Radon

AIR

No safe levelhealth-based level (EPA, verbatim)

A radioactive soil gas with no taste, color, or smell, and the second-leading cause of lung cancer in the U.S.

Settled science

Stasis Home ResearchEvery claim on this page is graded against peer-reviewed health research and linked to its primary source.Updated 2026-06

Radon is a radioactive gas that rises out of the ground and collects in homes, where it is the second-leading cause of lung cancer in the U.S. and the leading cause in people who never smoked. EPA states there is no known safe level, and attributes about 21,000 lung-cancer deaths a year to it. How much collects in your home depends on the geology beneath it, mapped by EPA radon zones, so the answer is to test rather than assume. The fix, when one is needed, is sub-slab depressurization.

What it is, and where it comes from

Radon-222 is a colorless, odorless, radioactive gas formed as uranium in soil and rock slowly decays. It seeps up out of the ground and into a home through cracks in the slab, plumbing and utility penetrations, sump pits, and gaps where the slab meets the wall, pulled in by the slight vacuum a heated house creates over the cooler soil beneath it. The gas itself is mostly breathed back out. The damage comes from its short-lived decay products, solid radioactive particles that stick to household dust and lodge in the lining of the airway when inhaled. How much collects indoors is set first by the uranium in the local geology, mapped nationally by EPA radon zones from Zone 1 (highest) to Zone 3 (lowest). A tighter, more energy-efficient building envelope traps whatever the soil emits, so newer construction is not automatically lower.

Why it matters

Once those radioactive particles deposit in the airway, they emit alpha radiation a fraction of a hair's width into the surrounding tissue. That energy lands entirely on the cells lining the lung, breaking both strands of their DNA in tight clusters that are hard to repair correctly, which is the mutation pathway to lung cancer. The risk climbs steadily with exposure and the evidence shows no clean threshold below which it disappears. Risk also depends heavily on smoking: a lifelong non-smoker's lifetime lung-cancer risk roughly doubles between clean air and heavy radon, while a smoker carries about thirteen times the baseline risk on top of that, one of the largest known combined effects of two carcinogens. For a never-smoker, radon is one of the few lung-cancer risks that is measurable and fixable in the home.

EPA states there is no known safe level of exposure to radon, and that there is significant risk at levels below its 4 pCi/L action level, a level set in 1986 as a feasibility compromise rather than a point at which exposure becomes safe.US EPA, What is EPA's Action Level for Radon and What Does it Mean? (no known safe level; significant risk below 4 pCi/L; consider fixing between 2 and 4 pCi/L)
Radon is the second-leading cause of lung cancer in the U.S. and the leading cause among people who never smoked, with EPA attributing roughly 21,000 lung-cancer deaths a year to residential radon, about 2,900 of them in never-smokers.US EPA, Assessment of Risks from Radon in Homes, EPA 402-R-03-003, 2003 (~21,000 U.S. lung-cancer deaths a year, ~2,900 in never-smokers)
A pooled analysis of thirteen European case-control studies found lung-cancer risk rising in proportion to long-term home radon with no detectable safe threshold, and that the absolute risk is far higher in smokers because radon and tobacco smoke act together.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 (no detectable threshold; radon-smoking interaction)
Sub-slab depressurization is EPA's recommended fix for slab-on-grade homes and reduces indoor radon by 80 to 99 percent, while EPA states sealing alone has not been shown to lower radon significantly or consistently.US EPA, Consumer's Guide to Radon Reduction (sub-slab depressurization 80-99% reduction; sealing alone not recommended as it has not been shown to lower radon significantly or consistently)

What we grade it against

Where the health research draws the line, versus the legal limit.
Contaminant	Health-based level	Legal limit	Source (health-based)
Radon, EPA action level (pCi/L)	0EPA: no known safe level; 4 pCi/L is a 1986 feasibility level, not a safe one	4action level at which mitigation is recommended (non-enforceable guidance)	US EPA, What is EPA's Action Level for Radon
Radon, EPA 'consider fixing' (pCi/L)	0EPA states there is significant risk below 4 pCi/L	2EPA suggests considering a fix between 2 and 4 pCi/L	US EPA, What is EPA's Action Level for Radon
Radon, WHO reference level (Bq/m3)	100~2.7 pCi/L; WHO's health-based reference, anchored on a no-threshold dose-response	none set	WHO, Handbook on Indoor Radon, 2009
Radon, WHO ceiling (Bq/m3)	300~8.1 pCi/L; the maximum WHO supports where the 100 Bq/m3 reference cannot be reached	none set	WHO, Handbook on Indoor Radon, 2009

Health-based levels come from peer-reviewed research and government risk scientists working without cost constraints. Legal limits are enforceable compromises. Your report grades to the health column.

What helps

Bigger retrofits

Sub-slab depressurization (SSD)
The standard engineered fix for a slab-on-grade home. A pipe is run through the slab and a quiet continuous fan holds the soil under the foundation at slight negative pressure, venting radon above the roofline before it can enter. EPA reports an 80 to 99 percent reduction. Sealing the slab penetrations and joints is part of this install because it lets the fan hold its pressure field.
Heat- or energy-recovery ventilation (HRV/ERV)
Dilutes indoor radon with tempered outdoor air, with a median reduction near 39 percent in the peer-reviewed data. It makes sense when a home also has high CO2, VOCs, or moisture, because one unit addresses several problems at once.
For radon alone it is meaningfully weaker than SSD per dollar, so it is the right answer only as a shared fix for multiple air problems, not as the first move against radon.

Free and behavioral

Sealing cracks and penetrations alone
Caulking slab cracks, sump caps, and foundation joints is a normal part of a full SSD install.
Sealing by itself is INSUFFICIENT. EPA states it has not been shown to lower radon significantly or consistently, with roughly a 10 to 30 percent effect in isolation. A caulk-and-call-it-done fix from a seller or handyman is not a substitute for SSD.
Triage testing with a continuous monitor
A single point-in-time reading can be off by several fold because radon swings with the time of day, weather, and season. A multi-day continuous monitor captures the real pattern, including the overnight peak, and the same instrument verifies the fix after a mitigation system turns on.

Radon is the one entry here whose weight depends on where you live. Prevalence is set by geology, which EPA maps into radon zones from Zone 1, the highest predicted indoor levels, to Zone 3, the lowest. In a Zone 1 region like the Colorado Front Range, uranium-rich geology, basement-heavy housing, and tight building envelopes make radon a first-order concern. In a Zone 3 coastal county like Orange County, California, surveys put only about one home in twenty at or above the action level, concentrated in specific geological pockets. We will not talk you into a fear your geology does not support, and we will not let a low-prevalence zone talk you out of a measurement. When a home's intake profile or geology raises the prior, we send a continuous monitor; when a reading comes back elevated, the radon section earns full rigor, the overnight trace, the never-smoker risk framing, and a referral for SSD. When it comes back low, we document the negative finding and move on. The science is settled even where the prevalence is low, which is exactly why you measure rather than assume in either direction.

Common questions

Is there a safe level of radon?

No. EPA states there is no known safe level of exposure to radon, and that there is significant risk even below its 4 pCi/L action level. That action level dates to 1986 and was set as a practical point at which fixing a home is almost always cost-effective, not a line below which exposure is harmless. Read it plainly: lower is better and the risk rises steadily with exposure.

Does radon depend on where I live?

Strongly. Prevalence tracks the uranium content of the local geology, which EPA maps into radon zones, from Zone 1 with the highest predicted indoor levels to Zone 3 with the lowest. The spread is wide: a Zone 1 market can run an order of magnitude higher in the share of homes at or above the action level than a Zone 3 coastal market. Orange County, California, for example, sits in Zone 3, and surveys there found about 5 percent of homes at or above the action level, concentrated in specific geological pockets. The zone sets the odds for your area. The only way to know your own home's number is to measure it.

Why does radon cause lung cancer and not other cancers?

Because the exposure path is breathing. Radon's radioactive decay products are inhaled and stick to the lining of the airway, where the alpha radiation they emit travels only a fraction of a hair's width, so all of the energy lands on lung tissue and none reaches other organs. That radiation breaks DNA in clusters that are hard to repair, which is the pathway to lung cancer. It is also the leading cause of lung cancer in people who never smoked.

Can I just seal the cracks in my foundation to fix radon?

No, sealing alone is not enough. EPA states that by itself sealing has not been shown to lower radon significantly or consistently, with only about a 10 to 30 percent effect. The engineered fix for a slab home is sub-slab depressurization, a pipe and a quiet fan that vent soil gas above the roof and cut radon by 80 to 99 percent. Sealing is a part of that install, not a replacement for it.

How is radon measured properly?

Over time, not in a single moment. Indoor radon swings with the time of day, the weather, and the season, so a snapshot reading can be off by several fold from the true average. A continuous monitor left in the lowest occupied living level for a multi-day window captures the real pattern, including the overnight peak when ventilation is lowest. The same instrument is also what verifies a mitigation system after it is installed.