How Radon Testing Works in a Colorado Home Inspection
On the Front Range, radon is the add-on buyers order more than any other during the inspection period, and geography is the reason. The EPA’s Map of Radon Zones places Boulder, Larimer, Jefferson, Denver, Douglas, El Paso, and most of the surrounding counties in Zone 1, the highest of three predicted-average bands. That does not guarantee your specific house is high, but it means the test earns its place on the order form. This page covers how the test slots into a home inspection, what the number means, how to keep a seller from skewing it, and how to act on the result before your objection deadline closes.
Why radon isn’t part of the standard inspection
A general home inspection follows the ASHI or InterNACHI Standards of Practice: the inspector visually assesses structure, roof, electrical, plumbing, HVAC, and finishes in a single visit. Radon falls outside that scope for a practical reason. Measuring it takes a minimum 48-hour monitoring window, a separate credential, and lab or instrument analysis the visual walkthrough can’t provide. So it’s sold as an add-on. Expect roughly $125 to $200 for a passive 48-hour test and $175 to $275 for a continuous monitor deployment; our breakdown of what radon testing costs gets into what drives the spread.
The workflow is straightforward. The inspector places the device in the lowest livable level at the start of the general inspection, finishes the walkthrough, and either returns after 48 to 96 hours or sends a courier to retrieve it. A passive canister ships to an accredited lab; a continuous monitor produces its report on pickup. The radon result usually arrives as a separate or supplemental document a day or two after the main report.
What the pCi/L number actually means
Radon is a colorless, odorless radioactive gas from the decay of uranium in soil and rock. It seeps into homes through foundation cracks, sump pits, crawlspace floors, and sometimes well water. The EPA reports it in picocuries per liter (pCi/L) and sets its action level at 4 pCi/L (150 Bq/m³). At or above that, the agency recommends fixing the home. Between 2 and 4 pCi/L it recommends considering a fix, because there is no known safe level of exposure. The 2009 World Health Organization reference level is tighter at 2.7 pCi/L.
The stakes are not abstract. The EPA estimates radon causes about 21,000 lung-cancer deaths a year in the U.S., making it the second-leading cause after smoking and the leading cause among people who never smoked, per the CDC and the American Lung Association. For a longer look at where a reading falls, see what counts as a safe radon level.
Passive canister or continuous monitor?
Two device types dominate inspection-period testing, and the difference matters more in a transaction than it does for casual homeowner testing.
| Feature | Passive charcoal canister | Continuous radon monitor (CRM) |
|---|---|---|
| Output | Single averaged pCi/L for the window | Hourly readings plus the average |
| Analysis | Mailed to accredited lab | On-device, report at pickup |
| Tamper visibility | None — can’t show an open window | Chart reveals abrupt drops |
| Typical add-on cost | $125–$175 | $175–$275 |
A canister gives you one number. A continuous monitor gives you a timeline, which is why it carries more weight when money is on the line: if the readings crash mid-test because a basement window was opened, the chart shows it. For a contested result or a jittery seller, the monitor is worth the extra cost.
The closed-house rules that make a result count
A short-term test is only valid under closed-house conditions, and this is the protocol element sellers most often break, usually not maliciously. The EPA’s short-term procedure calls for:
- Placement in the lowest livable area, at least 20 inches off the floor
- Away from drafts, exterior walls, and direct sunlight or heat
- Windows and exterior doors closed for 12 hours before the test starts and throughout
- HVAC running normally; no whole-house or attic fans
- No moving or covering the device
An open window during the test dilutes indoor air and pulls the reading artificially low, which is exactly the direction a seller benefits from. Placement mistakes matter too; our guide on where to place a radon test covers the spots that invalidate a result. If you have reason to think conditions weren’t held, ask for a re-test with a continuous monitor rather than arguing over a canister number you can’t audit.
Reading the result before your objection deadline
Colorado real estate contracts run on tight inspection-objection windows, so read the result the day it lands:
- Below 2 pCi/L: Low. No action typically needed.
- 2 to 4 pCi/L: The EPA suggests considering a fix. Many buyers treat this as a credit-to-consider item, not a hard objection.
- 4 pCi/L or higher: The EPA recommends mitigation. Buyers commonly request a seller-installed system before closing or negotiate a cash credit to install one after.
A radon mitigation system on a Colorado single-family home generally runs $1,200 to $2,500, so a 6 pCi/L reading gives you a concrete, defensible ask: system or credit. It is not a reason to walk. Mitigation is a mature, reliable fix; our overview of what radon mitigation involves explains how a sub-slab depressurization system works, and the broader radon testing guide covers homeowner testing beyond the transaction.
What an elevated reading does and doesn’t tell you
A high result does not mean the house is unsafe to occupy or that the neighborhood tests high. Radon entry is home-specific, driven by foundation type, the soil directly beneath the slab, and how the house is sealed and ventilated; two houses on the same block can read very differently. A 48-hour test is also a snapshot. Front Range levels usually climb in winter, when homes stay closed and the gas concentrates, so a July test can understate the heating-season number. After an elevated short-term result, the EPA recommends confirming with either a long-term (90-day-plus) alpha-track test or a multi-week continuous monitor.
Who is qualified to run the test in Colorado
Colorado is one of the few states that licenses radon professionals. Under HB21-1195, radon measurement and mitigation services have required a state license through the Department of Regulatory Agencies since July 1, 2022, and licensure is built on National Radon Proficiency Program (NRPP) or National Radon Safety Board (NRSB) certification. Ask your inspector for their current radon credential and confirm the lab or device is accredited. A number produced outside that system is just a number; it carries little weight in negotiation. The state Colorado radon program at CDPHE is the reference for local requirements and test-kit resources.
After mitigation: verify it worked
If a system goes in as a result of the inspection, don’t take performance on faith. The EPA recommends a post-mitigation test no sooner than 24 hours and within 30 days of activation; most mitigation contractors bundle one in. A correctly installed sub-slab system typically drops basement radon below 2 pCi/L, often below 1. Longer term, systems fail quietly — a stalled fan, a blocked vent, a settled slab — so a small consumer continuous monitor or a re-test every two years is cheap insurance against a slow rise you’d never smell or feel.
Frequently asked questions
How long does the radon test add to the inspection timeline?
The device sits for at least 48 hours, so plan for the result to arrive two to four days after the inspector’s visit. Order it early in your inspection window so the number is in hand before the objection deadline.
Can I use my own home radon kit instead of the inspector’s test?
You can, but in a transaction a hardware-store kit carries less weight than a test deployed and documented by a Colorado-licensed professional under verified closed-house conditions. For negotiation, the chain of custody and accreditation are part of what you’re paying for.
Does a passing radon test mean I never need to test again?
No. Levels shift with season, soil moisture, and any renovation that touches the foundation, basement, or HVAC. The EPA recommends re-testing every two years and after major work, whether or not you ever mitigate.
Should an elevated result make me terminate the contract?
Rarely. Mitigation is predictable and inexpensive relative to the purchase. The standard move is to request a seller-funded system or a credit and proceed, not to walk away.
This page reflects EPA, CDC, and Colorado (CDPHE/DORA) guidance current as of 2026; consult a licensed radon professional before any mitigation decision. Front Range buyers who want a vetted, licensed inspector or help reading a result before the deadline can reach us through our contact page.