MCLs and MCLGs

Establishing the Maximum Contaminant Level

The Maximum Contaminant Level (MCL) is the maximum concentration of a chemical that is allowed in public drinking water systems. The MCL is established by the U.S. Environmental Protection Agency (EPA). Currently there are fewer than 100 chemicals for which an MCL has been established; however, these represent chemicals that are thought to pose the most serious risk.

The EPA guidance for establishing an MCL states that "MCLs are enforceable standards and are to be set as close to the maximum contaminant level goals (MCLGs) (Health Goals) as is feasible and are based upon treatment technologies, costs (affordability) and other feasibility factors, such as availability of analytical methods, treatment technology and costs for achieving various levels of removal." The process of determining an MCL starts with an evaluation of the adverse effects caused by the chemical in question and the doses needed to cause such effects. The final result of this process is a safe dose (the dose thought to provide protection against adverse effects including a margin of safety), now called a Reference Dose (RfD) by the EPA. This evaluation is based on the results of animal experiments, and the research results are extrapolated to humans using standard EPA methods.

For chemicals that do not cause cancer, an MCLG is established by first converting the safe dose (RfD) to a water concentration. Then this number is divided by five based on the assumption that exposure to the chemical through drinking water represents only one-fifth of the possible exposure to this substance. Other sources of exposure may be air, soil, and food. In almost all cases, the MCLG value is the same one that is used as the MCL.

For chemicals believed to cause cancer, (known or probable humans carcinogens - EPA Class A or B), the MCLG is set at zero; i.e., no amount of chemical is considered acceptable. However, since zero cannot be measured, the MCL is based on the lowest concentration that can be measured on a routine basis. This is known as the Practical Quantitation Limit (PQL). Thus for known or probable carcinogens, the MCL is not a safe level but instead is the lowest measurable level.

For chemicals that are possible cancer-causing agents (EPA Class C); i.e., there is some evidence that they may cause cancer but this is not very convincing, a value equivalent to the MCLG is calculated as if they were not carcinogens. Then this value is divided by a factor of ten to give the final MCLG. This provides an additional margin of safety in case the chemical is later determined to be a carcinogen.

A sample MCL calculation can be seen here.

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This page was prepared by M. Kamrin, Michigan State University. April 1997 Extoxnet FAQs Team