Abstract Background Despite the adverse effects of ambient fine particulate matter (PM 2.5 ) on type 2 diabetes and the beneficial role of physical activity (PA), the influence of PM 2.5 on the relationship between PA and type 2 diabetes remains unclear. Methods In this prospective study with 71,689 participants, PA was assessed by a questionnaire and was categorized into quartiles for volume and three groups for intensity. Long‐term PM 2.5 exposure was calculated using 1‐km resolution satellite‐based PM 2.5 estimates. PM 2.5 exposure and PA's effect on type 2 diabetes were assessed by cohort‐stratified Cox proportional hazards models, individually and in combination. Results In 488,166 person‐years of follow‐up, 5487 incident type 2 diabetes cases were observed. The association between PA and type 2 diabetes was modified by PM 2.5 . Compared with the lowest quartile of PA volume, the highest quartile was associated with reduced type 2 diabetes risk in low PM 2.5 stratification (≤65.02 µg/m 3 ) other than in high PM 2.5 stratification (&gt;65.02 µg/m 3 ), with the hazard ratio (HR) of 0.75 (95% confidence interval [CI]: 0.66–0.85) and 1.10 (95% CI: 0.99–1.22), respectively. Similar results were observed for PA intensity. High PM 2.5 exposure combined with the highest PA levels increased the risk of type 2 diabetes the most (HR = 1.79, 95% CI: 1.59–2.01 for PA volume; HR = 1.82, 95% CI: 1.64–2.02 for PA intensity). Conclusion PA could reduce type 2 diabetes risk in low‐pollution areas, but high PM 2.5 exposure may weaken or even reverse the protective effects of PA. Safety and health benefits of PA should be thoroughly assessed for long‐term polluted residents.

Physical activity, long‐term fine particulate matter exposure and type 2 diabetes incidence: A prospective cohort study