Radon: what every homeowner should know

This site is mostly about mold. Radon belongs here anyway, and for the same reason carbon monoxide does: it’s an invisible air-quality problem that houses routinely mishandle, and the fix is a small amount of building science applied steadily. If you’ve read our moisture control guide, the mental model is already in place. Mold is a moisture problem. Radon is a soil-gas problem. Both are building problems first and health problems second, and both respond to mostly low-tech, well-understood interventions that a homeowner can either do or hire out with confidence.

The headline that should drive the rest of this article: radon is the second-leading cause of lung cancer in the United States, and the leading cause among people who have never smoked. The EPA puts the annual death toll at roughly 21,000 lung cancers per year — more than drunk driving, more than house fires, more than drowning. It is not an “evacuate now” emergency. It is a steady, dose-dependent risk that you reduce by testing your house once and, if necessary, spending one or two thousand dollars on a fan and a pipe.

Test every home. Not just the ones in EPA Zone 1. Every home. That is the single most important sentence in this article, and the rest of it is how and why.

What radon actually is

Radon is a naturally-occurring radioactive gas produced by the decay of uranium in soil and bedrock. Uranium is everywhere in trace amounts; it decays slowly into radium, which decays into radon. Radon itself has a half-life of about 3.8 days, which is long enough for it to seep out of the ground, find its way through cracks in your foundation, and accumulate inside your house.

It’s colorless, odorless, and tasteless. There is no human sense that detects it. You cannot smell a radon problem, you cannot see one, and you will not develop a headache from it the way you might from carbon monoxide. It is, in every detectable way, just air.

The pathway into a house is pressure-driven. The inside of a typical home runs slightly negative relative to the surrounding soil — the stack effect of a heated building, plus bath fans and dryers and range hoods, all pull air upward and outward. Soil gas, radon included, gets sucked in to fill the deficit. It enters through:

Cracks in concrete slab floors and foundation walls
Slab penetrations for plumbing
The sump pit (open, in direct contact with soil — often the single biggest entry point)
Crawl-space floors without sealed vapor barriers
The hollow cores of concrete-block walls
Construction joints between slab and footing

Once inside, radon decays into a series of short-lived isotopes collectively called “radon progeny” or “radon daughters” — polonium, lead, and bismuth. Unlike the gas itself, these are electrically charged solid particles that stick to dust and aerosols in the air. When you breathe, the progeny lodge in your bronchial tissue and deliver alpha radiation at point-blank range to the cells of your airway. That’s the mechanism. Repeated over years and decades, it raises the probability that one of those cells turns into a tumor.

The health risk, in plain numbers

The EPA’s working figure of ~21,000 radon-attributable lung cancer deaths per year in the US comes from epidemiological work pooling miner cohorts and residential case-control studies. Of those deaths, roughly 2,900 are in people who never smoked. Most of the rest are smokers, because radon and tobacco smoke act synergistically: the combined risk is much higher than either alone.

A few framings that may help calibrate:

At the EPA action level of 4 pCi/L, the lifetime lung-cancer death risk is approximately 7 in 1,000 for never-smokers and about 62 in 1,000 for smokers — roughly a 9× multiplier from the smoking interaction.
The risk is dose-dependent and approximately linear. Halving your radon level roughly halves your excess lung-cancer risk. The EPA and WHO both assume no safe threshold: lower is better, all the way down.
The harm is chronic, not acute. Nobody collapses from a radon-filled room the way they would from a CO leak. The damage is the slow probability shift over a 20–40 year exposure window.

This last point is what shapes the right emotional response. Radon is real and worth fixing, but it is not a fire alarm. A house at 8 pCi/L is not a house you flee in the night; it is a house you test once more to confirm, then hire a mitigation contractor for in the ordinary course of homeownership.

The action levels

There are three numbers worth knowing, and they say slightly different things.

Threshold	Source	What it means
4 pCi/L	EPA action level	”If you’re at or above this, fix it.” Operational US standard.
2–4 pCi/L	EPA “consider fixing” zone	Below the action threshold but not safe — mitigation worth considering, especially for smokers or households with children.
2.7 pCi/L (100 Bq/m³)	WHO reference level	More cautious; recommended worldwide. Some US radon professionals align to this.

The important framing: all three are policy thresholds, not biological thresholds. There is no level at which radon abruptly becomes dangerous, and no level at which it becomes harmless. The action numbers are where regulators decided the cost-benefit math clearly favors mitigation. Below 4 pCi/L is “lower priority,” not “safe.”

For most homeowners, “fix above 4, seriously consider fixing 2–4, keep testing below 2” is a defensible operational rule.

The radon-zone myth

The EPA published a county-level Map of Radon Zones in the early 1990s, classifying counties as Zone 1 (predicted average above 4 pCi/L), Zone 2 (2–4 pCi/L), or Zone 3 (below 2 pCi/L). The map is still widely circulated. It is also, taken on its own, deeply misleading.

EPA itself is clear that the map is a planning tool for code officials and not a substitute for testing your specific house. The reason is variability. The map shows county averages, and radon levels between adjacent houses can easily differ by 10× — even 20×. A Zone 1 county will contain houses well below 1 pCi/L. A Zone 3 county will contain houses above 10 pCi/L. What drives a particular house’s reading is the soil immediately under it, the foundation type, the slab condition, the sump situation, and the pressure dynamics of that specific building envelope. None of those things show up on a county map.

The cleanest operational message:

Regardless of what zone you’re in, test your house.

A $15 charcoal kit answers the question. The EPA’s radon zones page is a fine place to look up your state radon office (which often provides free or subsidized test kits), but it is not where the decision gets made. Your house’s actual reading is.

Testing options

There are four reasonable ways to measure radon in a home. They suit different situations and different budgets.

Short-term tests (2–7 days)

Almost always charcoal canisters: small disks of activated charcoal that adsorb radon during the exposure window, then go in the mail to a lab. Cost: about $15–30 including lab analysis, often less from your state radon office.

Short-term tests are useful for screening — a quick first look at whether you have a problem worth investigating further. They are less appropriate as the basis for a major decision because radon levels fluctuate substantially day-to-day with weather, season, and how the house is being operated. A single 48-hour test that returns 6 pCi/L on a wet, cold week could correspond to a true annual average of 3 pCi/L, or 8 pCi/L. You don’t know from one snapshot.

Short-term tests must be done under closed-house conditions: windows and exterior doors closed for at least 12 hours before the test and throughout the test, with normal HVAC operation.

Long-term tests (90+ days)

Usually alpha-track detectors: a small piece of plastic film that records permanent damage from alpha particles. After 90 days to a year of exposure, you mail it in for analysis. Cost: about $25–50.

This is the right primary test for a homeowner who isn’t under a real-estate deadline. A 90-day or 365-day average accounts for seasonal swings (radon is typically highest in winter, when stack effect is strongest and windows are closed) and gives you a number that actually represents your house. Long-term tests do not require closed-house conditions because they’re explicitly measuring lived-in conditions.

Continuous radon monitors (CRMs)

Electronic devices that read radon continuously and log it to an app. Consumer-grade units — Airthings View Radon, Ecosense RadonEye, Corentium Home — run $150–300 and have become the modern best practice for ongoing visibility. Pro-grade CRMs used by certified inspectors are pricier and more accurate but the same idea.

A CRM is not strictly more accurate than a properly conducted long-term alpha-track test, but it gives you something the alpha track never can: trend data. You can watch your basement readings rise and fall with weather, see what happens when you run the bath fan, and confirm at a glance that the mitigation fan you paid for is still doing its job. For a homeowner who’s serious about indoor air, a $200 CRM is the radon equivalent of the $15 hygrometer.

Professional measurement

A certified radon measurement professional, often the same person who does pre-purchase home inspections, will run a calibrated CRM in your house for 48 hours under closed-house conditions and produce a sealed report. Cost: $200–500. This is what’s typically required for real-estate transactions, where chain-of-custody and tamper- resistance matter.

Where to put the test

Lowest livable level of the house — basement if it’s finished or used for any meaningful time; first floor if not. Place the device 20 inches to 6 feet off the floor, away from drafts, exterior walls, windows, vents, and high-humidity rooms (kitchen, bath, laundry). Avoid running short-term tests during extreme weather (a major storm front passing through can swing readings dramatically) if you have flexibility on timing.

Real-estate testing: a separate context

The pre-purchase radon test deserves its own paragraph because the rules are different from regular homeowner testing.

In a real-estate transaction the test is short-term, conducted under strict closed-house conditions, with chain-of-custody documentation. The buyer (or their inspector) wants a reading they can act on within the closing window. It’s typically done with a professional-grade CRM that timestamps the data and detects tampering, because test tampering is a real thing in seller- disclosure contexts — opening windows during the test, moving the device near a fan, “accidentally” leaving the basement door open.

The cost of a radon test as part of a home inspection is small relative to the price of the house. Get one on every purchase, regardless of zone, regardless of whether the seller volunteers a prior reading. If the house is elevated, that’s negotiable — either the seller mitigates before closing, the price drops to cover it, or you walk in eyes-open. Pre-purchase radon testing is one of the cheapest pieces of insurance in residential real estate.

Mitigation: when and how

When to mitigate

The standard answer: at or above 4 pCi/L. Many radon professionals also recommend mitigation in the 2–4 pCi/L range, especially if anyone in the house smokes, if there are children spending significant time at the lower levels, or if you simply prefer the WHO-aligned threshold. There’s nothing wrong with mitigating a 3 pCi/L house. There’s a lot right with it.

What the system looks like

The dominant approach is sub-slab depressurization (SSD) — and it works so well it has become essentially the only approach worth discussing for a typical basement or slab-on-grade home. The system has five components:

A suction point cored through the basement slab into the gravel layer underneath. Larger or compartmentalized slabs sometimes need multiple points.
A 3- or 4-inch PVC pipe routed up through the house (often through a closet or chase) or along an exterior wall, terminating above the roofline.
An inline radon fan, typically in the attic or mounted outside, that runs continuously and creates slight negative pressure under the slab. Never in living space (it’s a sealed pipe carrying contaminated air).
A manometer — a small U-tube pressure gauge — mounted on the pipe at eye level so a homeowner can confirm at a glance that the fan is working.
Sealing of major obvious entry points: the sump-pit lid (a sealed, gasketed lid with grommets for any pump wiring), significant slab cracks, and the construction joint where slab meets wall.

The system works by reversing the pressure differential. Soil gas gets pulled up the pipe and discharged above the roof, where it disperses into outdoor air at concentrations that are not a concern.

For crawl spaces, the equivalent approach is sub-membrane depressurization — a heavy polyethylene membrane sealed to walls and around piers, with a suction pipe and fan underneath. Same principle.

Cost

Straightforward residential SSD installations run $1,200–$3,000 in most markets, with $1,500–$2,500 being typical. Crawl spaces, slab-on-grade homes with awkward geometry, and houses needing multiple suction points run $2,500–$5,000+. The operating cost of the fan itself is small — a 60–80W fan running continuously is roughly $10–20/month in electricity.

What to expect afterward

A correctly installed SSD system on a typical home brings the basement reading well below 4 pCi/L, and usually below 2 pCi/L. If post-mitigation testing still shows elevated levels, the contractor adjusts (more sealing, additional suction points, larger fan); a competent mitigator includes that follow-through in the scope.

The DIY question

Can you mitigate radon yourself? Sometimes. Should you? Usually not, and here’s the honest breakdown.

Reasonable DIY scope:

Sealing major entry points — gasketed sump-pit lid, sealing obvious slab cracks with appropriate polyurethane sealant, sealing around plumbing penetrations. Useful on its own (modest reductions) and useful as preparation for a professional active system.
Encapsulating a crawl space with a continuous vapor barrier (this also helps the moisture story — see below). A passive membrane alone won’t always solve a radon problem, but it’s a large piece of one.

Not reasonable DIY:

Active sub-slab depressurization systems. The fan sizing, the suction-point placement, the pipe routing, the exhaust location (above the eaveline, away from windows), the manometer installation, and especially the interaction with combustion appliances in the house all matter. A poorly designed SSD system can depressurize the house enough to backdraft an atmospheric-vent water heater or furnace, pulling combustion gases (including carbon monoxide) into living space. This is not theoretical; it’s a documented failure mode, and it’s why active systems should be installed by a certified mitigator who understands the building as a pressure system.

If your reading is borderline (2–4 pCi/L) and you want to start with sealing and a sump lid before deciding about an active system, that’s a reasonable progression. If you’re well above 4 pCi/L, hire it out.

The radon-and-moisture overlap

The pathways radon uses to enter a house are, to a striking degree, the same pathways water uses. The sump pit. The slab cracks. The crawl-space floor. The construction joint between slab and footing. The hollow concrete-block wall.

This means the building-science work that reduces radon often improves the moisture story too:

Encapsulating a crawl space with a continuous vapor barrier dramatically reduces both soil-gas entry and ground-source vapor.
Sealing the sump pit with a gasketed lid keeps radon out and also stops the steady evaporation of sump water into basement air (a meaningful contributor to summer basement humidity).
Sealing significant slab cracks reduces both radon entry and capillary water movement upward through the slab.

The reverse caution applies too: basement waterproofing changes radon dynamics. Interior French drains, sump replacements, slab patches, and exterior excavation can all shift the pressure regime of the house relative to the soil, sometimes spiking radon levels upward. If you’ve recently had basement waterproofing done, re-test. If you encapsulate a crawl space without a radon test, you may end up concentrating radon under the membrane — modern best practice is to design the encapsulation so a sub-membrane suction system can be activated later if post-encapsulation testing demands it.

The honest framing: most of the building-science instincts that protect you from mold also reduce radon risk, but radon has its own test and its own dedicated fix, and one does not substitute for the other. Don’t let a basement-waterproofing contractor sell you “radon reduction” as a bundled feature. It’s a separate scope with a separate verification.

Hiring a mitigator

The same conflict-of-interest patterns we covered in how to hire a mold pro without getting scammed apply here. The radon industry has fewer outright bad actors than mold remediation (the work is more standardized, the deliverable more verifiable), but the structural conflict — the same company tests, mitigates, and re-tests — is still worth flagging.

What to look for:

NRPP or NRSB certification. The two US radon professional certifications are the National Radon Proficiency Program (administered by AARST, the American Association of Radon Scientists and Technologists) and the National Radon Safety Board. NRPP is the more common credential. Either is fine; lack of either is a flag.
State licensure if your state requires it (Illinois, New Jersey, Pennsylvania, Kentucky, Iowa, Maine, Ohio, Florida, Minnesota, New Hampshire, New York, and a growing list — check your state radon office).
Multiple bids. Two or three written proposals against the same testing data. The radon trade pricing is reasonably consistent; significant outliers in either direction deserve scrutiny.
Written scope referencing the AARST standards — specifically SGM-SF for single-family mitigation. A contractor who can name the standard their work conforms to is a contractor who’s read it.
Manometer installed and labeled as part of the scope. This is not optional; it’s how you know the system is working five years from now.
Post-mitigation testing included in the bid.
Warranty on the system, typically 2–5 years on the fan and labor.

The structural recommendation, mirroring the mold-side guidance: ideally, the post-mitigation verification test is done by a different vendor than the one who installed the system — or DIY with an independent kit. The conflict of interest in same-vendor “test, mitigate, and re-test” is real, even when the contractor is honest.

Post-mitigation verification

AARST standards require a short-term test conducted 24 hours to 30 days after the fan is energized, under closed-house conditions, to confirm the system has brought radon below 4 pCi/L. The contractor should either do this or coordinate it explicitly.

A reasonable homeowner add-on is a long-term test (90 days or more) some months later, especially across the winter heating season when radon tends to peak. This confirms the system holds up under stack-effect stress.

After that, the system needs occasional verification — not daily attention, but not zero. See “living with radon” below.

Living with radon

A mitigated home is not a solved home. Fans fail (typical lifespan is 5–10 years; failure is silent unless you’re looking). Soil conditions change. Additions, foundation work, and HVAC changes can shift the pressure dynamics that the original system was designed around.

The radon equivalent of the weekly mold check is straightforward:

Glance at the manometer when you’re in the basement. If both legs of the U-tube are at the same level, the fan has stopped.
Keep a continuous radon monitor somewhere on a lived-in level. Look at it occasionally; let the long-term trend reassure you. A CRM that suddenly starts trending upward over weeks is the cheapest early warning you can buy.
Re-test every couple of years even with the CRM in place — an independent measurement is a useful cross-check. State radon offices often provide free or low-cost test kits annually.
Re-test after any work that touches the foundation, the basement slab, the HVAC, or the building envelope.

An unmitigated home in a marginal range (under 2 pCi/L) deserves the same long-term watching, just without the manometer. A CRM trending upward over a few seasons is information; a single high reading on a single bad-weather week is noise.

What to do today

In order:

Look up your state radon office. Most provide free or subsidized test kits; the EPA’s state contact page is the entry point. While you’re there, ignore the zone map for decision-making purposes.
Order a long-term alpha-track test ($25–50) if you’re not under any deadline. If you want a faster screen, a $20 short-term charcoal kit will tell you whether to bother with the long-term one.
If you have a basement and care about ongoing visibility, add a continuous radon monitor to your eventual purchase list. $150– 300 buys you a decade of peace of mind.
If your test comes back at or above 4 pCi/L, get two or three bids from NRPP- or NRSB-certified mitigators against the AARST SGM-SF standard. Expect $1,200–$3,000 for a typical basement install.
If your test comes back at 2–4 pCi/L, decide based on who lives in the house, whether anyone smokes, and your tolerance for the cost. Mitigation is reasonable in this range; so is re-testing long-term first.
If you’re buying a house, insist on a radon test as part of the inspection. Every time, every zone.

That sequence costs about $50 if your house is clean, and about $2,000 if it isn’t. Either way you stop wondering.

Moisture control: the complete guide to humidity and mold prevention — the building-science backbone for everything below grade
Combustion gases: CO, NO2, and the stuff that burns in your house — the other invisible indoor-air problem, with a more mature alarm story
Water quality at the tap — well water and radon, plus the rest of the drinking-water picture
Is my home making me sick? A symptom-to-cause guide — where radon sits on the differential
The 5-minute weekly mold check — the same maintenance instinct, applied to mold
How to hire help without getting scammed — conflict-of-interest patterns that translate to radon mitigators
Editorial standards — how we source and frame health claims on this site