Frequently Asked Questions:

4,000 pCi/L MEG for Radon in Well Water

Maine has a new Maximum Exposure Guideline (MEG) of 4,000 pCi/L radon in well water.

What is an MEG?

The Maine Center for Disease Control, Environmental and Occupational Health Program is responsible for developing Maximum Exposure Guidelines (MEG) for private wells. The MEG for radon has been 20,000 picocuries per liter (pCi/L); this MEG was established in the late 1970s. MEGs represent levels of chemicals in water below which there are minimal risks from lifetime ingestion of water. MEGs are used by multiple entities, but are primarily designed as guidance for homeowners with wells. A chemical that exceeds an MEG may result in a recommendation to reduce exposure.

Why is Maine lowering the MEG?

The MEG of 20,000 pCi/L of radon in water represents an unacceptably high cancer risk and needs to be lowered. Radon in water is mostly of concern because it escapes into indoor air whenever water is used. For every 10,000 pCi/L of radon in water, around 1 pCi/L is added to indoor air levels. Therefore, 20,000 pCi/L of water would add 2 pCi/L to indoor air radon level. In recent years, a number of studies of residential populations have confirmed that the added cancer risk associated with radon indoor air levels of 2 pCi/L approaches 1-in-100 (i.e., for every 100 people exposed to 2 pCi/L radon over a lifetime, 1 person would be expected to develop lung cancer). As a matter of ME-CDC policy, most MEGs for cancer-causing chemicals are set at very low cancer risks levels of 1-in-100,000. Thus, the current MEG for radon is unacceptably high and needs to be lowered.

What is the new MEG?

The new MEG for radon is 4,000 pCi/L. An MEG at this level represents approximately a 3 per 1,000 cancer risk. While this risk is far greater than typically considered acceptable for an MEG, it represents a contribution of only 20% to the 2 pCi/L indoor air guideline that Maine is now emphasizing for control of radon from soil gas. Given the prevalence of indoor air radon levels at and above 2 pCi/L due to soil gas, ME-CDC believes it is difficult to justify control of radon from water when levels are less than 4000 pCi/L; rather, resources should be directed toward reducing soil gas related sources.

How does Maine’s MEG compared to other New England states?

Several other states within New England use water guidelines that are similar to 4,000 pCi/L. New Hampshire recommends action at 2,000 pCi/L, Rhode Island at 4,000 pCi/L, Connecticut at 5,000 pCi/L and Massachusetts at 10,000 pCi/L. EPA has a proposed Alternative Maximum Contaminant Level of 4,000 pCi/L that would be relevant to public water supplies.

How do I interpret this new MEG?

While an MEG represents the point at which action should be considered by a homeowner, it does not necessarily mean the homeowner should install a treatment system. The only way to know how to most effectively reduce risk is by having both an air and water radon result and possibly consulting with the Maine Radon Program.