Radon Testing During a Home Inspection: What Buyers Need to Know

Radon is the second leading cause of lung cancer in the United States, according to the EPA. It is colorless, odorless, and has no taste. You cannot detect it without a test. If that is not sufficient reason to add radon testing to your home inspection, consider that mitigation — if levels come back above the action threshold — costs $1,000 to $2,000. That is a small number relative to the financial stakes of a home purchase and a meaningful number relative to the health stakes of the alternative.

Here is what the test measures, what the results mean, and what you do if the numbers require action.

How Radon Gets Into a Home

Radon forms naturally from the radioactive decay of uranium in soil and rock. It seeps upward through soil and enters homes through cracks in concrete slabs, gaps around service penetrations, construction joints, and porous block foundation walls. It accumulates in enclosed spaces — particularly basements, crawl spaces, and lower levels — because it is heavier than air.

Radon concentrations vary significantly by geography and by house. Homes in the same neighborhood can measure very differently based on local soil composition, foundation type, and construction quality. The only way to know your home's radon level is to measure it. Geographic radon zone maps (the EPA publishes one at the county level) identify high-risk areas, but buyers in low-risk zones have found elevated radon, and buyers in high-risk zones sometimes find acceptable levels. Testing is the only reliable answer.

High-risk geological zones include areas with granite, shale, or phosphate deposits — the Northeast, Mid-Atlantic, Midwest, and Rocky Mountain states have the highest prevalence. Colorado, Pennsylvania, Minnesota, Wisconsin, Iowa, and Montana have significant radon problems. However, elevated levels have been found in all 50 states.

Two Types of Radon Tests

Short-term tests are the most common option during a home inspection contingency. An activated charcoal canister is placed in the lowest livable level of the home (usually the basement) and left in place for 48 to 96 hours with windows and doors closed. The canister is then sent to a lab for analysis. Results come back within a few days. Short-term tests are fast enough to fit within a typical 7- to 14-day inspection contingency window. Cost: $150 to $350 when added to an inspection package.

Long-term tests use an alpha track detector left in place for 90 days to a year. They provide a more accurate picture of average annual exposure but are not practical during a transaction. Buyers sometimes conduct a long-term test post-closing for confirmation after a short-term test showed elevated but sub-actionable levels.

Some buyers purchase a continuous electronic radon monitor ($150 to $250 at hardware stores) for ongoing post-purchase monitoring. These devices provide rolling hourly averages and alert when levels spike.

What the Results Mean

The EPA measures radon in picocuries per liter of air (pCi/L). Here is how to read your results:

Below 2.0 pCi/L: The EPA's target level. Acceptable. No action needed.

2.0 to 3.9 pCi/L: Elevated but below the EPA action threshold. The EPA recommends fixing homes at this level if practical. At 2.0 pCi/L, your estimated lung cancer risk from radon is roughly equivalent to 100 chest X-rays per year of exposure. Many buyers choose to mitigate at this level.

4.0 pCi/L or above: EPA action level. The EPA recommends mitigation. At 4.0 pCi/L, the EPA considers the risk roughly comparable to smoking half a pack of cigarettes a day. At this level, mitigation is not optional for most buyers — it becomes a negotiation item.

Above 8.0 to 10.0 pCi/L: Urgent. The EPA reports that at these concentrations, the risk approaches or exceeds that of a full pack per day of smoking. The home requires professional mitigation before occupancy.

The average indoor radon level in US homes is approximately 1.3 pCi/L.

The Mitigation System: How It Works

If your test comes back at or above 4.0 pCi/L, the standard fix is an Active Soil Depressurization (ASD) system, also called sub-slab depressurization. It is not an exotic or experimental solution — it is a well-understood mechanical system with a high success rate.

Here is how an ASD system works:

1. A 3- to 4-inch hole is drilled through the concrete slab (or into the crawl space sub-grade).
2. A PVC pipe is sealed into the hole and routed vertically through the home or up an exterior wall to above the roofline.
3. A quiet, continuously running inline exhaust fan is installed in the attic or at an exterior location to draw air from beneath the slab.
4. The fan creates negative pressure below the slab, pulling radon gas from the soil and exhausting it above the roofline — away from windows and doors — before it can enter the living space.

A single suction point handles most homes. Properties with very high levels or complex foundation layouts may need multiple suction points. The system runs continuously using roughly the same electricity as a light bulb.

Installation typically costs $1,000 to $2,000. After installation, the EPA recommends re-testing to verify that levels have dropped below 2.0 pCi/L, and then re-testing every two years.

How to Handle Radon in Negotiations

If the short-term test returns below 4.0 pCi/L, radon is not typically raised in negotiations. Document the result and move on.

If the test returns at 4.0 pCi/L or above, you have two options:

Option 1 — Request seller-installed mitigation before closing. The seller hires a certified radon mitigator, the system is installed, and a follow-up test confirms results below the action threshold. This approach works but carries the same risk as all seller-performed repairs: the seller has incentive to use the cheapest contractor, and installation quality varies.

Option 2 — Request a closing credit equal to the cost of installation (typically $1,000 to $2,000, adjusted for your regional market). You select the mitigator, control the quality, and the system installation becomes your first post-closing project. Given the relatively modest cost, this is often the cleaner path.

In states with high radon prevalence — Colorado, Pennsylvania, Iowa, Minnesota — mitigation systems are extremely common, and their presence on a home is not a stigma. Many buyers of older homes in those states budget for mitigation as a matter of course, similar to budgeting for a water heater replacement in a 10-year-old home.

Should You Test Even if the House Has No Basement?

Yes. Slab-on-grade homes can have elevated radon because the slab sits directly on soil. Radon enters through construction joints, utility penetrations, and micro-cracks in the concrete. Slab homes sometimes measure lower than basement homes simply because the lower level is above-grade and more air-exchanged — but this is not always the case. Test regardless of foundation type.

Crawl space foundations have their own dynamic. If the crawl space is vented, radon may dissipate into the crawl space rather than into the living area. If it is unvented (an encapsulated crawl space), radon can accumulate and migrate upward. Always test.

The Home Inspection Checklist & Red Flag Guide includes a specialty inspection decision framework — covering radon, sewer scopes, mold, and structural evaluations — so you can quickly determine which add-on inspections are warranted for your specific property before the contingency window closes.

For Canadian buyers: Health Canada's action level is 200 Bq/m³ (roughly equivalent to 5.4 pCi/L). Provincial guidance varies — Alberta and Saskatchewan have high radon prevalence. The Canadian National Radon Proficiency Program (NRPP) certifies radon professionals. For UK buyers: Public Health England's action level is 200 Bq/m³. Wales, Devon, Somerset, and areas of England over granite geology have elevated radon concentrations. Check the UK Radon Association's maps for local risk. For Australian buyers: Indoor radon is generally lower than in North America, but the ARPANSA action level is 200 Bq/m³ in occupied spaces.