Ambient Particulate Matter Environmental Exposure and Upper Gastrointestinal Cancers
Open AccessThis dissertation entailed two related projects. Outdoor air pollution, especially particulate matter (PM) is a known carcinogen. Although an association between outdoor PM2.5 exposure and lung cancer has been clearly demonstrated, prior research investigating other cancer types is limited. Both inhaled and ingested particles may lead to cancer development. PM2.5 exposure may also increase the risk of developing UGI cancers, but few studies have evaluated this relationship.In the first study, we conducted a systematic review of the association between PM exposure and all GI cancers. We searched articles published up to December 1, 2020 and included original research that quantified exposure to PM in relation to GI cancer incidence or mortality. We evaluated the risk of bias of individual studies and the overall quality and strength of the evidence according to the Navigation Guide systematic review methodology. We identified fifteen studies, conducted within 13 countries, that met the inclusion criteria. We rated most studies as having “probably low” risk of bias and rated the overall body of evidence as having “moderate” quality and “limited” evidence for an association of PM and GI cancers. We based this determination on the generally positive but inconsistently statistically significant effect estimates reported by a small number of studies, and the fact that chance, bias, and confounding could not be ruled out with reasonable confidence. The second study assessed the association between PM2.5 exposure and the risk of developing UGI cancers. We evaluated this relationship among 496,517 NIH-AARP Diet and Health Study participants aged 50–71 years old at enrollment (1995–1996) across six states and two metro areas. Continuous, long-term PM2.5 exposure averages were estimated from 1980 to enrollment using a nationwide spatio-temporal prediction model at the census-tract level for the enrollment residential address. Esophageal and gastric cancers and their subtypes were ascertained via cancer registry linkage in follow-up through 2011. Multivariable Cox proportional hazards models were used to estimate HRs and 95% CIs for associations of exposure quartiles with cancer risk. We identified 1,005 incident cases of esophageal adenocarcinomas (EAC), 348 cases of esophageal squamous cell carcinoma (ESCC), 721 cases of gastric cardia adenocarcinoma (GCA), and 808 cases of gastric noncardia adenocarcinoma (GNCA). We observed statistically non-significant positive associations with EAC development across all PM2.5 quartiles, without monotonic trend (p-trend=0.4). Being in the highest quartile of PM2.5 exposure was associated with a modestly higher risk of developing ESCC compared with the lowest quartile of PM2.5 exposure, but this did not reach statistical significance (HR: 1.22; 95% CI: 0.75-1.99). We observed no significant associations between PM2.5 exposure and risk of GCA. There was a significantly increased risk of GNCA in the first exposure quartile (HR: 1.27; 95% CI: 1.01-1.59) but not in other exposure categories. Taken together, these dissertation findings demonstrate the importance of further evaluations of these associations by UGI cancer subtype and of additional studies in more heavily exposed populations. This dissertation also highlights the need to conduct more studies in the geographic areas most affected by UGI cancers, to evaluate the impact of different PM exposure assessment approaches on observed associations, and to include investigation of specific chemical components of PM.
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