Early studies showed that individuals who swam in waters with geometric mean coliform densities above 2300/100 mL for three days had higher illness rates. In the 1960s, these numbers were converted to fecal coliform concentrations assuming 18 percent of total coliforms were fecal. Consequently, the National Technical Advisory Committee in the US recommended the following standard for recreational waters in 1968: 10 percent of total samples during any 30-day period should not exceed 400 fecal coliforms/100 mL or a log mean of 200/100 mL (based on a minimum of 5 samples taken over not more than a 30-day period).