title: Swimmer illness associated with marine water exposure and water quality indicators: impact of widely used assumptions.
abstract: BACKGROUND
  Studies of health risks associated with recreational water exposure require investigators to make choices about water quality indicator averaging techniques, exposure definitions, follow-up periods, and model specifications; however, investigators seldom describe the impact of these choices on reported results. Our objectives are to report illness risk from swimming at a marine beach affected by nonpoint sources of urban runoff, measure associations between fecal indicator bacteria levels and subsequent illness among swimmers, and investigate the sensitivity of results to a range of exposure and outcome definitions.
  
  
  METHODS
  In 2009, we enrolled 5674 people in a prospective cohort at Malibu Beach, a coastal marine beach in California, and measured daily health symptoms 10-19 days later. Concurrent water quality samples were analyzed for indicator bacteria using culture and molecular methods. We compared illness risk between nonswimmers and swimmers, and among swimmers exposed to various levels of fecal indicator bacteria.
  
  
  RESULTS
  Diarrhea was more common among swimmers than nonswimmers (adjusted odds ratio = 1.88 [95% confidence interval = 1.09-3.24]) within 3 days of the beach visit. Water quality was generally good (fecal indicator bacteria levels exceeded water quality guidelines for only 7% of study samples). Fecal indicator bacteria levels were not consistently associated with swimmer illness. Sensitivity analyses demonstrated that overall inference was not substantially affected by the choice of exposure and outcome definitions.
  
  
  CONCLUSIONS
  This study suggests that the 3 days following a beach visit may be the most relevant period for health outcome measurement in recreational water studies. Under the water quality conditions observed in this study, fecal indicator bacteria levels were not associated with swimmer illness.
authors.affiliation: From the aSchool of Public Health, University of California, Berkeley, CA; bSouthern California Coastal Water Research Project, Costa Mesa, CA; cKaiser Permanente Division of Research, Oakland, CA; dUnited States Environmental Protection Agency, National Environmental Health Effects Research Laboratory, Chapel Hill, NC; eSurvey Research Center, University of California, Berkeley, CA; fOrange County Sanitation District, Orange County, CA; and gInstitute of Environment and Sustainability, University of California, Los Angeles, CA.
authors.forename: Benjamin F
authors.initials: BF
authors.lastname: Arnold
authors.forename: Kenneth C
authors.initials: KC
authors.lastname: Schiff
authors.forename: John F
authors.initials: JF
authors.lastname: Griffith
authors.forename: Joshua S
authors.initials: JS
authors.lastname: Gruber
authors.forename: Vincent
authors.initials: V
authors.lastname: Yau
authors.forename: Catherine C
authors.initials: CC
authors.lastname: Wright
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Wade
authors.forename: Susan
authors.initials: S
authors.lastname: Burns
authors.forename: Jacqueline M
authors.initials: JM
authors.lastname: Hayes
authors.forename: Charles
authors.initials: C
authors.lastname: McGee
authors.forename: Mark
authors.initials: M
authors.lastname: Gold
authors.forename: Yiping
authors.initials: Y
authors.lastname: Cao
authors.forename: Stephen B
authors.initials: SB
authors.lastname: Weisberg
authors.forename: John M
authors.initials: JM
authors.lastname: Colford
mesh_terms: D000293:Adolescent; D000328:Adult; D001493:Bathing Beaches; D002140:California / Q000453:epidemiology; D002648:Child; D002675:Child, Preschool; D003967:Diarrhea / Q000453:epidemiology / Q000209:etiology*; D004781:Environmental Exposure / Q000009:adverse effects*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007223:Infant; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D011998:Recreation; D012306:Risk; D013550:Swimming; D013997:Time Factors; D014871:Water Microbiology / Q000592:standards*; D060753:Water Quality / Q000592:standards*; D055815:Young Adult