Why testing is the first step

Radon is a naturally occurring radioactive gas with no color, odor, or taste. In Colorado, the state health department reports that approximately half of homes have radon levels above the EPA action level of 4.0 pCi/L, and it frames that as meaning half of Colorado homes should be mitigated to reduce lung cancer risk. That is a statewide pattern, not a prediction about your address. The Colorado Department of Public Health and Environment (CDPHE) encourages homeowners to test regardless of neighbors' results because radon levels vary from home to home.

The EPA's position is equally direct: the only way to know if radon is in your home is to test, and every home should be tested. Because the gas is invisible, no amount of familiarity with a house — its age, its foundation type, its neighborhood — substitutes for a measurement.

Short-term or long-term: two valid instruments

CDPHE describes two testing paths. Short-term tests take 48 to 120 hours to complete: the house is closed for 12 hours, then the device is activated or opened and left in place for 48 hours or more. Charcoal canisters are generally used, though electronic instruments may also be used. Long-term tests take 91 days to one year under normal living conditions, using alpha-track detectors or electronic detection instruments. CDPHE notes that long-term results give a more representative picture of true radon levels over time, because fluctuations from ambient temperature and barometric pressure are detected and factored into the final average.

You can test with an approved do-it-yourself device or hire a licensed radon measurement professional. CDPHE states plainly that it provides information and advice only and cannot test your home. EPA describes do-it-yourself kits as available online and at most home improvement and hardware stores, with some states and municipalities providing kits for free.

The closed-building condition is the part people get wrong

A test measures the air in a sealed house, so the sealing matters. CDPHE instructs residents to close all exterior windows and doors 12 hours before starting the test and keep them closed throughout. You can come and go as you normally would, but don't leave the door open for a long time. Most heating and AC units can operate as normal. The test should run a minimum of 2 days (48 hours).

Timing follows practical convenience: CDPHE says it is always a good time to test as long as you can achieve closed-building conditions, and that some people find this easier in winter.

Where to put the device

Place the test in the lowest inhabitable level of the home, such as the ground floor or basement, even if the basement is unfinished. It should sit on a surface 2 to 7 feet above the floor — a table or countertop — out of the reach of children or pets, in an open and centralized area where someone could spend a lot of time. For a real estate transaction, EPA recommends testing in the lowest area that could be modified to become a living area, such as an unfinished basement.

CDPHE's do-not list is specific: no crawl space, closet, kitchen, laundry room, or bathroom; not near exterior doors, windows, or walls; not on a granite countertop or slate-top pool table; not near air vents, fans, or direct sunlight; not where children or pets can disturb it. Do not operate a whole house fan during the test.

A worked hypothetical: two weeks in a Denver bungalow

The following scenario is illustrative, not a record of any real home or result. Suppose a homeowner with a finished basement and a first-floor living room wants a short-term test. On a Monday evening they close every exterior window and door and note the exact time — CDPHE's kit tips ask for the date and time down to the hour and minute. For this example, say the 12-hour closure begins Monday at 7:00 p.m. On Tuesday morning the kit is opened and placed on a basement table about 3 feet above the floor, away from the exterior wall and the floor vent. From Tuesday morning forward, no whole house fan runs, and doors are opened only for normal coming and going.

Say the device stays in place until Thursday morning, comfortably past the 48-hour minimum. The homeowner photographs the white barcoded sticker on the clear tray to keep the serial number for registration, then ships the kit to the lab the same day — CDPHE warns not to wait more than a day. When results come back, the number is read against official guidance rather than guessed at. That last step matters, because the result is a measurement of one period in one house, useful as a decision input and not as a medical conclusion.

Reading the number

EPA recommends taking action to fix your home if you discover radon levels above 4 picocuries per liter, and an EPA regional newsletter has stated the recommendation as four pCi/L or more. Because there is no known safe level of exposure to radon, EPA also recommends that Americans consider fixing their home for radon levels between 2 pCi/L and 4 pCi/L. CDPHE relays the EPA action level of 4.0 pCi/L as the threshold at which it says half of Colorado homes should be mitigated.

Those thresholds are policy guidance, not a diagnosis. If your result sits near a boundary, the honest next move is to consult the EPA's radon program or a certified professional about what the number means for your situation.

Retesting and record-keeping

EPA recommends that every home be tested at least every 5 years, or at least every 2 years to verify continued effectiveness of mitigation systems. CDPHE advises homeowners who already have radon mitigation systems to retest every two years to make sure the system is working properly. The two-year figure therefore appears in both contexts, but it carries a specific purpose for mitigated homes: confirming the system still performs.

Keep the basics from each test — the serial number, start and end times, and the reported result. CDPHE also notes that tenants are allowed to test their own units under Colorado law, and that as of August 7, 2023, Colorado landlords are required to disclose radon information when a tenant signs a lease. This guide does not interpret those legal requirements; the CDPHE testing and mitigation page links the relevant material.

After the result: where to go

If a short-term test comes back elevated, the next step is not a guess and not a purchase decision made on one number. Read the official guidance, decide whether a follow-up test or a professional evaluation fits, and keep a simple record of what was measured and when. Our next-steps guide covers what official sources say about interpreting results; the contractor questions page helps you prepare if you decide to talk to a licensed professional. CDPHE's January 2026 announcement also describes free test kits while supplies last and a Low-Income Radon Mitigation Assistance program with eligibility decided through an application — confirm current status with CDPHE directly rather than relying on any summary here. The most useful immediate step is the least glamorous one: schedule a test period when the house can stay closed, write down the dates, and treat the number you get as the start of a conversation with official guidance.