Was the SSM developed to serve as an acute criterion to protect swimmers against short term exposures?
No. The SSM values in the 1986 EPA Ambient Water Quality Criteria for Bacteria were not developed as acute criteria; rather, they were developed as statistical constructs to allow decision makers to make informed decisions to open or close beaches based on small data sets. This does not mean the SSM values serve no purpose outside of beach notification decisions. For example, they may give States and Territories the ability to make waterbody assessments where they have limited data for a waterbody. However, the SSMs were not designed to provide any more protection of health than provided by the geometric mean criterion. In developing the 1986 EPA Ambient Water Quality Criteria for Bacteria , EPA derived the SSM as upper percentiles of the frequency distributions around the geometric mean. The 1986 bacteria criteria document recognizes that there will be instances where the concentration of bacteria in one or more individual samples will be higher than the acceptable geometric mean conc
