cohort studies, and 3 were randomized controlled trials, as summarized in Table 2-3. Of the studies examined, the rate of certain symptoms or groups of symptoms was found to be significantly related to the count of fecal indicator bacteria in recreational water. Thus, there was a consistency across the various studies evaluated and gastrointestinal symptoms were the most frequent health outcome for which significant dose-related associations were reported. In marine waters, fecal streptococci or enterococci were the fecal indicators that best predicted gastrointestinal illness. In freshwaters, increased concentrations of fecal coliforms or Escherichia coli as well as fecal streptococci and enterococci were predictive of increased gastrointestinal illness risks. Staphylococci concentrations were also found to be predictive of increased risks of illness, including ear, skin, respiratory, and gastrointestinal illness. Although these latter relationships were attributed to the effects of bather density, this was not actually proven.

Based on the studies evaluated by Prüss (1998), strong and consistent associations have been reported between microbial indicators and various adverse health effects to include temporal and dose-response relationships. Furthermore, these studies have biological plausibility and analogy to clinical cases from drinking contaminated water. However, various biases commonly occur with epidemiologic studies as summarized in Table 2-4.

For marine bathing waters, randomized controlled trials in the United Kingdom (Fleisher et al., 1996; Kay et al., 1994) probably contained the least amount of bias. These studies also provide the most accurate measure of exposure, water quality, and illness compared to observational studies where an artificially low threshold and flattened dose-response curve (due to misclassification bias) were likely to have been determined. Therefore, the United Kingdom randomized controlled trials form the key studies for derivation of guideline values for the microbiological quality of recreational waters.⁸It should be recognized that these recommended guidelines values are from studies in temperate waters and are not characteristic of the tropical and subtropical waters found in many areas of the United States (e.g., the U.S. Gulf coast).

Based on analyses of data from numerous studies on the relationships between swimming-associated health effects and the microbial quality of bathing water, the WHO and other international as well as national entities have concluded that fecal streptococci and enterococci currently are the fecal indicator microorganisms that best predict health risks in recreational waters (WHO, 2001). Rather than classify recreational waters as either acceptable or unacceptable, WHO experts chose instead to establish a five-tiered classification system (i.e.,