Mapping risk: Spatial analysis of 2,4-D herbicide emissions and colorectal cancer.

Colorectal cancer remains a major public health concern in Arkansas and across the United States, disproportionately affecting rural and vulnerable populations. Beyond established lifestyle and genetic factors, environmental exposures, particularly agricultural chemicals, are increasingly recognized as potential contributors. In this ecological study, we screened 133 county-level estimated air pollutants to identify candidates associated with colorectal cancer incidence rates, focusing on Arkansas due to its agricultural economy and widespread herbicide use. Using publicly available environmental and cancer registry data, we applied spatial mapping, correlation tests, linear regression, and spatial regression models. Among screened pollutants, 2,4-Dichlorophenoxyacetic acid (2,4-D), a widely used herbicide, emerged as the top candidate. In Arkansas, 2,4-D emissions showed a strong positive association with colorectal cancer incidence rates (Spearman ρ = 0.338, p = 0.003; linear regression β1 = 6.28, p = 0.001), with high-emission counties aligning spatially with elevated incidence. This relationship was further confirmed in analyses of more than 2,500 U.S. counties, where 2,4-D emissions remained significantly associated with colorectal cancer (linear regression β1 = 5.94, p < 0.001; spatial regression β1 = 2.55, p = 0.001). Spatial regression improved model fit and accounted for geographic autocorrelation in both datasets. These findings suggest a potential association between 2,4-D emissions and colorectal cancer incidence rates, particularly in agricultural regions such as Arkansas. While causal inference is limited by the ecological design, the consistency of results warrants follow-up with individual-level, longitudinal, and mechanistic studies.