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BACKGROUND: Outdoor air pollution is a known lung carcinogen, but research investigating the association between particulate matter (PM) and gastrointestinal (GI) cancers is limited. OBJECTIVES: We sought to review the epidemiologic literature on outdoor PM and GI cancers and to put the body of studies into context regarding potential for bias and overall strength of evidence. METHODS: We conducted a systematic review and meta-analysis of epidemiologic studies that evaluated the association of fine PM [PM with an aerodynamic diameter of [less than or equal to]2.5 [micro]m (P[M.sub.2.5])] and P[M.sub.10] (aerodynamic diameter < 10 pm) with GI cancer incidence or mortality in adults. We searched five databases for original research published from 1980 to 2021 in English and summarized findings for studies employing a quantitative estimate of exposure overall and by specific GI cancer subtypes. We evaluated the risk of bias of individual studies and the overall quality and strength of the evidence according to the Navigation Guide methodology, which is tailored for environmental health research. RESULTS: Twenty studies met inclusion criteria and included participants from 14 countries; nearly all were of cohort design. All studies identified positive associations between PM exposure and risk of at least one GI cancer, although in 3 studies these relationships were not statistically significant. Three of 5 studies estimated associations with P[M.sub.10] and satisfied inclusion criteria for meta-analysis, but each assessed a different GI cancer and were therefore excluded. In the random-effects meta-analysis of 13 studies, P[M.sub.2.5] exposure was associated with an increased risk of GI cancer overall [risk ratio (RR) = 1.12; 95% CI: 1.01, 1.24]. The most robust associations were observed for liver cancer (RR = 1.31; 95% CI: 1.07, 1.56) and colorectal cancer (RR = 1.35; 95% CI: 1.08, 1.62), for which all studies identified an increased risk. We rated most studies with 'probably low' risk of bias and the overall body of evidence as 'moderate' quality with 'limited' evidence for this association. We based this determination on the generally positive, but inconsistently statistically significant, effect estimates reported across a small number of studies. CONCLUSION: We concluded there is some evidence of associations between P[M.sub.2.5] and GI cancers, with the strongest evidence for liver and colorectal cancers. Although there is biologic plausibility for these relationships, studies of any one cancer site were few and there remain only a small number overall. Studies in geographic areas with high GI cancer burden, evaluation of the impact of different PM exposure assessment approaches on observed associations, and investigation of cancer subtypes and specific chemical components of PM are important areas of interest for future research. https://doi.org/10.1289/EHP9620, Introduction Gastrointestinal (GI) cancers are a major cause of cancer burden and cancer death. Worldwide, there were an estimated 4.8 million new cases of GI cancers and 3.4 million GI [...]