EPA proposed the Radon in Drinking Water Rule in the Federal Register on November 2, 1999 (64 FR 59246). The proposed rule was designed to promote a multimedia approach that would reduce radon risks in indoor air, where the problem is the greatest, while protecting public health from the highest levels of radon in drinking water. Most radon exposure results from radon gas that enters indoor air from soil under homes and other buildings. Only approximately one to two percent of radon exposure comes from drinking water, which occurs primarily through inhaling radon gas that bubbles out of solution. Under the framework set forth in the 1996 amendments to the Safe Drinking Water Act (SDWA), EPA proposed that water systems comply with a lower maximum contaminant level (MCL) for radon in drinking water, or a higher, alternative maximum contaminant level (AMCL) for dissolved radon in drinking water combined with requirements for multimedia mitigation (MMM) programs to address radon that enters indoor air from soil under homes and buildings. Public water systems in States that adopt qualifying MMM programs would be subject to the AMCL, while those in States that did not adopt such programs would be subject to the MCL. Public water systems could also develop an MMM program with EPA approval in the absence of a State program. EPA proposed an MCL for radon in drinking water of 300 picocuries per liter (pCi/L) and an AMCL of 4,000 pCi/L. As part of the 2003 appropriations process, Congress directed EPA to report on the pending radon in drinking water regulations by August 19, 2003.