The absence of effect modification might reflect limited exposure heterogeneity in Belgium rather than a true null effect. Analyses will be extended with a June 2026 follow-up and more air pollution data (additional indicators).

Key messages

• In Belgium, travel-related physical activity cut anxiety scores nine months later. Air pollution did not blunt this benefit, supporting active mobility promotion as a mental health strategy.

• Longitudinal data linking air pollution, physical activity and mental health enables causal inference, moving beyond cross-sectional limitations that have hampered this field.