Combined Sewer Overflows and Gastrointestinal Illness in a Changing Climate: A Systematic Review
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Combined sewer overflow events (CSOs) often occur in urban areas due to heavy precipitation, which is expected to occur more frequently and at higher intensity due to climate change. CSOs can negatively impact wildlife, degrade recreational spaces, increase water nutrient loads leading to harmful algal blooms, and contaminate water bodies with toxins. After a CSO, humans can be exposed to pathogens from untreated wastewater through recreational activities and contaminated drinking water. Despite a general understanding that CSOs are deleterious, there is limited research on health impacts of CSOs. We conducted a systematic literature search of peer-reviewed epidemiological studies listed in PubMed and/or Web of Science that investigated associations between CSOs and human health outcomes. The abstract of each study returned from our search was screened by two reviewers to evaluate relevance to our research topic. Next, the full text of each study that passed the abstract screening was evaluated by two reviewers, according to pre-determined inclusion criteria. Our literature search and screening methods returned seven studies on the association between CSOs and gastrointestinal illness (e.g., gastroenteritis, diarrhea, nausea, vomiting, peristalsis, noninfectious gastroenteritis and colitis, etc.) in Europe and North America. Our selected peer-reviewed publications include three different study designs: time series, case-crossover, and retrospective cohort; and the study periods include years 2002 to 2014. We provide narrative summaries of each study, as well as study quality evaluation based on exposure assessment, quality of analysis, and risk of bias. We discuss what is known about the health risks associated with CSOs in urban areas, limitations of existing studies, and key knowledge gaps in the context of climate change.