Technical Fact Sheet: Proposed Radon in 

Drinking Water Rule 

The U.S. Environmental Protection Agency (EPA) is proposing new regulations to protect 

people from exposure to radon. The proposed regulations will provide States flexibility in how to 

limit the public's exposure to radon by focusing their efforts on the greatest public health risks 

from radon - those in indoor air - while also reducing the highest risks from radon in drinking 

water. The framework for this proposal is set out in the Safe Drinking Water Act as amended in 

1996, which provides for a multimedia approach to address the public health risks from radon in 

drinking water and radon in indoor air from soil. The Safe Drinking Water Act directs the EPA 

to propose and finalize a maximum contaminant level (MCL) for radon in drinking water, but 

also to make available an alternative approach: a higher alternative maximum contaminant level 

(AMCL) accompanied by a multimedia mitigation (MMM) program to address radon risks in 

indoor air. This framework reflects the unique characteristics of radon: in most cases, radon 

released to indoor air from soil under homes and buildings is the main source of exposure and 

radon released from tap water is a much smaller source of radon in indoor air. It is more costeffective to reduce risk from radon exposure from indoor air, than from drinking water. EPA 

strongly encourages States to take full advantage of the flexibility and risk reduction 

opportunities in the MMM program. 

What are the Public Health Concerns? 

Radon is a naturally-occurring radioactive gas that emits ionizing radiation. National and 

international scientific organizations have concluded that radon causes lung cancer in humans. 

Most of the radon in indoor air comes from the breakdown of uranium in soil beneath homes. 

Breathing radon from the indoor air in homes is the primary public health risk from radon, 

contributing to about 20,000 lung cancer deaths each year in the United States, according to a 

1999 landmark report by the National Academy of Sciences (NAS) on radon in indoor air. The 

U.S. Surgeon General has warned that radon in indoor air is the second leading cause of lung 

cancer. EPA and the U.S. Surgeon General recommend testing all homes and apartments located 

below the third floor for radon in indoor air. If you smoke and your home has high indoor radon 

levels, your risk of lung cancer is especially high. 

Radon from tap water is a smaller source of radon in indoor air. Only about 1-2 percent of radon 

in indoor air comes from drinking water. However breathing radon released to air from 

household water uses increases the risk of lung cancer over the course of your lifetime. Ingestion 

of drinking water containing radon also presents a risk of internal organ cancers, primarily 

stomach cancer. This risk is smaller than the risk of developing lung cancer from radon released 

to air from tap water. Based on a second 1999 NAS report on radon in drinking water, EPA 

estimates that radon in drinking water causes about 168 cancer deaths per year, 89 percent from 

lung cancer caused by breathing in radon released from water, and 11 percent from stomach 

cancer caused by drinking radon-containing water. 

Who Must Comply with the Proposed Rule? 
The proposed radon in drinking water rule applies to all community water systems (CWSs) that 

use ground water or mixed ground and surface water (e.g., systems serving homes, apartments, 

and trailer parks). The proposed rule would not apply to CWSs that use solely surface water, nor 

to non-transient non-community public water supplies and transient public water supplies (e.g., 

systems serving schools, office buildings, campgrounds, restaurants, and highway reststops). 

What does the Rule Propose to Require? 

The rule proposes a maximum contaminant level goal (MCLG), a maximum contaminant level 

(MCL), an alternative maximum contaminant level (AMCL), and requirements for multimedia 

mitigation (MMM) program plans to address radon in indoor air. The proposal also includes 

monitoring, reporting, public notification and consumer confidence report requirements, 

proposed best available technologies and analytical methods. 

Maximum Contaminant Level Goal (MCLG), Maximum Contaminant Level 

(MCL), and Alternative Maximum Contaminant Level (AMCL)

The proposed MCLG for radon in drinking water is zero. This is a non-enforceable goal. 

The proposed regulation provides two options for the maximum level of radon that is allowable 

in community water supplies. The proposed MCL is 300 picoCuries per liter (pCi/L) and the 

proposed AMCL is 4,000 pCi/L. The drinking water standard that would apply for a system 

depends on whether or not the State or CWS develops a MMM program. The regulatory 

expectation of CWSs serving 10,000 persons or less is that they meet the 4,000 pCi/L AMCL 

and be associated with an approved MMM program plan - either developed by the State or by the 

CWS. 

The enforceable MCL or AMCL would apply under the following circumstances: 

Small CWSs: Proposed regulatory expectation for systems that serve 10,000 or fewer 

people 

Does State develop MMM 

program?

Does CWS develop local MMM 

program?

CWS Complies with:

yes not needed AMCL: 4000 

pCi/L* 

>no yes** AMCL: 4000 pCi/L 

* Small systems may elect to comply with the MCL of 300 pCi/L 

** Small systems may elect to comply with the MCL of 300 pCi/L, instead of developing a local 

MMM program. 

Large CWSs: Proposed compliance options for systems that serve more than 10,000 people 

Does State develop MMM Does CWS develop local MMM CWS Complies 
program? program? with:

yes not needed AMCL: 4000 

pCi/L* 

no yes AMCL: 4000 pCi/L

no no MCL: 300 pCi/L 

* Large systems may elect to comply with the MCL of 300 pCi/L 

Monitoring Requirements

CWSs must monitor for radon in drinking water according to the requirements described in the 

table below and report their results to the State. If the State determines that the radon level in a 

CWS is below 300 pCi/L, the system only needs to continue meeting monitoring requirements 

and is not covered by the requirements regarding MMM programs. 

Type Frequency Condition

Initial Four consecutive 

quarters of 

monitoring for one 

year. 

At each entry point to the distribution system which is 

representative of each well after treatment and/or storage 

Routine One sample per year If running average from four consecutive quarterly samples is 

less than MCL/AMCL, and at the discretion of State. 

Reduced One sample every 

three years 

If average from four consecutive quarterly samples is less than 

&#frac12; the MCL/AMCL, no samples exceed the 

MCL/AMCL, and State determines the system is "reliably and 

consistently below MCL/AMCL." 

Four consecutive 

quarters of 

monitoring 

If the MCL/AMCL for radon is exceeded in a single sample, 

when monitoring annually. Can return to one sample per year if 

meet routine monitoring conditions, listed above. 

Increased