Radon Testing: What Homeowners Need to Know First
Radon testing means putting a small detector in the lowest lived-in level of your house, keeping doors and windows shut, and reading the result in picocuries per liter of air. The Environmental Protection Agency sets 4.0 pCi/L as the level at which you should fix the house, and suggests considering a fix between 2.0 and 4.0. A short term kit takes 2 to 7 days, a long term kit runs 90 days or more, and no house is safe or unsafe by reputation. Two identical homes on the same street can read very differently, so the only way to know yours is to test it.
Radon is a colorless, odorless gas from decaying uranium in soil. You cannot smell it, notice it, or tell from a musty basement whether you have it.
Updated September 2026
Sizing a basement project or a mitigation quote? Use our free Square Footage Calculator to get the exact slab area of the level you are testing, since sub-slab systems are designed around floor area, slab condition, and how many separate slabs the house has.
Why radon testing matters more than most homeowners expect
Radon decays into short lived particles that attach to dust and lodge in lung tissue. The EPA and the Surgeon General identify it as the second leading cause of lung cancer in the United States after smoking, and the leading cause among people who never smoked. That is the whole reason this is a standard part of a home inspection in many states.
The practical takeaways are simpler than the science:
- It is a soil gas, not a building material problem. It enters through the floor, not the walls or the furniture.
- It varies by house, not by neighborhood. Your foundation type, the soil under it, and how the house pulls air determine your number.
- It varies by season. Winter readings usually run higher because the house is sealed and the stack effect is strongest.
- It is fixable. A properly designed mitigation system routinely brings a high reading down to a low one, and it is not an exotic technology.
If you have ever wondered why your basement feels stuffy and you cannot place the smell, that is a separate issue covered in musty basement smell. Radon has no smell at all. The two problems often share a cause (soil air moving into the house) but they are not the same test.
How radon testing works, start to finish
Every method is doing the same thing: capturing what is in the air over a known period, then converting that to picocuries per liter.
The four kit types you will actually encounter:
| Kit type | How long it runs | How you get the result | Best for |
|---|---|---|---|
| Charcoal canister | 2 to 7 days | Mail to a lab | A fast first screen or a real estate deadline |
| Alpha track detector | 90 days to 12 months | Mail to a lab | The most honest picture of your yearly average |
| Electret ion chamber | 2 days to several months | Lab or licensed measurement pro | Professional testing, both short and long term |
| Continuous radon monitor | Continuous, hourly readings | Reads on the device | Watching how the number moves, and testing a mitigation fix |
The sequence:
- Close the house 12 hours before you start, and keep it closed the whole time.
- Place the detector correctly (see the placement rules below).
- Log the start date and time on the card. Labs reject kits with missing or impossible times.
- Leave it completely alone. No moving it, no covering it, no opening a window "just for an hour."
- Log the end time, seal the kit, and mail it the same day. A charcoal kit sitting in a hot car for a week gives a meaningless number.
Where to put the kit, and where not to
Placement is where most home tests go wrong. The goal is a spot that represents the air people actually breathe in the lowest livable level.
Put the detector:
- In the lowest level you live in or could reasonably finish. A finished basement counts. An unfinished basement you plan to finish counts. A crawl space does not.
- 20 inches to 6 feet above the floor. Radon is not a heavy layer sitting on the slab, but very low placement can read the slab surface rather than the room.
- At least 3 feet from exterior doors and windows, and 3 feet from an exterior wall.
- At least 4 inches from any other object, with air moving freely around it.
- Away from vents, fans, fireplaces, direct sun and heat. Drafts and heat both skew charcoal kits.
Do not put the detector:
- In a kitchen, bathroom, laundry room, or utility room. High humidity ruins charcoal kits and those rooms have their own exhaust fans.
- On a windowsill, a heat register, or the top of an appliance.
- In a closet, a cabinet, or behind furniture.
- In a crawl space or an attached garage.
Closed house conditions, in plain terms: exterior windows and doors stay shut except for normal in and out traffic. Whole house fans, window fans and attic fans stay off. Furnaces, air conditioners and heat recovery ventilators run normally. Do not run a fireplace or wood stove unless it is your only heat.
What the number means
| Result | What it means | What to do |
|---|---|---|
| Under 2.0 pCi/L | Low, close to typical indoor air | Retest every few years, and after major work |
| 2.0 to 3.9 pCi/L | Elevated but under the action level | The EPA suggests considering mitigation. A long term test tells you more |
| 4.0 to 7.9 pCi/L | At or above the action level | Confirm with a second test, then mitigate |
| 8.0 to 19.9 pCi/L | Well above the action level | Confirm with a short term test, then mitigate promptly |
| 20 pCi/L and up | Very high | Do a second short term test right away and mitigate as soon as you can |
A couple of numbers worth knowing. The world average outdoor level is around 0.4 pCi/L, so zero is not the target and no system reaches it. In countries using metric units, radon is measured in becquerels per cubic meter, and 4.0 pCi/L is about 148 Bq/m3. The EPA's homeowner guidance lives at epa.gov/radon if you want the primary source.
Worked example: a first test that came back high
A 1,450 square foot ranch has a full basement, half of it finished as a family room. The owner runs a charcoal kit in February.
Setup. House closed Saturday evening. Kit placed Sunday morning on a bookshelf in the finished half, 4 feet off the floor, 5 feet from the nearest window, 6 inches clear of the wall. Start time written on the card. The kit runs 48 hours, gets sealed Tuesday morning, and goes in the mail that afternoon.
Result. 6.8 pCi/L.
What happens next. A single short term test in winter is a screen, not a verdict, so the owner runs a second short term kit in the same spot. It reads 5.9 pCi/L. Two tests over 4.0 is a clear answer, and the average of 6.35 becomes the before number.
The fix. A mitigation contractor cores a hole through the 4 inch basement slab, digs a small suction pit under it, runs 3 inch or 4 inch PVC up through the rim or outside the house to above the roof line, and sets an inline fan in the attic or on the exterior. Floor cracks and the sump pit get sealed so the fan pulls from under the slab rather than from the room. A visible manometer on the pipe shows the system is running.
The proof. A post-mitigation test 24 hours to 30 days after the fan starts reads 0.9 pCi/L. That test is the part people skip, and it is the only evidence the system works.
Where the square footage mattered. The slab is roughly 1,450 square feet in one pour, which one suction point can usually serve. A house with the same total area split into three separate slabs at different levels can need more than one suction point, and that changes the design entirely. Measure the slab, not the house. Our square footage calculator handles the L-shaped and split-level cases that are easy to get wrong on paper.
Foundation type changes everything
| Foundation | How radon gets in | Testing note | Typical fix |
|---|---|---|---|
| Full basement | Slab cracks, the sump pit, the floor-to-wall joint, utility penetrations | Test in the basement, finished or not | Sub-slab depressurization |
| Slab on grade | Slab cracks, perimeter joint, plumbing penetrations | Test on the main floor | Sub-slab depressurization |
| Crawl space | Exposed soil, vents, the floor above | Test in the living space above, never in the crawl | Sub-membrane depressurization over sealed plastic |
| Mixed, such as a house with an addition | All of the above, separately | Test the lowest lived-in area of each section | Often more than one suction point |
Crawl space houses are the ones people get wrong. You do not test in the crawl, because nobody breathes there. You test the room above it. A sealed crawl space with a taped membrane and a suction point under it is the standard fix, and it overlaps heavily with the work described in crawl space encapsulation cost.
Things that change your result, honestly
- Season. Winter tends to read higher. A closed, heated house pulls harder on the soil.
- Wind and storms. A big pressure swing can move a short term reading noticeably.
- Sump pumps and drain tile. An open sump pit is a direct opening into the soil gas layer. A sealed lid with a gasket often drops readings on its own.
- Radon fans that are off. If you bought a house with a system, check the manometer before you test anything.
- New windows and air sealing. Tightening a house can raise indoor radon because there is less accidental dilution.
- Basement finishing. Covering a slab with flooring changes the paths, and a finished basement means more hours spent in the highest concentration part of the house. Test before you start the work described in cost to finish a basement.
- New foundation cracks. A new crack is a new pathway. If you are already watching one, see foundation cracks when to worry.
Radon testing during a home sale
Real estate transactions have their own rules and their own pressure.
- Use a licensed measurement professional where your state requires one. Some states do, and a DIY kit may not be accepted by the parties.
- Two detectors side by side is the standard for a transaction. Placing them 4 inches apart and comparing results catches a bad kit.
- Tamper controls matter. Professionals use continuous monitors that log hourly, so a window opened on day two shows up in the data.
- Test early in the inspection window. A 48 hour test plus lab time can eat a contingency period fast.
- Do not test in an empty house that has been open for painting. The result will not reflect how the house behaves when occupied.
- A seller's old test is not your test. Ask when it ran, in which season, who ran it, and whether anything has changed since. Then run your own.
If a mitigation system is part of the deal, get the scope in writing the same way you would for any other trade. Our guides on how to hire a contractor and contractor red flags apply directly, and the scope for a radon system should name the number of suction points, the pipe diameter and route, the fan model, whether the sump and floor cracks are sealed, whether a manometer is installed, and who pays for the post-mitigation test.
A simple testing schedule you can keep
- Test once now, short term, in the lowest lived-in level.
- If it comes back under 2.0, retest every 3 to 5 years, and any time you finish a basement, replace windows, add or change an HVAC system, or open a new foundation penetration.
- If it comes back 2.0 to 3.9, run a long term alpha track test for 90 days or more to get your real annual average before deciding.
- If it comes back 4.0 or higher, run a confirming short term test, then mitigate.
- After mitigation, test again within 30 days, then every 2 years to confirm the fan is still doing its job.
- Put the fan check on your seasonal list. The manometer takes 5 seconds to read, and it belongs on your home maintenance checklist.
Frequently asked questions
What is a safe radon level in a house?
There is no level that is completely risk free, since outdoor air itself averages around 0.4 pCi/L. The EPA action level is 4.0 pCi/L, and the agency suggests considering a fix between 2.0 and 3.9. Most well designed mitigation systems bring a house down to the 0.5 to 2.0 range, so under 2.0 is a realistic target rather than a wish.
How long does a radon test take?
A short term charcoal kit runs 2 to 7 days, plus a few days for lab processing. A long term alpha track detector runs 90 days to a year and gives a much better picture of your annual average, because it smooths out weather and season. A continuous monitor gives hourly numbers immediately and is what most professionals use.
Can I do radon testing myself or do I need a professional?
You can do it yourself for your own information, and a properly placed kit is genuinely reliable. Hire a licensed measurement professional for a real estate transaction, when your state requires certification, or when you want tamper resistant hourly data. The physical test is the same, the difference is chain of custody and documentation.
Does opening windows lower radon?
Temporarily, yes, and that is exactly why it invalidates a test. Ventilation dilutes the gas while the windows are open and the level climbs back once the house is closed. Living with the windows open all winter is not a mitigation strategy, and it is why closed house conditions are required for 12 hours before and throughout the test.
Do basement waterproofing or encapsulation fix radon?
Not by themselves, though they help. Sealing cracks, covering a sump pit and laying a taped vapor barrier all reduce the number of entry paths, and sometimes drop a borderline reading below the action level. A real fix adds active suction under the slab or the membrane so soil gas is pulled outside instead of into the house. Related reading: basement waterproofing cost.
Where should I place a radon test kit in a two story house with a basement?
In the basement, if it is finished or could reasonably be finished, at 20 inches to 6 feet above the floor, 3 feet from exterior walls and windows, and away from kitchens, bathrooms and laundry areas. If the basement is a true crawl space you will never occupy, test on the lowest floor people actually use. Bedrooms on upper floors do not need their own kit for a screening test.
How often should I retest after installing a mitigation system?
Test 24 hours to 30 days after the fan starts to confirm it worked, then every 2 years afterward. Also retest any time the fan is replaced or serviced, after major renovation, or if the manometer stops showing a pressure difference. Radon fans run continuously and do eventually wear out, usually quietly.
Does a radon problem affect my house's value?
An untested house is an unknown, and an unknown is what slows a sale. A documented test plus a working system with a post-mitigation result is usually treated as a solved problem rather than a defect. Buyers react far worse to a high reading discovered during inspection than to a system that is already installed and proven.
