Maximum Contaminant Levels for Drinking Water

MCL is the Maximum Contaminant Level, or concentration of a chemical, that is allowed in public drinking water supplies.  MCL standards are set by the U.S. Environmental Protection Agency (EPA). There are currently fewer than 100 chemicals with a set MCL. These are chemicals that are thought to pose the most serious health risk. The EPA standard for establishing an MCL says, "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 for determining an MCL for a particular chemical starts with an evaluation of the adverse effects caused by the chemical and the doses needed to cause such effects.  As a result, a safe dose, 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. 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, carcinogens (EPA Class A or B), the MCLG is set at zero, meaning no amount of chemical is acceptable. Since zero cannot be measured, the MCL is based on the lowest concentration that can be measured. This is called 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), meaning 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 method for calculating MCL can be seen here.