Two houses can sit side by side on the same street, built the same year by the same crew, and test completely differently for radon. One comes back at 1.1 picocuries per liter. The other comes back at 9. Nothing you can see from the curb tells you which is which.
That is the whole problem. Radon is a radioactive gas that seeps up out of soil and rock, and you cannot see it, smell it or taste it. The Environmental Protection Agency puts the stakes plainly on its radon health risk page: “Overall, radon is the second leading cause of lung cancer.” EPA estimates radon contributes to about 21,000 lung cancer deaths in the United States every year.
A test kit costs less than takeout. EPA’s own fact sheet notes that many sell for under $25.
Where radon comes from and how it gets inside
Uranium sits in ordinary rock and soil nearly everywhere. As it decays it produces radium, and radium decays into radon. Radon is a gas, so it moves. Outdoors it disperses into a very large volume of air and comes to almost nothing.
Under a house, it has somewhere else to go.
Warm air rises and leaks out of the upper parts of a building. That pulls replacement air in at the bottom, through cracks in a slab, gaps around pipes, sump openings, crawlspace floors and the seam where a wall meets its footing. Builders call this the stack effect, and it works the way a chimney does. Your house is gently sucking on the ground underneath it, and soil gas comes along for the ride.
That is why the lowest lived-in level usually tests highest, and why radon is not a problem of old houses or of houses with basements. EPA’s basic facts sheet is blunt: elevated levels have turned up in every state, in new homes and old ones, tight ones and drafty ones, with basements and without.
Radon then decays into solid particles, and those particles are the part that matters. Breathe them in and some lodge in lung tissue, where they emit alpha radiation at very close range. The damage lands on the DNA of the cells lining your airways, and it accumulates across decades.
What 4, 2, 1.3 and 0.4 mean
Radon is measured in picocuries per liter of air, written pCi/L. Four numbers do most of the work.
4 pCi/L is EPA’s action level. The agency recommends homes be fixed at or above it.
2 to 4 pCi/L is the range where EPA recommends you consider fixing anyway. The reason is worth sitting with: there is no known safe level of radon. The 4 is the point where the agency thinks the repair is clearly worth making, not a boundary between dangerous and safe.
1.3 pCi/L is roughly the average indoor level in American homes. That average is the basis for EPA’s 21,000 estimate.
0.4 pCi/L is roughly the average outdoors, about a tenth of the action level. You cannot get a house below outdoor air, and EPA’s radon reduction guide says most homes can be brought to 2 pCi/L or lower with current methods.
Two clarifications keep people out of trouble.
First, the action level is a federal recommendation, not a requirement. No agency will make you test or fix an existing home. Some states and local governments do have their own rules, including disclosure at a sale, and those carry legal weight. Installers are certified through private programs rather than by EPA. Three kinds of authority, and only one can be enforced against you.
Second, the 21,000 figure is a modeled estimate, not a tally of death certificates. Radon never appears as a cause of death on a form. The number combines measured radon levels in American homes with risk estimates from studies of miners and householders, and EPA’s own risk assessment carries a wide range around it. Treat it as a well-supported estimate of scale, not a count.
Radon and smoking multiply each other
Here is the fact that gets buried.
Radon and tobacco smoke do not simply add up. They multiply. A smoker in a high-radon house is not at the smoker’s risk plus a small amount. The combined risk lands far above what either exposure produces alone.
The clearest demonstration comes from a collaborative analysis of 13 European case-control studies, pooling individual records for 7,148 people with lung cancer and 14,208 people without, published by Darby and colleagues in the BMJ in 2005. The authors estimated the absolute risk of lung cancer by age 75 at three radon concentrations. For lifelong non-smokers, the risks ran roughly 0.4 percent, 0.5 percent and 0.7 percent as radon rose. For cigarette smokers, the figures were about 25 times greater at every level: roughly 10 percent, 12 percent and 16 percent.
Read those two rows again. The same jump in radon costs a non-smoker about three-tenths of a percentage point in absolute risk and costs a smoker about six points. Same gas, same house.
A combined analysis of seven North American case-control studies, published by Krewski and colleagues in Epidemiology in 2005, found the same direction of effect at ordinary residential concentrations, with lung cancer odds rising alongside radon and the result matching what the miner studies had predicted.
EPA’s health risk page translates this into household terms. Out of 1,000 people exposed over a lifetime at the 1.3 pCi/L national average, the agency estimates about 20 smokers and about 2 never-smokers would die of lung cancer. At 4 pCi/L, those figures rise to roughly 62 and roughly 7.
The practical conclusion runs against intuition. People sometimes decide a radon test is beside the point in a household where someone smokes, reasoning that the smoking is the real problem anyway. That is backwards. The household with a smoker in it has more to gain from fixing a radon problem than any other house on the block, because that is where the two exposures compound. Quitting still does more than mitigation ever will, and help for that exists. Both things are true at once.
One more finding from the European analysis deserves a line. The dose-response relationship looked linear with no threshold, and it held up when the authors looked only at homes below 200 becquerels per cubic metre, a little above 5 pCi/L. The risk does not switch cleanly off anywhere.
Running a test that tells you something
Two families of test exist, and the trade-off between them is real.
Short-term tests sit in the house from 2 to 90 days, depending on the device. They are fast and cheap, and they are what a real estate deal runs on. Their weakness is that radon levels swing with weather, season, soil moisture and how the house is operated. A two-day reading during a windy week is a snapshot of a windy week.
Long-term tests sit for more than 90 days. They average out those swings and come far closer to what you breathe year after year. Their weakness is that you wait a season for the answer.
EPA’s citizen’s guide lays out a sequence that handles the trade-off. Start short-term. If the result is 4 pCi/L or higher, follow up with either a long-term test or a second short-term test. A long-term follow-up at 4 or above means fix the house. With a second short-term test, average the two and consider fixing if the average reaches 4. If the first result came in above roughly twice the action level, EPA says run the second test right away rather than waiting out a season.
Placement is not a detail. Put the device on the lowest level you live on: the basement if you use it regularly, the first floor if you do not. Pick a room people occupy. Skip kitchens, bathrooms and laundry rooms, where humidity and air movement distort the reading. Keep the device at least 20 inches off the floor, away from drafts, exterior walls, direct heat and anything likely to knock it over.
Then there is the closed-house rule, which is where most do-it-yourself tests go wrong. For short tests of two or three days, EPA says close windows and outside doors at least 12 hours before you start and keep them closed throughout, except for ordinary coming and going. Run heating and cooling normally, but do not run fans that pull in outside air. A radon fan already installed stays on. Short tests also should not run during severe storms or unusually high wind.
Open a window during the test and you have not measured your house. You have measured your house with a window open, which is not how you live in January.
Winter conditions matter for the same reason. The house is sealed, the heating system drives the stack effect hard, and frozen or saturated ground outside pushes more soil gas toward the one warm dry opening available.
The reader correction: your neighbor’s number is not your number. A National Cancer Institute fact sheet says this directly, noting that neighboring homes can have very different levels and that a neighbor’s result predicts yours poorly. Soil varies over short distances. So do the foundation, the slab, the sump and how tightly the house was built. EPA’s radon zone maps are planning tools for states and builders, not a substitute for testing one address.
What a radon fix involves
The dominant method has an intimidating name and a simple idea behind it. It is called sub-slab depressurization, or soil suction.
A contractor cuts a hole through the basement slab or foundation floor and sets a pipe into the gravel or soil beneath. The pipe runs up through the house, or along the outside, and vents above the roofline. A fan on that pipe runs continuously.
That is the whole trick. The fan makes the pressure under your slab slightly lower than the pressure inside your house. Soil gas now finds an easier path into the pipe than into your living room, so it goes up and out. Sealing cracks helps the system work better, but EPA’s reduction guide is clear that sealing on its own is not a reliable fix.
EPA’s guide gives typical reductions of 50 to 99 percent for sub-slab suction and calls it the most common and usually most dependable approach for homes with basements or slabs. Crawlspaces get a variation: heavy plastic sheeting over the soil with suction drawn from underneath, also 50 to 99 percent.
This is a solved engineering problem. The methods are standardized, the standard of practice for existing low-rise homes is written down as ASTM E2121, and installers train and certify through private programs, most often the National Radon Proficiency Program or the National Radon Safety Board.
On price, EPA declines to publish a dollar figure, and so will this article. What the agency says across several documents is that most homes can be fixed for about what other common home repairs cost, and that the number depends on house size, foundation type and method. Get more than one written estimate.
Once a system is in: the fan runs continuously and should never be switched off, the warning device on it is usually a simple pressure gauge worth glancing at, and fans last five years or more. Retest within 30 days of installation, at least every two years after, and again after significant remodeling.
Questions to ask before you sign a mitigation contract
These separate a good job from a cheap one.
- What radon level are you guaranteeing, for how long, and does the guarantee transfer if I sell?
- Is diagnostic testing included in this price, or billed on top of it?
- Where will the pipe run, where will the fan sit, and where does the exhaust discharge relative to my windows and my neighbor’s?
- What warning device are you installing, and will you show me it works before you leave?
- Will you run the post-installation test, or should I hire someone independent?
- Which program certifies you, and can I call two customers from last year?
Testing when a house changes hands
A radon test during a sale is a short-term test under deadline pressure, which is exactly the situation where a result is easiest to spoil.
EPA’s guide for home buyers and sellers describes three accepted approaches, all running at least 48 hours: two identical devices side by side at the same time, two identical devices run back to back, or a continuous monitor. In all three the decision rule is the same, which is to fix if the average reaches 4 pCi/L.
The same guide lists how testers guard against interference: motion and proximity detectors, logs of barometric pressure and temperature, tamper-evident window seals, and a written agreement from the occupant not to disturb conditions. EPA notes interference may invalidate a test and is illegal in some states.
Think about why that list has to exist. During a sale, the person with physical control of the house is usually the person with the strongest financial reason for the number to come back low. A window cracked open on a mild afternoon leaves no trace in the result. It simply lowers it.
That does not mean sellers cheat, and most do not. But a buyer has an easy structural answer. Hire your own qualified tester, settle in the contract who pays for testing and who pays for mitigation, and have the device placed on the lowest level you intend to use rather than the lowest level the seller uses. If you plan to turn the basement into a bedroom, that is the level your decision rests on.
Sellers have the stronger play. Test early, before listing, with no clock running. If the house needs a system, install one and keep the paperwork. A documented mitigation system is a selling point rather than a negotiation.
Buyers looking at a previously tested home should ask four things: what the number was, who ran the test, which level it was run on, and whether anything structural or mechanical has changed since. A test from before a furnace replacement or a basement renovation is a historical document, not a measurement.
Well water and new construction
If your water comes from a private well, radon can arrive dissolved in it and escape into the air when you shower or run the dishwasher. EPA’s citizen’s guide is direct about the relative scale: soil is usually a far larger source than water, and the risk from breathing radon released by water outweighs the stomach cancer risk from swallowing it. Test your water if your air test comes back high and you are on a well. On a public system, ask your utility. Point-of-entry treatment handles water before it enters the house and addresses the airborne risk; a point-of-use device at the kitchen tap treats drinking water only and does nothing about the shower.
For new construction, the features are cheap and dull: clean gravel under the slab, plastic sheeting over it, a sealed vent pipe running from the gravel up through the building and out the roof, and careful caulking of the foundation. That assembly is a passive system, and EPA notes many builders already do pieces of it for moisture control. If a test after move-in still reads 4 pCi/L or above, a mitigator adds a fan and the passive system becomes an active one.
The step people skip: test the new house anyway. Radon-resistant features reduce entry. They do not guarantee a number.
When to involve a professional, and what to do this week
Radon causes no symptoms. No cough, no headache, no smell, nothing that tells you to act. That is precisely why it has to be a scheduled task rather than a response to something you notice.
A few situations call for a clinician rather than a contractor. A persistent cough, coughing up blood, unexplained shortness of breath or chest pain all warrant a medical appointment regardless of your radon reading, and none of them are how radon exposure announces itself. If you smoke or used to, and especially if you live in a house that has tested high, bring the radon history to your clinician and ask whether lung cancer screening makes sense for you. They will not have that history unless you hand it to them.
The task itself is small. Buy a test kit, or get one from your state radon program, which often sells them cheaply. Put it on the lowest level you live on, at least 20 inches off the floor, in a room you use. Close the windows 12 hours beforehand and keep them closed. Mail it back when the package says to.
In a few weeks you will know. Under 2 pCi/L, you are done for a couple of years. Between 2 and 4, you have a judgment call EPA says is worth making deliberately, particularly if anyone in the house smokes. At 4 or above, you have a result to confirm with a second test and a well-understood repair waiting on the other side of it.
Keep exploring: water testing, water quality report, laundry detergent and fabric softener.
Sources
- U.S. Environmental Protection Agency. “What is EPA's Action Level for Radon and What Does it Mean?.” epa.gov
- U.S. Environmental Protection Agency. “Radon.” epa.gov
- U.S. Environmental Protection Agency. “Health Risk of Radon.” epa.gov
- U.S. Environmental Protection Agency. “A Citizen's Guide to Radon (EPA 402/K-12/002).” epa.gov
- U.S. Environmental Protection Agency. “Consumer's Guide to Radon Reduction: How to Fix Your Home (EPA 402/K-10/005).” epa.gov
- U.S. Environmental Protection Agency. “Home Buyer's and Seller's Guide to Radon.” epa.gov
- U.S. Environmental Protection Agency. “Basic Radon Facts (EPA 402/F-16/002).” epa.gov
- U.S. Environmental Protection Agency. “How to Address Radon When Building a New Home.” epa.gov
- U.S. Environmental Protection Agency. “Radon Publications, Webinars and Videos.” epa.gov
- U.S. Environmental Protection Agency. “Radon Risks and Solutions.” epa.gov
- National Cancer Institute. “Radon and Cancer.” cancer.gov
- PubMed. “Darby S et al. "Radon in homes and risk of lung cancer: collaborative analysis of individual data from 13 European case-control studies." BMJ, 2005. PubMed. DOI: 10.1136/bmj.38308.477650.63.” doi.org
- PubMed. “Krewski D et al. "Residential radon and risk of lung cancer: a combined analysis of 7 North American case-control studies." Epidemiology, 2005. PubMed. DOI: 10.1097/01.ede.0000152522.80261.e3.” doi.org