EPA evaluates recent research findings about the health effects of drinking water contaminants to identify the national primary drinking water regulations for which there is potential to revise the maximum contaminant level goalmaximum contaminant level goalThe maximum level of a contaminant in drinking water at which no known or anticipated adverse effect on the health of persons would occur, and which allows an adequate margin of safety. Maximum contaminant level goals are non-enforceable health goals. (MCLG). This includes determining if a new health effects assessment for the contaminant has been completed or will be completed early in the current six-year review development cycle. New health effects assessments are reviewed to determine if changes should be made to the current reference dose (RfD) or cancer risk for a contaminant for which the MCLG is based.

• A reference dose is an estimate of a daily oral intake that is likely to be without an appreciable risk of harmful effects during a lifetime. RfDs are used to calculate the MCLG for a non-carcinogen contaminant.
• Cancer risk is the estimated increased risk of developing cancer from a lifetime exposure to a contaminant at a certain level. These values are used to estimate risk levels for cancer causing contaminants.

If the health effects information supports a revision to the MCLG, information about available analytical and treatment methods as well as contaminant occurrence is reviewed to determine if changes should be made to the contaminant's maximum contaminant levelmaximum contaminant levelThe highest level of a contaminant that is allowed in drinking water as delineated by the National Primary Drinking Water Regulations. These levels are based on consideration of health risks, technical feasibility of treatment, and cost-benefit analysis. (MCL).

During the review, EPA also identifies whether new health effects data and information is available on contaminants for which no recent or ongoing assessments exist. EPA may identify these contaminants as potential nominees for a new health assessment.