Research conducted by scientists from the Harvard T.H. Chan School of Public Health has resulted in a groundbreaking compilation of millions of indoor radon measurements collected between 2001 and 2021, shedding light on the widespread and often underestimated risks associated with radon exposure in the United States. Through innovative use of predictive modeling, this new database reveals that nearly 25% of the U.S. population may be living in homes where radon concentrations exceed 148 Bq/m³ — a threshold recognized by the U.S. Environmental Protection Agency (EPA) as being associated with elevated risks of lung cancer.
What is Radon and How Does it Impact Public Health?
Radon is a naturally occurring radioactive gas that emerges from the decay of uranium found in the earth’s crust. It is an odorless, colorless gas that can easily travel through the soil and make its way into homes and buildings, often accumulating in basements or ground-level spaces. Radon exposure is the second leading cause of lung cancer in the U.S., and globally, it is responsible for approximately 220,000 deaths each year. According to the EPA, radon is directly linked not only to lung cancer but also to other serious health risks, including breast cancer, stomach cancer, and stroke.
In the United States, it is estimated that 21,000 deaths annually are linked to radon-induced lung cancer, a staggering number when considering that the gas is entirely preventable. Prolonged exposure to elevated levels of radon is a major contributor to cancer risk, particularly among smokers, whose risk is significantly amplified when combined with radon exposure.
New Insights from High-Resolution Radon Mapping
For many years, radon risk maps provided broad overviews of areas deemed to have high or low potential for radon, often based on county-level averages. However, these maps were criticized for offering limited insight into actual risks faced by homeowners because the data was sparse, often based on small sample sizes. Furthermore, population growth, climate change, and variations in building construction have altered radon patterns over time, making older methods of mapping increasingly unreliable.
To address this issue, Harvard researchers launched a comprehensive new study designed to offer a higher-resolution, community-specific radon mapping system that would more accurately reflect current exposure risks. The study, titled “High-resolution national radon maps based on massive indoor measurements in the United States”, was published in the Proceedings of the National Academy of Sciences and provides essential updates to the state of knowledge about radon exposure.
Using a vast collection of radon measurement data (4.48 million radon measurements from homes) alongside 186 contributing factors (including geological, meteorological, and architectural parameters), the researchers used a statistical tool called a random forest algorithm to generate highly detailed maps predicting radon concentrations at the zip code level. By analyzing factors like the amount of uranium in bedrock, temperature, and even societal factors like median household income, the team built a model capable of identifying hotspots of radon exposure with unprecedented precision.
The inclusion of factors such as the size and age of homes, the presence of basements, as well as local soil compositions allowed the model to capture the complex interplay of variables that impact radon accumulation within buildings. Areas where high-permeability soils (e.g., sandy or gravelly soils) are prevalent or where shallow bedrock exists tend to see higher radon concentrations as the gas can migrate more easily upward into homes.
Findings from the Radon Risk Map
The results from the new radon mapping exercise are startling, revealing that nearly 26.8% of Americans—about 83.8 million people—are living in areas where radon concentrations might exceed the 148 Bq/m³ threshold. This threshold, according to the EPA’s radon guidelines, suggests the need for a radon mitigation system to reduce indoor radon levels and mitigate health risks.
Interestingly, this high-risk population does not just live in traditionally identified “high-radon zones” but is scattered throughout regions that were once thought to be low risk. For example, some areas that have historically been categorized as Zone 1 (those with radon levels below 37 Bq/m³) were found to host communities where local conditions (such as specific construction practices and local geology) lead to significantly higher radon concentrations than previously expected.
The mapping identified five key zones based on the average radon concentrations:
- Zone 1 (below 37 Bq/m³): This is the lowest radon zone covering the central valley of California, large parts of Texas and Louisiana, and the Southern U.S. It encompasses large cities such as Los Angeles, Dallas, and parts of the Eastern Seaboard.
- Zone 2 (37–74 Bq/m³): This zone spans areas including parts of Oklahoma, New Mexico, and the Pacific Northwest, among others.
- Zone 3 (74–111 Bq/m³): A larger area, encompassing most of Colorado, parts of New York, Pennsylvania, and much of the Western U.S..
- Zone 4 (111–148 Bq/m³): These zones represent slightly higher concentrations of radon, extending through parts of Ohio, Illinois, Wisconsin, and Colorado.
- Zone 5 (above 148 Bq/m³): Areas in this zone are under the most concern, including parts of South Dakota, North Dakota, Nebraska, and eastern Pennsylvania, where radon levels are the highest. Particularly troubling are parts of Ohio, where local levels can climb as high as 246 Bq/m³—far exceeding the thresholds that would typically trigger mitigation action.
Radon Variability Within Zones
While the national averages indicate significant variability across regions, local patterns and individual home characteristics within these zones were found to drive considerable variability in radon levels. In Zone 2, for example, approximately 33.4 million residents live in households where radon concentrations might be approaching dangerous levels. Surprisingly, even some areas within traditionally low-radon zones (Zone 1) exhibited higher-than-expected radon levels due to local geological formations, specific construction practices, or demographic factors.
Additionally, the seasonality of radon exposure plays a significant role in potential risks. The study’s authors note that colder weather during winter leads to reduced ventilation, especially in homes with basements. Radon concentrations tend to increase when homes are sealed to conserve heat, thus trapping radon gas in the lower levels of the house, causing an increase in indoor concentrations.
Practical Implications for Public Health and Building Practices
Given these findings, the researchers underscore the importance of comprehensive radon testing in homes across all zones, even in traditionally low-risk areas. They advocate for more tailored intervention strategies that incorporate high-resolution radon maps into public health efforts aimed at preventing the adverse health impacts of radon exposure.
Some of the key practical takeaways for public health policy and building codes include:
- Increased radon mitigation efforts: For homes with elevated radon levels, particularly those in Zone 5 and other high-risk zones.
- Homebuyer education: More robust programs to inform homeowners and real estate professionals about radon risks, especially when properties have higher radon potential.
- Enhanced testing and regulation: Community-specific testing efforts, possibly backed by local or state regulations, could help pinpoint individual high-risk locations for radon exposure.
- Stronger construction standards: New building codes, particularly in higher-risk regions, can ensure the incorporation of ventilation systems and radon-resistant materials in homes.
Conclusion
This high-resolution radon map offers a significant leap forward in the fight to reduce radon-related lung cancer deaths and other health risks associated with the gas. By addressing previously uncharted high-risk areas, the study urges both public health officials and homeowners to take immediate action in identifying and mitigating radon exposure. With accurate mapping and informed public health strategies, significant progress can be made in safeguarding millions of people from the harmful effects of radon.
Reference: Longxiang Li et al, High-resolution national radon maps based on massive indoor measurements in the United States, Proceedings of the National Academy of Sciences (2025). DOI: 10.1073/pnas.2408084121