Nationwide, enhanced standards, which took effect for loan applications after June 30, 2023, aim to improve air quality, with some 2025 updates beginning to refine these requirements. Recently, significant tightening of radon testing requirements for multifamily housing (five or more units), backed by Fannie Mae and Freddie Mac, includes increasing testing of ground-floor units from 10% to 25%, mandating oversight by an Environmental Professional (EP), and implementing strict tenant notification protocols.
This SCS Engineers blog by Project Professional Tyler Overton outlines recent expansions and tightening of radon testing requirements for multifamily housing in California, emphasizing updated protocols, legal compliance, and public health implications.
Expanded Radon Testing Protocols
The U.S. Environmental Protection Agency (EPA) has broadened radon testing mandates for multifamily housing, now requiring testing in 100 percent of ground-contact residential units and at least 10 percent of upper-floor units in multistory buildings for new construction and rehabilitation projects funded by HUD or other federal sources. This marks a significant shift from previous practices that sampled only 10 percent of ground-floor units. These protocols are based on updated methodologies from the American Association of Radon Scientists and Technologists (AARST) and ANSI/AARST standards, which California-certified radon professionals must follow. In addition to HUD projects, lender policies, investor standards, and local regulations, these requirements may also be enforced, making radon testing standard for both affordable and market-rate multifamily developments.
Importance of Radon Testing
Radon is a naturally occurring radioactive gas that infiltrates buildings through foundation cracks and other openings, accumulating especially in ground-contact units. It is colorless and odorless, requiring testing for detection. The EPA identifies radon as the second leading cause of lung cancer in the U.S. and the primary cause among non-smokers. Long-term exposure heightens cancer risk, underscoring the importance of radon management in residential properties, particularly multifamily housing, where many occupants may be affected.
California’s Certification and Compliance Requirements
California law mandates that radon testing and mitigation services be performed only by individuals certified through the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB), with their certifications registered with the California Department of Public Health (CDPH). Unauthorized radon services are prohibited, and violations are misdemeanors subject to fines. Compliance is thus a regulated activity with legal consequences for property owners and developers.
Radon Mitigation Procedures
If radon levels exceed action thresholds, mitigation is required. The primary method in multifamily buildings is active soil depressurization, which vents radon gas safely above the roof using piping and fans. Other measures include sealing foundation penetrations and improving sub-slab ventilation. Mitigation must comply with ANSI/AARST standards and may also relate to California Residential Code Appendix F, which addresses radon-resistant construction in new buildings. Post-mitigation testing and often long-term system monitoring are necessary to ensure effectiveness.
Geographic Considerations and Testing Necessity
Although California is not generally classified as a high-radon state, certain counties, especially in Northern California, the Sierra Nevada foothills, and parts of the Central Coast, have moderate to elevated radon potential. Both the EPA and the California Geological Survey stress that elevated radon levels can occur anywhere, making testing the only reliable way to assess radon levels. Consequently, geographic exemptions are discouraged, and standardized testing is becoming the norm for multifamily housing regardless of location.
Public Awareness and Health Promotion
Public health agencies promote radon awareness and testing as crucial for reducing risks. The CDPH offers guidance, mapping, and referrals to certified professionals, while the EPA runs national campaigns encouraging testing in all homes. California’s CDPH supports a public library program lending free short-term digital radon monitors. Despite these initiatives, radon risk remains underestimated in the state, underscoring the need for greater homeowner awareness and proactive radon management in multifamily housing.
Future Outlook
California’s evolving radon framework reflects a growing focus on environmental health in housing. With stricter testing protocols and mandatory involvement of certified professionals, radon safety is becoming a baseline expectation. For HUD or federally funded multifamily projects, understanding radon origins, risks, and mitigation is essential. As standards tighten, proactive compliance is critical to safeguard residents and preserve long-term asset value.
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About the Author: Meet Tyler Overton, an SCS project professional who supports clients by managing environmental projects at engineering, development, solid waste firms, and nonprofits. At SCS Engineers, his focus is on due diligence and environmental site assessments. His responsibilities include conducting Phase I environmental site assessments, regulatory compliance projects, groundwater and stormwater sampling programs, and Phase II subsurface investigations, including soil, soil vapor, and groundwater sampling.
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