5.2 Interpreting Results & Action Recommendations

5.2 Interpreting Results & Action Recommendations

Translating raw radon concentration numbers into clear, standardized recommendations is one of the core responsibilities of a Radon Measurement Professional (RMP). Clients rely on RMPs to evaluate health risks, explain regulatory benchmarks, and outline concrete next steps. Recommendations must adhere strictly to guidance set forth by the U.S. Environmental Protection Agency (EPA) and ANSI/AARST standards, ensuring consistent consumer guidance across all testing scenarios.

Action Level Framework and Health Benchmarks

The U.S. EPA has established 4.0 picocuries per liter (pCi/L) as the national Action Level for indoor radon gas. It is critical to communicate to clients that 4.0 pCi/L is not a safe threshold below which no risk exists; rather, it is a technology-based and health-based regulatory benchmark where mitigation is strongly advised.

In 2009, the World Health Organization (WHO) established a lower global reference level of 2.7 pCi/L (100 Bq/m³), highlighting that significant health risks—specifically radon-induced lung cancer—persist at levels between 2.0 and 4.0 pCi/L. The average outdoor radon level in the United States is approximately 0.4 pCi/L, while the average indoor level is estimated at 1.3 pCi/L.

Standardized Result Tiers and Client Recommendations

Tier 1: Results Less Than 2.0 pCi/L (< 2.0 pCi/L)

• Interpretation: The measured radon concentration is low and well below the EPA Action Level.
• Action Recommendation: No immediate radon mitigation is required.
• Retest Guidance: Recommend retesting the building every 2 years to account for structural settling, ground movement, or changes in living habits. Immediate retesting is recommended if:
  1. Major structural renovations or additions occur.
  2. HVAC equipment, ductwork, or exhaust ventilation systems are replaced or substantially modified.
  3. Unoccupied lower levels (e.g., an unfinished basement) are finished or converted into living spaces.

Tier 2: Results Between 2.0 pCi/L and 4.0 pCi/L (2.0 – 3.9 pCi/L)

• Interpretation: The radon level is below the EPA Action Level of 4.0 pCi/L, but exceeds ambient outdoor concentrations and approaches the WHO reference level (2.7 pCi/L).
• Action Recommendation: Inform the client that they should consider fixing the building. While mitigation is not mandatory under standard real estate guidelines, reducing levels below 2.0 pCi/L significantly lowers long-term lung cancer risk.
• Risk Modifiers: Emphasize that mitigation is strongly encouraged if occupants smoke cigarettes, have young children, or spend extended daily hours in ground-contact spaces.
• Retest Guidance: Re-evaluate with a long-term test (90 to 365 days) to determine the true annual average, or retest with a short-term test within 1 to 2 years.

Tier 3: Results Equal to or Greater Than 4.0 pCi/L (≥ 4.0 pCi/L)

• Interpretation: The radon concentration meets or exceeds the EPA Action Level of 4.0 pCi/L, posing an elevated lifetime risk of lung cancer.
• Action Recommendation: Issue a direct and unambiguous written recommendation to fix the building. Urge the client to hire a certified Radon Mitigation Professional (RMP/RMT) to design and install an active soil depressurization (ASD) system or other appropriate engineering controls.
• Timeframe for Action:
  • For initial short-term tests between 4.0 and 8.0 pCi/L: Perform a follow-up test (either short-term or long-term) to confirm average levels prior to mitigation, OR proceed directly to mitigation if in a real estate transaction.
  • For test results exceeding 8.0 pCi/L (or during real estate sales): Recommend immediate mitigation without delay.

Post-Mitigation Verification Testing Rules

When a radon mitigation system is installed, a post-mitigation clearance test is mandatory to confirm system effectiveness. RMPs must enforce specific timing and operational rules for verification:

1. Timing Window: Post-mitigation testing must be conducted no earlier than 24 hours after the active mitigation system has been powered on and operational, and no later than 30 days post-installation. The 24-hour waiting period ensures that soil gas dynamics reach equilibrium and radon levels stabilize.
2. Independent Testing: Ideally, verification testing should be conducted by an independent certified RMP who was not involved in the system installation, avoiding conflicts of interest.
3. Operational State: The mitigation fan must operate continuously during the entire test window. Closed-building protocols remain fully applicable.

Retest Frequency and Ongoing Maintenance Requirements

Radon entry rates vary over time due to foundation cracking, soil settling, seasonal water table fluctuations, and mechanical system degradation.

• All Unmitigated Homes: Retest every 2 years, even if previous tests yielded low results.
• Mitigated Homes: Retest every 2 years to verify that the active mitigation system, fan performance, and suction pit seals remain functional.

Decision Matrix for Radon Result Recommendations