Radon testing requirements for apartment buildings and multifamily housing have tightened significantly, with new federal standards now mandating that developers test 25% of ground-floor units instead of the previous 10%, and requiring oversight by certified environmental professionals. These changes, backed by mortgage giants Fannie Mae and Freddie Mac, represent a major shift in how the real estate industry approaches this invisible health threat. For developers, property owners, and investors, understanding these new protocols is now essential to staying compliant and protecting residents.

Why Are Radon Testing Standards Becoming Stricter?

Radon is a naturally occurring radioactive gas that seeps into buildings through foundation cracks, soil contact, and other openings. Because it is colorless and odorless, the only way to detect it is through testing. The U.S. Environmental Protection Agency (EPA) identifies radon as the second leading cause of lung cancer in the United States, and the primary cause among non-smokers. Long-term exposure to elevated radon levels significantly increases cancer risk, which is why public health agencies are pushing for more comprehensive testing in residential properties.

The tightening of standards reflects growing recognition that radon poses a widespread public health risk. Enhanced standards that took effect for loan applications after June 30, 2023, aim to improve indoor air quality across the nation, with additional refinements continuing through 2025 and beyond. For multifamily housing in particular, where a single building may house dozens or hundreds of residents, the stakes are especially high.

What Are the New Testing Requirements for Multifamily Buildings?

The EPA has significantly expanded radon testing mandates for multifamily housing developments, especially those funded by the U.S. Department of Housing and Urban Development (HUD) or other federal sources. The new protocols require 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. This marks a dramatic shift from previous practices.

Beyond HUD-funded projects, lender policies, investor standards, and local regulations are also enforcing these requirements, making radon testing standard practice for both affordable and market-rate multifamily developments. The testing protocols are based on updated methodologies from the American Association of Radon Scientists and Technologists (AARST) and ANSI/AARST standards, which all California-certified radon professionals must follow.

• Ground-Floor Testing: Testing requirements for ground-floor units have increased from 10% to 25% in multifamily buildings, with 100% testing required for ground-contact residential units in federally funded projects.
• Upper-Floor Testing: At least 10% of upper-floor units in multistory buildings must now be tested for new construction and rehabilitation projects.
• Professional Oversight: An Environmental Professional (EP) must oversee all testing and mitigation work, and strict tenant notification protocols must be implemented.
• Certification Requirements: Only individuals certified through the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB) may perform radon services in California, with certifications registered with the California Department of Public Health (CDPH).

How to Comply with New Radon Testing and Mitigation Standards

• Hire Certified Professionals: Ensure that all radon testing and mitigation work is performed by individuals certified through the NRPP or NRSB. Unauthorized radon services are prohibited and violations are misdemeanors subject to fines.
• Conduct Comprehensive Testing: Test 100% of ground-contact units and at least 10% of upper-floor units in multistory buildings, following AARST and ANSI/AARST standards and methodologies.
• Implement Active Soil Depressurization: If radon levels exceed EPA action thresholds, the primary mitigation method in multifamily buildings is active soil depressurization, which vents radon gas safely above the roof using piping and fans.
• Conduct Post-Mitigation Testing: After mitigation work is complete, perform follow-up testing and establish long-term system monitoring to ensure the effectiveness of radon reduction measures.
• Notify Tenants: Implement strict tenant notification protocols to inform residents about radon testing, results, and any mitigation measures being taken.

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. All mitigation work must comply with ANSI/AARST standards and may also relate to California Residential Code Appendix F, which addresses radon-resistant construction in new buildings.

Does Geographic Location Matter for Radon Testing?

Although California is not generally classified as a high-radon state, certain counties have moderate to elevated radon potential. Northern California, the Sierra Nevada foothills, and parts of the Central Coast are known to have higher radon risks. However, 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 in any given location.

Because radon risk cannot be predicted by geography alone, geographic exemptions from testing are being discouraged. Standardized testing is becoming the norm for multifamily housing regardless of location. This shift reflects the understanding that radon is a widespread concern that requires proactive management across all regions and property types.

Public health agencies are working to increase awareness about radon risk. The California Department of Public Health (CDPH) offers guidance, mapping, and referrals to certified professionals, while the EPA runs national campaigns encouraging testing in all homes. California's CDPH also supports a public library program that lends free short-term digital radon monitors to residents. Despite these initiatives, radon risk remains underestimated in the state, underscoring the need for greater homeowner awareness and proactive radon management in multifamily housing.

For developers and property owners, the 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 residential development. As standards continue to tighten, proactive compliance is critical to safeguard residents, avoid legal liability, and preserve long-term asset value in the competitive multifamily housing market.