Effect of ambient air pollution on the incidence of colorectal cancer among a diabetic population: a nationwide nested case–control study in Taiwan

Objectives An increasing number of studies had shown that air pollution exposure may aggravate blood glucose control in patients with diabetes, an independent risk factor for colorectal cancer (CRC) proposed by some researchers. This study aimed to investigate the impact of exposure to ambient particulate matter with aerodynamic diameters ≤2.5 μm (PM 2.5 ) on the incidence of CRC among a diabetic population. Design A nested case–control study. Setting A subset data retrieved from the Taiwan’s National Health Insurance Research Database. Participants We identified patients with newly diagnosed diabetes (n=1 164 962) during 1999–2013. Participants who had subsequently developed an incident of CRC were placed into the case group, while controls were matched to the cases at a 4:1 ratio by age, gender, date of diabetes diagnosis and the index date of CRC diagnosis. Methods and outcome measures All variables associated with the risk of CRC entered into a multinomial logistic regression model. The dose–response relationship between various average concentrations of PM 2.5 exposure and the incidence of CRC was estimated by logistic regression. Results The study included a total of 7719 incident CRC cases matched with 30 876 controls of random sampling. The mean annual concentration of PM 2.5 was 35.3 µg/m 3 . After adjusting for potential confounders, a dose–response relationship was observed between the CRC risks and each interquartile increase of PM 2.5 concentration (Q1–Q2: 1.03 (0.95–1.11), Q2–Q3: 1.06 (0.98–1.15), ≥Q3: 1.19 (1.10–1.28) in model 2. The adjusted ORs (95% CI) of CRC incidence for each 10 µg/m 3 increment of PM 2.5 was 1.08 (1.04–1.11). Moreover, a faster growing adapted Diabetes Complications Severity Index (aDCSI) score was noticed in CRC group compared with the controls, which also showed a significant association in our multivariate analysis (adjusted OR=1.28, 95% CI 1.18 to 1.38). Conclusions Long-term exposure to high concentrations of PM 2.5 may contribute to an increased incidence of CRC among diabetic populations.