Specifying the Associations between PM2.5 Constituents and Gastrointestinal Cancer Incidence: Findings from a Prospective Cohort Study in Beijing, China

This study aimed to test the association between PM_2.5 and the incidence of gastrointestinal (GI) cancer, and further to detect the primary constituents on this association. A sum of 142,982 participants without GI cancer at baseline were derived from the National Urban Cancer Screening Program in Beijing (2013-2019). The 5 year averaged concentrations of PM_2.5 mass and its five constituents, namely, black carbon (BC), ammonium (NH₄⁺), nitrate (NO₃^-), organic matter (OM), and inorganic sulfate (SO₄^2-), were estimated by using a hybrid machine learning model. The Cox proportional hazard model with fixed effects was used to explore the associations between PM_2.5 mass and its constituents with the incidence of GI cancer. The double-exposure linear model, the mixture exposure model of quantile-based g-computation, and an explainable machine learning model were utilized to evaluate the importance of different PM_2.5 constituents. Long-term exposure to PM_2.5 mass and its constituents was linearly associated with GI cancer; the estimated hazard ratio and 95% confidence interval (95% CI) of per standard deviation increment were 1.367 (95% CI: 1.257 to 1.487) for PM_2.5 mass, 1.434 (95% CI: 1.307 to 1.574) for BC, 1.255 (95% CI: 1.169 to 1.349) for NH₄⁺, 1.217 (95% CI: 1.139 to 1.301) for NO₃^-, 1.410 (95% CI: 1.287 to 1.546) for OM, and 1.410 (95% CI: 1.288 to 1.542) for SO₄^2-. By using multiple methods, results indicated that SO₄^2- and BC were the most important constituents. Results indicated that long-term exposure to PM_2.5 was associated with a high incidence of GI cancer, and BC and SO₄^2- were robustly identified as the primary constituents.