Association between combined sewer overflow events and gastrointestinal illness in Massachusetts municipalities with and without river-sourced drinking water, 2014-2019

Background Combined sewer overflow (CSO) events release untreated wastewater into surface waterbodies during heavy precipitation or snowmelt. Combined sewer systems serve approximately 40 million people in the United States, primarily in urban and suburban municipalities in the Midwest and Northeast. Predicted increases in heavy precipitation events driven by climate change underscore the importance of quantifying potential health risks associated with CSO events. Objectives The aims of this study were: to 1) estimate the association between CSO events (2014-2019) and emergency department (ED) visits for acute gastrointestinal illness (AGI) among Massachusetts municipalities that border a CSO-impacted river, and 2) determine if associations differ by municipal drinking water source. Methods A case time series design was used to estimate the association between daily cumulative upstream CSO discharge in the prior four days and ED visits for AGI, adjusting for temporal trends, temperature, and precipitation. Associations between CSO events and AGI were also compared by municipal drinking water source (CSO-impacted river vs. other sources). Results Extreme upstream CSO discharge events (>95th percentile by cumulative volume) were associated with a cumulative risk ratio (CRR) of AGI of 1.22 (95% CI: 1.05, 1.42) over the next four days for all municipalities, and the association was robust after adjusting for precipitation (1.17 [0.98, 1.39]). In municipalities with CSO-impacted drinking water sources, the adjusted association was somewhat less pronounced following 95th percentile CSO events (1.05 [0.82, 1.33]). The adjusted CRR of AGI was 1.62 in all municipalities following 99th percentile CSO events (95% CI: 1.04, 2.51) and not meaningfully different across strata defined by drinking water source. Discussion In municipalities bordering a CSO-impacted river in Massachusetts, extreme CSO events are associated with higher risk of AGI within four days. The largest CSO events are associated with increased risk of AGI regardless of drinking water source. ### Competing Interest Statement Dr. Wellenius serves as a consultant to the Health Effects Institute (Boston, MA) and Google, LLC (Mountain View, CA). All other authors declare they have nothing to disclose. ### Funding Statement This project was supported by an Early Stage Urban Research Award from the Boston University Initiative on Cities. BMH was partially supported by National Institute of Environmental Health Sciences (NIEHS) grant T32ES014562 and a National Science Foundation Research Traineeship (NRT) grant to Boston University (DGE 1735087). ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: This study was designated as exempt from review by the Boston University Medical Campus Institutional Review Board (IRB number: H-42193). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Health outcome data were made available to the authors by the Center for Health Information and Analysis under a data use agreement and cannot be shared by the authors. Exposure data that originated from public sources are available from the corresponding author upon reasonable request.