EPA has set an enforceable regulation for hexachlorocyclopentadiene, called a maximum contaminant level (MCL), at 0.05 mg/L or 50 ppb. MCLs are set as close to the health goals as possible, considering cost, benefits and the ability of public water systems to detect and remove contaminants using suitable treatment technologies. In this case, the MCL equals the MCLG, because analytical methods or treatment technology do not pose any limitation.
The Phase V Rule, the regulation for hexachlorocyclopentadiene, became effective in 1994. The Safe Drinking Water Act requires EPA to periodically review the national primary drinking water regulation for each contaminant and revise the regulation, if appropriate. EPA reviewed hexachlorocyclopentadiene as part of the Six Year Review and determined that the 0.05 mg/L or 50 ppb MCLG and 0.05 mg/L or 50 ppb MCL for hexachlorocyclopentadiene are still protective of human health.
States may set more stringent drinking water MCLGs and MCLs for hexachlorocyclopentadiene than EPA.