The Gas You Can’t Detect Without Testing: Radon Exposure Assessment
By Paul E. Rosenfeld, M.S., Ph.D., Principal Environmental Chemist, RoseyFields LLC, Santa Monica, California
The second leading cause of lung cancer in the United States
Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil and rock, and it is the second leading cause of lung cancer in the United States after smoking, and the leading cause among non-smokers, according to EPA’s own risk assessment, making it one of the few environmental carcinogens whose primary exposure source is entirely natural rather than industrial (US EPA, A Citizen’s Guide to Radon).
Why radon exposure is almost entirely an indoor air problem
Radon gas migrates from soil into buildings through foundation cracks, gaps around pipes, and other openings, accumulating to levels far higher indoors than in outdoor air, meaning radon exposure assessment is fundamentally an indoor air investigation, in sharp contrast to most other contaminants addressed in this case study series, which involve industrial emission sources and outdoor-to-indoor transport.
The regulatory action level and its limitations
EPA recommends remediation when indoor radon concentrations reach or exceed 4 picocuries per liter, but the agency has been explicit that this is an action level reflecting practical remediation feasibility, not a demonstrated safety threshold, since EPA’s own risk models estimate meaningful lung cancer risk even at concentrations below this action level (US EPA, Radon Action Level).
Geographic and geological variability in radon potential
Because radon originates from underlying geology, radon potential varies enormously by region, with certain areas of the country, particularly those with uranium-bearing granite or shale formations, experiencing systematically higher indoor radon concentrations than other regions, making regional geological data relevant background information for any radon exposure assessment (US EPA, EPA Map of Radon Zones).
Why radon exposure reconstruction depends heavily on building-specific factors
Unlike outdoor air contaminants where a single dispersion model can estimate community-wide exposure, radon concentration varies dramatically between individual buildings, and even between different rooms or floors of the same building, depending on foundation type, construction quality, ventilation, and specific soil conditions immediately beneath that structure, meaning building-specific historical testing data, where available, is far more valuable than a generalized regional estimate.
The epidemiological foundation linking radon to lung cancer
The scientific basis for radon’s carcinogenicity traces substantially to occupational uranium miner cohort studies documenting a clear exposure-response relationship between radon progeny exposure and lung cancer, subsequently confirmed and extended to residential exposure through pooled analyses of case-control studies conducted in North America and Europe (National Research Council, BEIR VI; Krewski et al., 2005).
IARC’s classification and the synergistic effect with smoking
IARC classifies radon and its decay products as carcinogenic to humans (Group 1), and the epidemiological literature has specifically documented a synergistic, rather than simply additive, relationship between radon exposure and cigarette smoking, meaning a smoker’s radon-attributable lung cancer risk is substantially higher than a non-smoker’s risk at the same radon exposure level (IARC, Radiation, Vol 100D).
Reconstructing historical exposure without historical testing data
Because residential radon testing has become common only in recent decades, reconstructing historical exposure for a plaintiff who occupied a building for years before any testing occurred typically requires combining whatever building-specific test results exist for any period, regional radon potential data, and information about any structural changes, such as foundation work or ventilation modifications, that could have altered radon entry rates over the relevant occupancy period.
Radon mitigation systems and their exposure-reduction effectiveness
Where a building has an installed radon mitigation system, typically a sub-slab depressurization system similar in concept to the vapor intrusion mitigation systems used for chlorinated solvent contamination, historical exposure reconstruction must account for the system’s installation date and documented performance, since pre- and post-mitigation exposure periods can differ by an order of magnitude or more within the same structure.
Why radon remains a distinctive and continuing exposure assessment need
Because radon is a naturally occurring hazard present to some degree in nearly all buildings rather than one tied to a specific industrial defendant, radon exposure assessment more often arises in the context of real estate transactions, landlord-tenant disputes, and building-specific negligence claims than in the industrial defendant litigation addressed elsewhere in this series, but the underlying exposure reconstruction and dose-response methodology remains equally rigorous. For more on Dr. Rosenfeld’s indoor air exposure assessment work, visit RoseyFields.com.
References
1. US Environmental Protection Agency. A Citizen’s Guide to Radon. Washington, DC: US EPA.
2. US Environmental Protection Agency. EPA Assessment of Risks from Radon in Homes. EPA 402-R-03-003. Washington, DC: US EPA; 2003.
3. National Research Council. Health Effects of Exposure to Radon: BEIR VI. Washington, DC: National Academies Press; 1999.
4. Krewski D, Lubin JH, Zielinski JM, et al. Residential radon and risk of lung cancer: a combined analysis of 7 North American case-control studies. Epidemiology. 2005;16(2):137-145.
5. IARC Working Group on the Evaluation of Carcinogenic Risks to Humans. Radiation. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Vol 100D. Lyon, France: International Agency for Research on Cancer; 2012.
6. Darby S, Hill D, Auvinen A, et al. Radon in homes and risk of lung cancer: collaborative analysis of individual data from 13 European case-control studies. BMJ. 2005;330(7485):223.
7. US Environmental Protection Agency. Map of Radon Zones. Washington, DC: US EPA.
8. Agency for Toxic Substances and Disease Registry. Toxicological Profile for Ionizing Radiation. Atlanta, GA: US Department of Health and Human Services; 1999.
9. US Environmental Protection Agency. Home Buyer’s and Seller’s Guide to Radon. Washington, DC: US EPA.
10. American Association of Radon Scientists and Technologists. Radon Measurement and Mitigation Standards. Fletcher, NC: AARST.
11. National Institute for Occupational Safety and Health. Radon Progeny Exposure in Underground Mines. Cincinnati, OH: CDC/NIOSH.
12. US Environmental Protection Agency. Protocols for Radon and Radon Decay Product Measurements in Homes. Washington, DC: US EPA.
13. Rosenfeld PE. RoseyFields LLC: Indoor Air Quality and Radon Exposure Assessment. Santa Monica, CA: RoseyFields LLC; 2026.
14. US Environmental Protection Agency. Revisions to the Guideline on Air Quality Models. 40 CFR Part 51, Appendix W. Fed Regist. 2017;82(10):5182-5235.