Radon is a radioactive gas from uranium in soil. It seeps through slabs, sump pits, and crawlspaces and concentrates in the lowest level where air sits still. You cannot see it or smell it. A crack in the foundation is not a radon reading, and a radon reading is not a structural defect. The only fact that matters is a number from a proper test, in picocuries per liter.

The EPA's action level is 4.0 pCi/L. That is not a bright line between safe and unsafe. EPA also says to consider fixing homes between 2 and 4. Many buyers treat 4 as the negotiation trigger because that is the number mitigators and the EPA pamphlet use. World Health Organization guidance is lower. Know which number your offer is using before you argue about it.

What it is

A soil gas, higher in some geologies (parts of the Appalachians, the upper Midwest, the Rockies) and still present, house by house, in places the county average calls low. The county map is a prior, not a test. Two neighbors can differ by a factor of five because of the slab, the sump, and the stack effect. Winter tests often read higher because the house is closed and the heating system pulls soil gas in. A 48-hour test with the windows open is not a test.

How to spot it in the listing

You usually cannot. Remarks mention radon only when a system is already installed and the seller wants credit for it, or when a prior test was low and they think it helps. "Radon mitigation system" with a photo of a PVC pipe and a fan on the exterior is a sign someone already measured a problem and piped it. Ask for the pre-mitigation number, the post-mitigation number, and whether the fan is running. A pipe with no fan, or a fan that is silent and hot, is a system in name only.

If the listing is silent, assume you need your own test in any house with a basement, a slab on grade, or a crawlspace. The research guide covers the EPA map and the county health department. Use the map to decide how fast you test, not to skip the test.

How to spot it at the showing

Look for a white PVC riser leaving the basement wall or the slab, a manometer (a U-tube) on the pipe, and a fan, often in the attic or outside. Ask to hear the fan. Look at the sump: a sealed lid with a pipe tied in is part of a system. An open sump in a basement is a classic entry path. None of this tells you the number. It tells you whether you are inheriting a machine.

The real risk

Long-term exposure increases lung cancer risk. EPA's figures are about lifetime exposure, not a weekend of showings. The buying decision is whether you will live there for years without measuring. A short-term test can miss a high house or flag a moderate one. If the result is close to 4, a longer test or a second short-term test is the honest move. Do not average a seller's test from 2019 with yours and call it science.

Mitigation, when the house is a straightforward basement, is a fan and a pipe that depressurize the soil under the slab. It is boring and it works. It does not fix a wet basement or a cracked foundation. Those are different contracts.

Insurance and lending

Standard homeowners policies do not treat radon as a covered peril and do not refuse houses for it. Lenders on conventional loans almost never require a test. FHA and VA do not require radon testing in most of the country, though a buyer can still order one. The friction is resale and, in some states, disclosure. Several states require a radon disclosure or a pamphlet. If you skip the test, you may be the seller explaining a blank disclosure later. Confirm the rule for the state before you waive.

Typical cost ranges

Typical ranges, not quotes.

Short-term test by a certified tester, or a quality device you place correctly: often $150 to $400 for a professional test. Cheap mail-order kits exist. Follow the closed-house conditions or the number is noise.
Sub-slab depressurization on a simple basement: commonly $800 to $2,500.
Complicated houses (crawlspaces, additions, very tight slabs, multiple foundations): often $2,500 to $5,000 or more.
Fan replacement years later: a few hundred dollars. Budget it the way you budget an exhaust fan, not a foundation.

Pass, negotiate, or investigate

Investigate on every basement or slab house you actually might buy. Put the test in the inspection window. Negotiate when the result is at or above the level you wrote into the offer, usually 4.0, and ask for a system installed by a certified mitigator with a post-test, or a credit in the typical range above. A credit is reasonable when you would rather hire the contractor. Pass is rare for radon alone if the seller will mitigate. Pass when the result is very high, the house has a complicated foundation, the seller refuses both a system and a credit, and you do not want the machine or the argument. A high number is a fan, not a tear-down, unless the rest of the house is already a pass. The inspection checklist should include the test order on day one of the contingency, because labs take time.

Radon: The Basement Gas You Cannot Smell and Should Still Test For