Invisible Foes: How Air Pollution and Lifestyle Conspire in the Rise of Colorectal Cancer

Abstract Background Air pollution is a recognized risk factor for colorectal cancer (CRC), but its specific impact in China, especially among older populations, remains underexplored. Methods We used multivariable Cox models with restricted cubic splines to assess the dose–response relationship between air pollution and CRC risk. Subgroup analyses by sex, residence (urban vs. rural) and pre-existing health conditions (hypertension, diabetes) were conducted. Mediation analysis assessed the indirect effects of smoking and alcohol on CRC risk. Results Exposure to PM2.5, NO2 and SO2 was linked to increased CRC risk. For PM2.5, the hazard ratio (HR) for the highest exposure quartile was 2.35 (95% CI: 1.85–3.03) in urban residents and 1.93 (95% CI: 1.45–2.56) in rural residents. For NO2, the HR per 10 µg/m³ increase was 1.82 (95% CI: 1.56–2.12), and for SO2, it was 1.56 (95% CI: 1.31–1.85). Smoking mediated 14.03% of the effect of PM2.5 on CRC, while alcohol consumption mediated 5.12%. Subgroup analysis showed higher susceptibility in females (HR = 3.47, 95% CI: 2.63–4.63) compared to males (HR = 2.98, 95% CI: 2.25–3.79), and individuals with hypertension (HR = 2.67, 95% CI: 1.98–3.60) or diabetes (HR = 2.10, 95% CI: 1.55–2.86) were at significantly higher risk. Conclusion Long-term exposure to PM2.5, NO2 and SO2 significantly increases CRC risk in older Chinese adults, with smoking as a key mediator. Females, urban residents and those with hypertension or diabetes are especially vulnerable. Public health strategies should focus on reducing pollution in these high-risk groups.