Montana Radon Guide for Helena Homeowners (2026)

Updated October 23, 2026. This guide is general information, not legal or engineering advice. Rules change, so confirm with the agency listed before you start work.

Quick reference

Topic	What to know	Source
Local test data	Lewis and Clark County: 52.1% of 1,034 tests (1997 data) and 434 of 836 tests (2005 data) at or above 4 pCi/L	MSU radon report for Montana DEQ
County average	13 pCi/L, per the Montana State Radon Office	Lewis and Clark Public Health newsletter (2012)
EPA zone	Zone 1 (highest potential); EPA says test every home regardless of zone	EPA map of radon zones
Statewide	About half of Montana homes tested are at or above 4 pCi/L	Montana DEQ (January 2026)
Action level	Mitigate at 4 pCi/L or higher; test twice before acting	Montana DEQ
Mitigator licensing	None; DEQ lists NRPP/NRSB-certified providers but doesn't certify them	Montana DEQ
Contractor registration	Construction contractors with employees must register; verify at (406) 444-7734	Montana Dept. of Labor & Industry
Home sales	Radon disclosure statement plus any known test results (MCA 75-3-606)	Montana Code Annotated
Retesting	Every 2 to 5 years; at least every 2 years with a system	Montana DEQ
State help	DEQ Radon Control Program hotline 1-800-546-0483	Montana DEQ

How much radon do Helena-area homes have?

A lot of them have more than the EPA's action level. A Montana State University report prepared for the state compiled radon results by county. For Lewis and Clark County, its 1997 data covered 1,034 tests with a mean of 7.9 pCi/L, 52.1 percent at or above 4 pCi/L and a top result of 115.1 pCi/L. Its 2005 data covered 836 tests, with 434 (51.9 percent) at or above 4 pCi/L; the report flags the 2005 maximum of 2,574 pCi/L as an assumed sampling error, which also inflates that year's average. These are tests homeowners chose to run, not a random survey, but the share of high results is consistent across both data sets.

In January 2012, the Lewis and Clark Public Health newsletter reported an average indoor radon level of 13 pCi/L for the county, citing the Montana State Radon Office. Statewide, Montana DEQ said in January 2026 that about half of Montana homes tested are at or above 4 pCi/L and that the state's geology makes it prone to elevated radon.

Why is radon common here?

DEQ says geology typically determines the radon concentrations in an area. The Boulder Batholith, a body of granite (quartz monzonite) roughly 75 miles long, lies between Butte and Helena, and a 1956 study published by the U.S. Geological Survey described uranium in veins in its northern part. Uranium is the starting point of the decay chain that produces radon. Geology sets the background, but it can't predict a single house.

What does the EPA zone map mean for my house?

Only so much. The EPA's 1993 Map of Radon Zones places Lewis and Clark County in Zone 1, the category with a predicted average indoor screening level above 4 pCi/L. The EPA says the map was made to help governments target resources, that it should not be used to decide whether an individual home needs testing, and that all homes should be tested regardless of zone. DEQ adds that elevated homes have been found in all three zones.

How and when should I test?

Test the lowest level of the house that is suitable for living in, with the house closed up. If that level isn't used much, DEQ suggests testing a higher-use room too. If a result is elevated, DEQ says to test twice before acting. After a system goes in, test within the first month but no sooner than 24 hours after it's switched on, use an independent tester rather than the installer, and retest at least every two years. DEQ encourages homes without a system to test every two to five years. DEQ's Radon Control Program, created by the Montana Legislature through the 1993 Radon Control Act, runs a toll-free hotline and has offered discounted mail-in kits during January's National Radon Action Month while supplies last.

What about radon in well water?

If your home is on a private well, DEQ notes that radon in water is released into indoor air during showering, dishwashing and cooking, at roughly 1 pCi/L of air for every 10,000 pCi/L in the water. DEQ recommends testing well water through a certified lab and, if needed, treating it where it enters the house with aeration or granular activated carbon.

Who can install a radon system in Montana?

Montana doesn't license radon mitigators. DEQ publishes lists of mitigation and measurement providers who hold NRPP or NRSB certification, but says it does not certify or require proof of certification for those contractors and does not regulate their conduct. DEQ encourages hiring a trained, certified contractor, getting at least three written bids, and checking insurance and references.

Montana's Department of Labor and Industry requires all construction contractors with employees to register. Ask the company for proof of registration and confirm it's in good standing on the department's website or by calling (406) 444-7734.

What must a seller disclose?

Montana Code 75-3-606 requires a radon disclosure statement on at least one document signed before or with an offer to buy inhabitable property, acknowledged by the buyer's signature. If the seller knows the building was tested, the seller must provide the results and any evidence of mitigation. The law doesn't require a test and doesn't create a contingency or a right to cancel unless the contract says so.

Common questions

Is Lewis and Clark County a high-radon area?

Yes, by the available data. About half of county tests in the state's compiled data were at or above 4 pCi/L, and the EPA places the county in Zone 1. Test every home anyway, since individual houses vary.

Does Montana license radon mitigation contractors?

No. DEQ lists NRPP- and NRSB-certified providers but doesn't certify them. Contractors with employees must hold a Montana construction contractor registration.

Where can I get help or a test kit in Helena?

Call Montana DEQ's Radon Control Program hotline at 1-800-546-0483 to ask about discounted kits, or buy a kit at a hardware store.

Montana radon guide for Helena and Lewis and Clark County (2026)