The essentials in 30 seconds
EPA recommends fixing your home when one long-term test, or the average of two short-term tests, shows 4 picocuries per liter (pCi/L) or more, and suggests considering it between 2 and 4 pCi/L. The usual fix is active sub-slab depressurization: a pipe through the floor slab and a fan that runs nonstop, pulling radon from under the house and venting it above the roof. Systems like this typically cut levels by 50 to 99 %. Hire a state-certified or privately certified mitigator, then run your own post-mitigation test within 30 days, and retest at least every two years.
- The fan must stay on all the time; a gauge, usually a manometer, shows it is working.
- Sealing cracks helps a system work but, on its own, does not reliably lower radon.
- In the UK, UKHSA advises reducing radon above 200 Bq/m³, ideally to below 100 Bq/m³.
When EPA says you should install a radon reduction system
Your decision starts with a reliable test. If you have only one short-term result, see our guide to radon testing for the follow-up test EPA recommends before you spend on mitigation.
| Result | What EPA recommends |
|---|---|
| 4 pCi/L or more (one long-term test or the average of two short-term tests) | Fix your home |
| 2 to 4 pCi/L | Consider fixing; levels below 4 still carry some risk |
| Below 2 pCi/L | Most homes can be brought down to around this level; retest in the future |
| About 1.3 pCi/L | Average indoor level in US homes |
| About 0.4 pCi/L | Typical outdoor level |
There is no known safe level of radon, so the target after mitigation is as low as the system can reasonably reach. Many contractors guarantee a negotiated level, for example 2 pCi/L or less.
How a radon mitigation system works
Air pressure inside a house is usually a little lower than in the soil around the foundation, so the house acts like a gentle vacuum, drawing soil gas, radon included, through cracks, joints, sumps and gaps around pipes. A soil suction system reverses that: a fan creates a stronger vacuum under the house than the house creates inside, so radon is caught before it enters and released above the roof, where it dilutes quickly.
The parts of a typical active system:
- Suction point: a hole through the slab (or from outside, under the slab) into the gravel or soil beneath. Often one is enough; the number depends on how easily air moves under the slab.
- Vent pipe: a sealed pipe running up through the house, garage or along an outside wall.
- Radon fan: installed in an unconditioned space such as the attic or garage, or outside the house, never in or below a lived-in room.
- Exhaust: above the roof surface, so radon cannot drift back in through windows.
- Warning device: a manometer (a U-shaped liquid gauge), dial gauge, light or alarm that shows the system is pulling suction.
Before installing, a contractor may run diagnostic tests, such as puffing chemical smoke into holes and cracks, or a soil communication test with a vacuum cleaner in one hole and smoke at another, to see how air moves under the slab.
Radon mitigation methods by foundation type
| Method | Where it is used | Typical reduction |
|---|---|---|
| Active sub-slab suction (sub-slab depressurization) | Basements and slab-on-grade homes; the most common and usually most reliable method | 50 to 99 % |
| Passive sub-slab suction | New homes built radon-resistant; relies on natural air currents | 30 to 70 % |
| Drain-tile suction | Homes with perforated drain pipe around the foundation | 50 to 99 % |
| Sump-hole suction | Basements with a sump; the sump is capped and still drains water | 50 to 99 % |
| Block-wall suction | Basements with hollow block walls, often combined with sub-slab suction | 50 to 99 % |
| Sub-membrane suction | Crawl spaces: a heavy plastic sheet over the soil with a pipe and fan beneath it | 50 to 99 % |
| Natural crawl space ventilation | Crawl spaces, by opening or adding vents | 0 to 50 % |
| House (basement) pressurization | Tight basements isolated from outdoors and upper floors | 50 to 99 % |
Homes with a basement under part of the house and a slab or crawl space under the rest may need a combination. If you have a crawl space and moisture problems as well, sub-membrane suction can be planned together with crawl space encapsulation; the radon pipe goes under the vapor barrier. For a sump that also handles groundwater, see our sump pump guide.
Other methods and their limits
- Sealing cracks and openings is a standard part of most systems and saves conditioned air, but EPA does not recommend sealing alone: it has not been shown to lower radon significantly or consistently, and settling opens new gaps.
- House or room pressurization blows air into the basement to keep radon out. It depends on doors and windows staying shut and can bring in moisture, so it is a fallback.
- Heat recovery ventilators (HRVs) add fresh air while recovering heat, and work best on a basement only. Filters need cleaning and changing twice a year, and an HVAC professional should check the balance yearly.
- Opening windows helps temporarily, but radon levels usually return to where they were within about 12 hours of closing up.
How to choose a certified radon mitigation contractor
Start with your state radon office: many states require radon professionals to be licensed, certified or registered and can give you a list. In states without rules, ask whether the contractor holds a credential from a private proficiency program, such as the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB), and whether they follow ASTM E2121, the standard practice for installing radon mitigation in existing homes.
Questions from EPA's checklist:
- Can you show references and before-and-after test results from past jobs?
- Can you explain the work, how long it takes and exactly how the system will work?
- Did you inspect the house structure and review how my radon test was done?
- Will you run diagnostic tests before designing the system?
- Will you install a warning device and test after installation?
- Do you guarantee a level of 4 pCi/L or below, and for how long?
- Do you carry liability insurance, a bond and the local licenses?
Get the agreement in writing before work starts. It should describe the work, the time needed, permits and building codes, responsibility for damage and cleanup, any guarantee on the radon level, hardware warranties, whether warranties transfer if you sell, and what you need to prepare.
What a correct installation looks like
Use this list on handover day:
-
01
Labels
Every part of the system is clearly labeled, so nobody changes it by mistake.
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02
Exhaust height
The pipe vents above the roof surface, at least 10 feet above the ground, and at least 10 feet from windows, doors or other openings unless it discharges at least 2 feet above them.
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03
Fan location
The fan is outside the living space, in an unconditioned area or outdoors, and an outdoor fan is rated for exterior use.
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04
Electrical work
Connections meet the local electrical code.
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05
Warning device
A manometer or alarm is fitted where you can see or hear it, and the contractor shows you it works.
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06
Instructions
You receive written operating and maintenance instructions and copies of warranties.
Testing after the system is installed
Test within 30 days of installation, but no sooner than 24 hours after the system starts running with the fan on. EPA recommends a two- to seven-day measurement with closed-house conditions: windows and doors shut for 12 hours before and during the test, apart from normal coming and going. Your contractor may test too, but an independent test, or one you run yourself with a kit from a qualified laboratory, avoids any conflict of interest. Our radon testing guide explains where to place the kit.
Living with a radon fan and manometer
- Look at the gauge regularly. Note the reading the mitigator shows you at handover; if the gauge changes clearly or an alarm sounds, call a contractor.
- Leave the fan running. It must run continuously for the system to work, all year round.
- Plan for the fan to age. Fans may last five years or more, and manufacturer warranties tend to run up to about five years; after that it may need repair or replacement.
- Retest at least every two years, and after major work such as finishing a basement, which can also affect warranties.
- Selling the home? Tell buyers about the system and hand over its operating and maintenance information.
| What you notice | Possible cause | What to do |
|---|---|---|
| Gauge reading has changed or alarm sounds | Fan stopped, power cut to the fan or a disconnected pipe | Check the breaker and call your mitigator |
| Fan is louder than usual or rattles | The fan may be wearing out or have worked loose | Ask the installer to inspect it |
| New test above 4 pCi/L despite a working fan | New entry routes or changes to the house | Ask the mitigator to review the design |
| Basement now used as a bedroom or playroom | Exposure in a new lived-in level | Retest on that level |
Radon reduction in the UK
UKHSA uses becquerels per cubic metre. It advises reducing radon in homes where the yearly average is above the Action Level of 200 Bq/m³, ideally to below the Target Level of 100 Bq/m³, and to consider action between the two, especially if a smoker or ex-smoker lives there. For reference, 1 pCi/L equals 37 Bq/m³, so EPA's 4 pCi/L is about 148 Bq/m³.
UKHSA ranks an active radon sump with a fan as the most effective method, best under solid floors. Positive ventilation (a small fan blowing in air, usually from the loft) suits lower levels; homes with suspended timber floors often use natural or fan-assisted underfloor ventilation. Simple sealing on its own does not reduce radon, and complete sealing of wooden floors can cause rot. Above 1,000 Bq/m³, UKHSA suggests contacting it for advice.
Radon in well water
Radon from soil is usually a far bigger risk than radon from water, but private wells can carry it. If your air test is high and you have a well, test the water too. Whole-house aeration removes 95 to 99 % and needs yearly cleaning; granular activated carbon removes 85 to 95 % at moderate levels but can collect radioactivity and needs careful disposal. Point-of-use filters at a tap leave the radon released by showers and appliances untreated.
Radon mitigation: frequently asked questions
What radon level requires mitigation?
EPA recommends fixing the home at 4 pCi/L or more and considering it between 2 and 4 pCi/L. In the UK the Action Level is 200 Bq/m³.
How does a radon mitigation system work?
A fan pulls air from beneath the slab or crawl space membrane through a pipe and vents it above the roof, so radon is removed before it can enter the house.
Can I turn off my radon fan?
No. It must run continuously for the system to work, and radon levels rise again when it stops.
How long does a radon fan last?
EPA says fans may last five years or more, after which they may need repair or replacement. Check the gauge regularly to spot a failure.
How do I read a radon manometer?
Note the reading set at installation. A clear change from that reading means the system may have stopped working; call your mitigator.
How soon should I test after radon mitigation?
Within 30 days of installation, but at least 24 hours after the fan starts. A two- to seven-day test is recommended.
Can I install a radon mitigation system myself?
EPA recommends a qualified contractor, because a poor design can raise radon levels. If you go ahead, take a course through your state radon office first.
Does sealing cracks reduce radon?
Sealing helps a suction system work better, but on its own it has not been shown to lower radon reliably.
Sources
Official radon guidance, checked on 27 October 2026.
- US EPA · Consumer's guide to radon reduction: how to fix your home ↗Action levels, how soil suction works, methods by foundation type and their typical reductions, contractor checklist and contract, installation requirements, post-mitigation test, fan life and maintenance, radon in well water.
- US EPA · A citizen's guide to radon ↗Health risk, average indoor and outdoor levels, no known safe level, retesting every two years.
- UKHSA · UKradon: Action Level and Target Level ↗200 and 100 Bq/m³ and when to act between them.
- UKHSA · UKradon: how to reduce radon levels ↗Radon sumps, positive ventilation, underfloor ventilation, limits of sealing and advice above 1,000 Bq/m³.
Got a high radon result?
Keep your test reports and a sketch of your foundation ready before you call a certified mitigator.