Locate the reference dose (RfD) for oral exposure provided by the EPA. Navigate your web browser to the EPA's Integrated Risk Information System. Locate the chemical name for the contaminant and click on "Quickview." Record the RfD in milligrams per kilogram per day.
Multiply the RfD by the weight of the person drinking the water in kilograms. Divide weight in pounds by 2.2 to find weight in kilograms. Use 70 kilograms to determine the MCL for the average person. For example, 0.5 mg/kg-day times 70 kg equals 35 mg/day.
Divide the result by the amount of water the person drinks each day. Use 2 liters per day as an average water intake. For example, 35 mg/day divided by 2 liters/day equals 17.5 mg/liter.
Multiply the result by 0.2 to adjust for the assumption that water accounts for only 20 percent of total consumption. For example, 17.5 mg/liter times 0.2 equals 3.5 mg/liter. Record the result as the maximum contaminant concentration unless the contaminant is a known carcinogen.
Divide the result from above by 10 if the contaminant is a Class C carcinogen. For example, 3.5 mg/liter divided by 10 equals 0.35 mg/liter. Set the MCL equal to the practical quantitation limit (PQL) if it is a Class B or A carcinogen. The EPA uses the lowest concentration that can be measured routinely, the PQL, as the MCL for Class A or B carcinogens because zero cannot be measured.