New radon map of US finds 25% of Americans are in high risk cancer-causing radon hotspots

National radon gas maps and database of high risk areas

Researchers from the Harvard T.H. Chan School of Public Health have mapped out radon hotspots having developed a comprehensive national database containing millions of multi-day indoor radon measurements spanning two decades, from 2001 to 2021. Their findings suggest that approximately 25% of the U.S. population could be living in homes with radon concentrations exceeding 148 Bq/m³—a level associated with significant cancer risks.

What causes radon gas in homes?

Radon is a naturally occurring radioactive gas that originates from the decay of uranium deposits found in the earth's crust. As uranium-238 breaks down, radon gas is released and can migrate upward through soil, often pushed by the water levels in the ground, and eventually entering gaps in buildings where it can accumulate to hazardous levels.

Radon exposure is recognized as the second most common cause of lung cancer, responsible for an estimated 21,000 deaths annually in the US alone, and around 220,000 deaths worldwide. It is also linked to other health concerns, including breast cancer, stroke, and stomach cancer.

The U.S. Environmental Protection Agency (EPA) recommends implementing radon mitigation measures if radon levels in indoor spaces reach or exceed 148 Bq/m³. However, past efforts to map radon exposure risks have relied on limited data and typically provided broad county-wide averages, which are insufficient for identifying specific high-risk areas.

The evolving interplay of population growth, changes in housing construction practices, climate shifts, and increased radon mitigation awareness has altered the landscape of radon risk over time. Public health experts have called for more granular mapping techniques to better identify communities most in need of intervention.

In the last few decades, regulations in 35 states have mandated radon testing and disclosure during real estate transactions. These requirements have resulted in the collection of tens of millions of measurements, creating a unique opportunity to reassess radon risks across the country.

New radon maps released for the US  

A recent study titled "High-resolution national radon maps based on massive indoor measurements in the United States," published in the Proceedings of the National Academy of Sciences, utilized over 4.48 million indoor radon measurements collected between 2001 and 2021. Researchers combined this data with a predictive modeling approach to generate detailed radon exposure maps for communities across the mainland United States.

To estimate radon levels in the maps at a community level, the study employed a random forest algorithm trained on 186 different variables. These predictors included geological factors (such as uranium content in soil and bedrock), weather conditions (like soil moisture levels and barometric pressure), socioeconomic data (e.g., median household income), and housing characteristics (including the age, size, and presence of basements in homes). This multifaceted approach allowed for a robust analysis of the factors contributing to radon exposure across different environments.

The model demonstrated strong predictive accuracy, allowing for the production of highly-accurate radon maps of the US with a mean absolute error of 22.6 Bq/m³ for zip-code-level observations. It also provided estimates for areas lacking direct measurements by leveraging the known predictor variables for those regions.

Find radon testers and radon mitigation products here 

Where do the maps say are high risk areas for radon gas exposure?

The national average indoor radon concentration was estimated at 53.3 Bq/m³, slightly higher than the EPA’s previous estimate of 48.1 Bq/m³. However, significant regional differences were identified, leading to the classification of five distinct radon zones on the new maps:

• Zone 1 (below 37 Bq/m³): Includes California’s Central Valley, much of Texas, Louisiana, Mississippi, southern Arkansas, and parts of the Southeastern U.S. Louisiana and Texas reported the lowest state averages at 24.7 Bq/m³ and 29.7 Bq/m³, respectively.
• Zone 2 (37–74 Bq/m³): Spans areas such as Oklahoma, northern Texas, portions of New Mexico and Arizona, the Pacific Northwest, and regions surrounding California’s Central Valley.
• Zone 3 (74–111 Bq/m³): Covers much of Colorado, Utah, northern Nevada, and extends to parts of the Northeast, including Pennsylvania, New Jersey, and New York.
• Zone 4 (111–148 Bq/m³): Concentrated in regions like Colorado, the Midwest, and parts of Wisconsin, Illinois, Iowa, and Ohio.
• Zone 5 (above 148 Bq/m³): Found in areas like South Dakota, Nebraska, parts of Iowa, eastern Pennsylvania, and central Ohio. Newark, Ohio, reported the highest average radon concentration in the country at 246 Bq/m³.

Approximately 3.7 million people live year-round in most-at-risk Zone 5, with seasonal peaks bringing this number to nearly 10 million during winter. Seasonal highs are driven by reduced ventilation in homes during colder months, which traps radon gas in basements and recirculated air.

Even within low-radon zones on the maps, localized hotspots can occur due to unique geological and environmental factors. Areas with high uranium content in the soil or regions with permeable sandy or gravelly soils are particularly prone to elevated radon levels. Additionally, shallow bedrock combined with rainwater can transport uranium isotopes downward, further contributing to radon problems.

Homes with basements are especially vulnerable, as radon tends to infiltrate through cracks and openings in foundation walls. In colder climates, these spaces often see higher radon concentrations due to limited ventilation.

How do you get rid of radon in homes?

There are different techniques available to homeowners for radon mitigation depending on the level of finishing in your basement.  Find more here about the best and cheapest radon mitigation techniques.  For new homes it is always wise to have a radon barrier at least, and even better is a radon evacuation stack, certainly for higher risk areas. 

The study estimated that 83.8 million people, or roughly 27% of the U.S. population, live in homes where radon levels are to be considered dangerous and could exceed 148 Bq/m³. Interestingly, many of these individuals reside in areas categorized as low-radon zones (Zones 1 and 2). Zone 2 alone accounts for 33.4 million residents exposed to elevated radon levels, while Zone 3 adds another 20 million. These concentrations often occur in densely populated regions where overlapping risk factors converge.

Overall, the study and new maps highlight modest declines in radon levels in some historically high-risk areas, possibly reflecting the impact of improved mitigation practices, stricter building codes, and heightened public awareness. However, the findings emphasize the importance of widespread radon testing and targeted interventions to address scattered high-risk communities effectively.

The new radon mapping model offers significant potential for guiding public health initiatives, enhancing building codes, and improving radon mitigation efforts. Furthermore, it could provide valuable data for future studies exploring the long-term health impacts of radon exposure.