When is the best time for radon testing in Denver?
Late fall and winter, when your windows stay shut and your furnace runs, give the most telling radon results for a Denver home. A short term kit costs about $20 and takes two to four days. If your result is 4 pCi/L or higher, the EPA recommends fixing your home, and the Colorado health department says a mitigation system here usually runs about $1,000 to $2,000.
Radon testing in Denver is one of the cheapest, most useful things you can do for your house this winter. Radon is an invisible, odorless gas that seeps up from the soil, and the Colorado Department of Public Health and Environment (CDPHE) says it shows up at elevated levels in one out of every two Colorado homes. That includes bungalows in Platt Park, ranch homes in Hampden and new builds in Centennial alike. Your neighbor’s result tells you nothing about yours, so the only way to know is to test.
This guide covers what radon is, why Colorado homes run high, why the cold months are the right time to test, how to read a result, what a fix costs, and how radon fits into buying or selling a home in South Denver.
What Is Radon and Why Do So Many Colorado Homes Test High?
Radon is a radioactive gas that forms naturally as uranium in soil and rock breaks down. Outdoors it thins out quickly. Indoors it can collect, because a house pulls air up from the ground through cracks in the slab, gaps around pipes, sump pits and crawl spaces.
Colorado’s geology makes this common. In a January 2025 press release, CDPHE said nearly half of homes in the state have radon above the EPA action level, that Colorado ranks 45th among all states for the share of test results above that level, and that more than 500 lung cancer deaths in Colorado each year are attributed to radon. The agency compared living at the state’s average radon level to having about 200 chest X rays a year.
Nationally, the EPA’s radon health page (updated June 2026) calls radon the number one cause of lung cancer among people who have never smoked and the second leading cause overall, responsible for about 21,000 lung cancer deaths every year in the United States.
Age and style of house do not predict radon. Older Denver homes with stone or unfinished basements can test high, and so can brand new homes with tight construction. Two houses on the same block in Washington Park can have very different numbers.
Why Late Fall and Winter Are the Best Time for Radon Testing in Denver
A radon test is only as good as the conditions it runs in. CDPHE’s guide to testing your home says to close all exterior windows and doors 12 hours before you start a short term test and keep them closed the whole time. That is easy in December and hard in June.
Winter helps in a second way. When the furnace runs and warm air rises and leaks out the top of the house, the lower levels pull in replacement air, some of it from the soil underneath. Closed windows plus a running furnace make a reasonable picture of what your family breathes during the months you spend the most time indoors. A summer test with windows open and a swamp cooler running can read lower than what you live with in January.
January is National Radon Action Month, and CDPHE usually pushes testing hard then. In January 2026, KKCO 11 News reported that CDPHE was giving away free radon test kits while supplies lasted. If you want to test now rather than wait, you do not need to hold out for a giveaway; a low cost kit works the same way.
Testing pairs well with the rest of your cold weather list. Our guide on how to winterize your home in Denver covers sprinkler blowouts, tree care and the sidewalk snow rule, and a radon kit in the basement is a natural add on to that weekend.
Short Term vs Long Term Radon Tests: Which One Should You Use?
There are two kinds of tests, and both have a place.
| Test type | How long it runs | Best for |
|---|---|---|
| Short term | 48 to 120 hours, with the house closed for 12 hours first | A quick first read, and home purchases on a deadline |
| Long term | 91 days to one year, under normal living conditions | The truest picture of your yearly average before you spend money on a fix |
Those time frames come from CDPHE’s testing guide. The state points residents to low cost kits sold through National Radon Program Services, a program run under an agreement between the EPA and Kansas State University. As of this writing, its short term kit costs $20 and its long term kit costs $30, lab analysis included. CDPHE also notes that do it yourself kits sell at home improvement stores for about $10 to $50.
How to run a good test
- Place the kit on the lowest level you live on or could finish, such as a basement, even an unfinished one.
- Follow the placement directions that come with your kit, and leave it undisturbed for the full test.
- Close windows and outside doors 12 hours before you start and keep them closed, apart from normal coming and going.
- Run your furnace normally. Skip whole house fans and window units during the test.
- Mail the kit to the lab promptly when the test ends, following the kit’s instructions.
A simple plan: start with a short term test this winter. If it comes back high, run a second short term test or a long term test to confirm before you hire anyone.
What Do Your Radon Results Mean?
Radon is measured in picocuries per liter of air, written pCi/L. The EPA’s action level page (updated October 2026) gives two lines to remember:
- 4 pCi/L or more: the EPA recommends fixing your home.
- Between 2 and 4 pCi/L: the EPA recommends that you consider fixing it.
One test is a snapshot. Levels move with weather, season and how the house is used. That is why a high short term number is worth confirming with a second test, and why a long term test is the better basis for a decision when time allows.
How Radon Mitigation Works and What It Costs in Colorado
The good news is that radon is very fixable. CDPHE says the most effective method is a depressurization system under the slab, or under a sealed plastic membrane if you have a crawl space. A contractor drills a hole through the basement floor, runs a pipe from below the slab up and out of the house, and installs a fan that pulls soil gas from under the house and vents it outside before it can get inside. Sealing major cracks and openings helps the system work.
On cost, CDPHE’s testing guide says a mitigation system in Colorado usually costs about $1,000 to $2,000 unless the house has difficult design problems. A home with a mix of foundations, such as a basement beside a crawl space, can fall into that harder category. After the system goes in, test again to confirm the level dropped, and retest from time to time so you know the fan is still doing its job.
Choosing a contractor
CDPHE keeps a list of radon contractors who are certified by the National Radon Proficiency Program or the National Radon Safety Board and licensed by the Colorado Department of Regulatory Agencies. You can check a license through the state’s radon professionals licensing page. CDPHE is clear that it does not recommend or endorse any contractor on its list, so get more than one bid and ask each company how it will confirm the results.
Help for income qualified homeowners
CDPHE runs a Low Income Radon Mitigation Assistance program for owner occupied homes that test high. A January 2024 update from the state’s environmental public health tracking team said the program had received 1,083 applications and mitigated 375 homes by that point, and that CDPHE received a significant increase in funding in 2023. Income limits and award amounts are spelled out in the application instructions on the CDPHE page, so check there for the current rules before you apply.
Radon When You Buy or Sell a Home in South Denver
Radon is a standard question in a South Denver home purchase, and Colorado law sets some ground rules.
What sellers must disclose
Under Colorado’s radon disclosure law for home sales, a buyer of a home has the right to know whether it has been tested for radon. The purchase contract must state in bold type that CDPHE strongly recommends all home buyers have an indoor radon test before buying. Sellers must disclose what they know: whether tests were done, the most recent results, any mitigation, and whether a system is installed. They must also hand over the current CDPHE radon brochure. The law does not require a seller to test; it covers only what the seller knows.
What buyers can do
Because the seller may never have tested, the simplest step is to add a short term radon test to your home inspection during the inspection period in your contract. The test needs the house closed for 12 hours first and at least 48 hours to run, so schedule it early, especially on a tight timeline. If the result comes back at or above 4 pCi/L, common options include:
- Asking the seller to install a mitigation system by a licensed contractor before closing, with a test afterward showing the level dropped.
- Asking for a price reduction or a credit at closing so you can choose your own contractor.
- Walking away under the inspection objection terms of your contract, if the seller will not agree.
A house that already has a working system is a plus. Ask for the install paperwork and the most recent test, and check that the fan is running. If you are shopping in a specific neighborhood, our buyer guides for buying a home in Platt Park, buying a home in Washington Park and buying a home in Englewood cover the rest of the inspection and offer process.
Advice for sellers
If you plan to list in the spring, testing this winter gives you time to fix a problem on your own schedule instead of under a buyer’s deadline. A recent clean test, or a mitigation system with a passing follow up test, answers the question before it comes up in negotiations.
Renters
CDPHE notes that since August 7, 2023, Colorado landlords must give tenants radon information when they sign a lease, including any known test results and mitigation, plus the CDPHE brochure. Tenants may also test their own units.
Frequently Asked Questions
How much does radon testing in Denver cost?
A short term kit from National Radon Program Services, the source CDPHE links to, costs $20 including lab analysis, and a long term kit costs $30. CDPHE says kits at home improvement stores run about $10 to $50.
What radon level should worry me?
The EPA recommends fixing your home at 4 pCi/L or more and suggests considering a fix between 2 and 4 pCi/L. Confirm a high short term result with a second test, ideally a long term test, before you pay for mitigation.
How much does radon mitigation cost in Colorado?
CDPHE says a mitigation system in Colorado usually costs about $1,000 to $2,000, unless the house has difficult design problems. Income qualified homeowners can apply to CDPHE’s Low Income Radon Mitigation Assistance program for help.
Does a Colorado seller have to test for radon?
No. Colorado law requires sellers to disclose any radon tests, results and mitigation they know about and to give buyers the CDPHE radon brochure, but it does not require them to test. That is why ordering your own test during the inspection period is the surest way to know.
Can I test for radon in summer?
You can, as long as you keep windows and outside doors closed for 12 hours before and during a short term test. That is harder in Denver summers, which is why late fall and winter, with the furnace running and the house closed up, usually give the more useful reading.
Sources: Colorado Department of Public Health and Environment radon testing guide, radon program and Low Income Radon Mitigation Assistance pages, and January 2025 press release; Colorado Environmental Public Health Tracking update, January 2024; EPA radon pages updated June and October 2026; National Radon Program Services test kit page; the Colorado statutes on radon disclosure in home sales and leases; KKCO 11 News, January 6, 2026.
