Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil and rock. It's invisible, odorless, and tasteless — the only way to detect it is through testing. The EPA estimates radon is responsible for about 21,000 lung cancer deaths every year in the United States, making it the second leading cause of lung cancer after smoking and the leading cause among non-smokers. About 2,900 of those deaths occur among people who have never smoked ([EPA](https://www.epa.gov/radon/health-risk-radon)).
The EPA's action level is 4 pCi/L (picocuries per liter) — equivalent to 150 Bq/m³. If your home tests at or above this level, the EPA recommends fixing it. But the EPA also notes that there is no known "safe" level of radon exposure, and recommends considering mitigation at levels between 2 and 4 pCi/L ([EPA](https://www.epa.gov/radon/health-risk-radon)).
WHO says the EPA action level is too high
The World Health Organization's Handbook on Indoor Radon recommends a national reference level of 100 Bq/m³ (2.7 pCi/L) — significantly lower than the EPA's 4 pCi/L threshold. If 100 Bq/m³ isn't achievable under country-specific conditions, the WHO says the reference level should not exceed 300 Bq/m³ (8.1 pCi/L) ([WHO](https://iris.who.int/bitstream/handle/10665/44149/9789241547673_eng.pdf)).
The WHO's rationale is based on the linear no-threshold model: lung cancer risk increases approximately 16% for every 100 Bq/m³ increase in long-term radon exposure, with no evidence of a safe threshold. The worldwide average indoor radon concentration is 39 Bq/m³. The EPA responded to the WHO recommendation by noting it "has been recommending for years that homeowners should also consider fixing their homes when the radon level is between 2 and 4 pCi/L" and that "there is significant risk at levels below 4 pCi/L" ([EPA](https://www.epa.gov/radon/does-2009-world-health-organizations-recommendation-lower-radon-action-level-affect-epas)).
| Organization | Action/Reference Level | Recommendation |
|---|---|---|
| EPA (United States) | 4 pCi/L (150 Bq/m³) | Fix your home; consider fixing at 2-4 pCi/L |
| WHO (Global) | 100 Bq/m³ (2.7 pCi/L) preferred | Set national reference level at 100; max 300 Bq/m³ |
| ICRP (International) | 300 Bq/m³ upper limit | Should not exceed 300 Bq/m³ for dwellings |
| Worldwide average | 39 Bq/m³ | Baseline indoor concentration |
How HVAC systems affect radon
HVAC systems can either increase or decrease indoor radon levels depending on how they interact with building pressure. The key mechanism is depressurization: when a home's air handling system creates negative pressure relative to the soil beneath the foundation, it actively pulls radon-laden soil gas inside through cracks, joints, and gaps.
Several common HVAC-related factors contribute to depressurization:
- Leaky return ducts in basements or crawlspaces:If return ducts leak in unconditioned spaces, they exhaust conditioned air, creating negative pressure that draws soil gas inside. This is one of the most overlooked radon entry pathways.
- Bathroom and kitchen exhaust fans:These blow air out of the house continuously when running, creating negative pressure that pulls radon from the soil.
- Clothes dryers and combustion appliances:Any device that exhausts air to the outside contributes to depressurization. The EPA specifically warns that mitigation contractors should check for combustion appliance backdrafting before installing depressurization-based systems ([EPA](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=10004BO9.TXT)).
- Stack effect in winter:Cold weather makes warm indoor air rise and escape through the upper portion of the house, creating negative pressure at the foundation that pulls radon in. This is why radon levels often spike in winter.
On the positive side, ASHRAE Standard 62.2 (Ventilation for Acceptable Indoor Air Quality in Residential Buildings) includes provisions that help address radon. The 2025 edition adds a requirement for ground cover over exposed earth in crawl spaces — directly targeting radon entry from bare soil — and raises minimum filtration from MERV 6 to MERV 11 ([ASHRAE](https://www.ashrae.org/technical-resources/bookstore/standards-62-1-62-2)).
HRV and ERV systems for radon.The EPA recognizes heat recovery ventilation (HRV) as a radon reduction method, but with caveats. HRVs dilute indoor radon by introducing outdoor air while recovering heat from exhausted air. They're "more effective at reducing radon when used to ventilate only the basement" and effectiveness is "limited by radon concentration and the amount of ventilation air available for dilution." Typical installation cost: $1,200-$2,500, with annual operating costs of $75-$500. The EPA notes HRVs can "significantly increase heating and cooling costs" and should run continuously for radon control. HRVs are a secondary approach — sub-slab depressurization is the primary method ([EPA](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=10004BO9.TXT)).
Mitigation methods ranked by effectiveness
The EPA's Consumer's Guide to Radon Reduction details multiple mitigation techniques. Radon reduction systems can reduce levels by up to 99%, with most homes reducible to 2 pCi/L or below. The average contractor installation cost is about $1,200 ([EPA](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=10004BO9.TXT)):
| Technique | Radon Reduction | Install Cost | Annual Operating Cost | Best For |
|---|---|---|---|---|
| Sub-slab depressurization (active) | 50-99% | $800-$2,500 | $50-$200 | Basement/slab homes — most common & reliable |
| Sub-membrane depressurization | 50-99% | $1,000-$2,500 | $50-$250 | Crawlspace homes with exposed earth |
| Drain-tile suction | 50-99% | $800-$1,700 | $50-$200 | Homes with existing drain tile loops |
| Sump hole suction | 50-99% | $800-$2,500 | $50-$250 | Homes with sump pump in basement |
| Block wall suction | 50-99% | $1,500-$3,000 | $100-$400 | Hollow block foundation walls |
| Passive sub-slab suction | 30-70% | $550-$2,250 | Energy penalties only | New construction with radon-resistant features |
| House/basement pressurization | 50-99% | $500-$1,500 | $150-$500 | Only after other methods insufficient |
| Heat recovery ventilation (HRV) | Variable | $1,200-$2,500 | $75-$500 | Secondary method, basement-only ventilation |
| Sealing alone | Not reliable | $100-$2,000 | None | Not recommended as sole method by EPA |
The EPA is explicit that sealing cracks alone "has not been shown to lower radon levels significantly or consistently" because "it is difficult to identify and permanently seal all locations where radon enters" and "normal settling of the house can open new entry routes." Sealing is recommended as a supplement to other methods, not a standalone solution ([EPA](https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=10004BO9.TXT)).
Not an endorsement.References to EPA mitigation techniques, WHO reference levels, and ASHRAE ventilation standards are informational only. This article does not recommend specific radon mitigation products, brands, or contractors. HVAC Zone Inc is brand-neutral. Radon mitigation should be performed by a certified radon professional. Testing is the only way to know your radon level — contact your state radon office or the EPA for certified testing resources.
Testing: the only way to know
The EPA recommends testing all homes for radon — including new homes, homes with radon-resistant construction features, and homes in all radon zones. Testing can be done with short-term tests (2-7 days, useful for quick screening) or long-term tests (90+ days, more representative of year-round exposure). The EPA recommends using the results of one long-term test or the average of two short-term tests to determine whether mitigation is needed at 4 pCi/L or higher ([EPA](https://www.epa.gov/radon)).
Radon levels can vary significantly between neighboring homes, so EPA radon zone maps are a starting point — not a substitute for testing. New Jersey, where HVAC Zone operates, has significant Zone 1 (highest radon potential) areas in the northwest counties and Zone 2 areas throughout the rest of the state.
The bottom line
Radon is the second leading cause of lung cancer, killing about 21,000 Americans per year. The EPA's 4 pCi/L action level is higher than the WHO's recommended 100 Bq/m³ (2.7 pCi/L), and there is no known safe level. HVAC systems can worsen radon through depressurization from leaky return ducts, exhaust fans, and the stack effect — or help mitigate it through balanced ventilation and HRV systems in basements. The primary mitigation method is sub-slab depressurization: a PVC pipe and fan system that creates a vacuum beneath the foundation, venting radon outdoors before it enters. It costs $800-$2,500 installed, reduces radon 50-99%, and is effective in most homes. Every home should be tested — testing is simple, inexpensive, and the only way to know if radon is a problem.
Concerned about radon in your home? Request a consultation — we can help you understand how your HVAC system affects indoor pressure, evaluate whether duct leakage or ventilation imbalances are contributing to radon entry, and connect you with certified radon testing and mitigation resources.