Time-varying exposure to ambient air pollution and mortality among colon cancer patients in northern Thailand: a 15-year retrospective cohort study

Background Northern Thailand experiences severe annual air pollution (PM 2.5 > 35 μg/m 3 ), primarily due to agricultural burning. While ambient PM 2.5 has been linked to gastrointestinal cancer deaths, its effect on colon cancer survival in high-exposure areas remains understudied. Methods In this 15-year retrospective cohort study, we used data on 5,018 patients with colon cancer (2003–2018) obtained from the Chiang Mai Cancer Registry. Time-varying exposure to PM 2.5 , PM 10 , NO₂, SO₂, CO, and O₃ was evaluated by exploiting annually updated Copernicus Atmosphere Monitoring Service data and linking them to residential districts. Multivariable time-dependent Cox models adjusted for demographics, tumor characteristics, and treatment were used in the analysis. Results Over 18,650 person-years of follow-up (PYFU), 72% of patients died (mortality rate: 19.4 per 100 PYFU). Exposure to PM 2.5 ≥ 37.5 μg/m 3 (a regionally relevant threshold) was independently linked to double the risk of all-cause mortality (adjusted hazard ratio (aHR) = 1.96, 95% confidence interval: 1.49–2.58, p < 0.001), as was having a low body mass index (aHR = 1.54) and being male (aHR = 1.23). Patients who died had significantly higher mean time-updated PM 2.5 exposure (with consistent annual exposure differences) compared to the survivors (37.7 vs. 35.5 μg/m 3 ; p < 0.001). No significant associations were found between colon cancer and PM 10 , O₃, NO₂, SO₂, or CO exposure. The PM 2.5 effect size was larger than estimates from regions with lower exposure levels. Conclusion Long-term exposure to ambient PM 2.5 markedly increases the risk of death among colon cancer patients in northern Thailand. As a modifiable risk factor, PM 2.5 mitigation urgently needs to be incorporated into environmental and survivorship care strategies in high-exposure areas.