Potential confounders in a cohort study of water recreation in the Chicago area

Background: Several prospective cohort studies of the health risks of swimming at beaches have published in the past decade. Little is known about the health risks of limited contact water recreational activities, such as boating, fishing, rowing and paddling.

Methods: CHEERS, the Chicago Health, Environmental Exposure, and Recreation Study (CHEERS) is a prospective cohort study conducted at lakes, rivers, and channels in the Chicago. Three groups of participants have been enrolled: 1) those who engage in limited contact water recreation at surface waters with point sources of fecal pollution, 2) those who engage in the same activities at waters without point sources of fecal pollution and 3) those who engage in outdoor physical activities, at the same times and locations as the water exposed subjects. Field and telephone follow-up interviews were conducted, and measures of water quality were made. Survival analyses describe the onset of gastrointestinal illness as a function of study group.

Results: From 2007 through July 13, 2009, a total of 11,119 participants were enrolled. Analyses of the 2007 and 2008 data identified variables that were associated with study group, with health endpoints, or with both. These include age, race/ethnicity, dietary exposures, recent antibiotic use, and a history of gastrointestinal disease.

Discussion: Key variables have been identified that will require further evaluation as potential confounders of associations between health risk and study group. Adjusted survival models will characterize the risks attributable to limited contact water recreation and to exposure to point sources of fecal pollution.