Frequent Questions

Why was the inital MCLG for selenium under the National Interim Primary Drinking Water Regulations revised?

The MCLG for selenium was originally 0.01 mg/L as part of the National Interim Primary Drinking Water Regulations based upon signs of selenium toxicity at an intake of 0.7-7 mg/day and an assumed selenium intake of 200 ug/day (50 FR 46936,46974; November 13, 1985). EPA proposed to revise the MCLG to 0.05 mg/L under the Phase II Proposed Rule. The revision was based on results from a human study which examined selenium toxicity and deficiency effects in China (54 FR 22062, 22078; January 30, 1991). The human study examining selenium toxicity provides a no-effect level and suggests that 0.400 mg/L of selenium/person/day is a maximal daily safe intake of selenium. Assuming average adult consumption of 2 L/day and the average dietary intake in the U.S. of 0.125 mg/person/day, the combined ingestion of water containing 0.05 mg/L and a typical U.S. diet would result in a total daily exposure of 0.225 mg selenium/person, which is well below the 0.400 mg selenium that the study suggests is safe. EPA believes that 0.05 mg/L is adequate to protect both the general public and those who may take selenium supplements(56 FR 3526, 3539; January 30, 1991) .
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