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Building Science

Does Radon Really Cause Lung Cancer?

Establishing the risk for homeowners based on radon level, exposure time, and personal behaviors

Understanding the penetrating power of radiation is key to understanding if radon really does cause lung cancer [public domain]

In a recent article on the result of my radon test, I referred to the U.S. Environmental Protection Agency’s (EPA) work on the health effects of radon. They claim that about 21,000 people in the U.S. each year get lung cancer from indoor exposure to radon. In the comments, a few people questioned the EPA claim. How do we know that radon really does cause lung cancer, they ask? The EPA based their conclusion on data from miners working underground. How can radon exposure in homes be related to the higher radiation levels they received?

So I’ve dug in a little bit. I’m going to keep this as brief as I can and relatively simple. If you want to dig into the background work on this yourself, click the links I provide throughout the article and explore the resources at the end.

Radioactivity data

We have more than a century’s worth of data on exposure to all kinds of radiation. When Henri Becquerel discovered radioactivity in 1896, the study of its effect on humans began. It was unintentional at first because the early researchers exposed themselves to high doses without knowing the dangers. Many suffered and died from diseases associated with radiation exposure.

Radium, the parent of radon in the uranium-238 decay chain, became popular for glow-in-the-dark watch faces starting in 1908. That led to the famous case of the “Radium Girls.” They were factory workers who painted the watch faces, using their lips to put a point on their brushes. As a result, they ingested a lot of radium. And they made it worse by painting their fingernails and even their teeth for fun. Many didn’t live long enough to get cancer, though, because the high doses gave them radiation poisoning.

"Radium girls" painting watch faces with radioactive glow-in-the-dark paint [pubic domain]
“Radium Girls” painting watch faces with radioactive glow-in-the-dark paint [public domain].
Another big batch of data came from the atomic bombs dropped on Hiroshima and Nagasaki at the end of World War II. Then we have all the people who were exposed to radiation during the testing of nuclear weapons. (The documentary Radio Bikini is a scary look at the testing done in 1946 at the Bikini Atoll.) We also have all the monitoring done for occupational exposure.

Radioactive decay

You may think none of that’s related to radon exposure in homes, but it is. We need to start with the physics of radioactive decay, though. I covered some of this in the last article already, but let’s review.

A radioactive element has an unstable nucleus that will decay in one of several ways: alpha, beta, gamma, or neutron emission. We describe the rate of decay in terms of a half-life, the time it takes for half of a sample to decay. We know the decay chains of radioactive elements. Radon-222, for example, decays into polonium-218 by alpha emission. Its half-life is 3.82 days. We know these things out to many decimal places.

Alpha particles are helium nuclei, with two protons and two neutrons. Alpha particles are large (compared to beta particles), energetic, and easily stopped. Alpha particles hitting you from outside aren’t a big deal. They don’t get through your clothing. If they hit your skin, they’re absorbed in the outer layers where they don’t cause problems.

Biological effects of alpha particles

The problem with alpha particles arises when they’re emitted inside your body. They have enough mass, energy, and momentum to damage a lot of cells. And that’s where radon comes in. Literally. It comes in through your lungs. Radon is an alpha emitter, but radon’s emissions aren’t the ones thought to cause lung cancer. Radon’s 3.82 day half-life causes it to hang around in your indoor air for a few days before decaying. It’s an electrically neutral atom, though, so it doesn’t stick to your lung tissue when you inhale it. And most radon atoms will leave before decaying when you exhale.

The alpha emissions that occur in your lungs are mainly the ones from radon’s progeny, polonium-218 and polonium-214. They are not neutral, so they stick to the lung tissue when you inhale them. Then they just sit there, waiting to decay and emit their alpha particles. That’s where the trouble begins.

An alpha particle has a lot of energy, so it rips through the cells in your lung tissue like an NFL running back through a middle school football team. It ionizes atoms along the path, which can kill cells or worse, mutate them.

Going from high dose to low dose

Let’s do a quick summary:

  • Radioactive elements emit ionizing radiation that can kill or mutate cells.
  • Alpha particles are inconsequential with external exposure but damaging from internal exposure.
  • Radon has two electrically charged progeny that can be inhaled and release alpha particles in the lungs.
  • Much of our data on the health effects of radiation come from high dose samples like the Radium Girls, those exposed during nuclear weapons detonations, and occupational exposures.

Scientists also have tracked enough people who were exposed to lower doses that they have a model for how to extrapolate to lower doses. It’s called the linear no-threshold model. Unlike many scientific terms, you can tell from the name exactly what it means. First, the effects of radiation exposure are linear with dose. If a dose of X results in Y cases of cancer among 100,000 people, a dose of 0.1 X will result in 0.1 Y cases of cancer.

The other part is the threshold. For a long time, scientists have debated whether or not there is a threshold of exposure below which radiation isn’t harmful. The majority of scientists now believe there is no safe threshold. Any exposure to ionizing radiation, they say, can result in cancer, leukemia, or other health problems.

The National Academy of Sciences, the National Council on Radiation Protection and Measurements, and other organizations involved in the health effects on radiation support the linear no-threshold model. Why? Because there’s a lot of data behind it.

Effects of radiation exposure

In my last radon article, I discussed the units used to measure radioactivity in your indoor air. A picocurie per liter tells you the concentration of radon in the air. Curies are a measure of how many radioactive decays occur per unit time. But that doesn’t tell you what effect it might have on your body. For that we use other units.

The sievert is a unit for the effective radiation dose. It accounts for what type of radiation you’re exposed to and what part of the body is exposed. It’s the unit that governments set limits for in occupational exposure.

The chart below gives you an idea of the scale of effective radiation dose we get from different sources. Radon makes up a big chunk of the exposure. (Thoron, by the way, is a special name for the radon-220 isotope, so it’s still radon. The primary isotope is radon-222.) Those of us who have had significant doses of medical radiation skew the chart. If that’s not you, you’re mainly getting background radiation, which is mostly radon.

Sources of radiation exposure [Source: National Council on Radiation Protection & Measurements (NCRP), Report No. 160

And over the course of a year, the average person in the U.S. gets an effective dose of 2.28 millisievert (mSv) from radon. The chart below puts that in perspective so we can get to the issue of whether or not radon really does cause lung cancer.

Radiation dose scale

So, 2.28 mSv is pretty low on the chart. It’s well below the 100 mSv where scientists can state definitively that your chance of getting cancer increases due to exposure. But remember: The linear no-threshold model says there’s no threshold. And the linear part means that the chances drop linearly with dose.

The EPA’s documentation

Beir VI: Health Effects of Exposure to Radon

The EPA published a 98-page report titled EPA Assessment of Risks from Radon in Homes in 2003. Their report is a fine-tuning of a 592-page report from the National Academy of Sciences titled Biological Effects of Ionizing Radiation (BEIR) VI Report: “The Health Effects of Exposure to Indoor Radon.” (Links to both documents are below.) These are high-level scientific reports done by professional scientists.

Both reports use data on radon exposure to miners working underground and extrapolate down to lower doses that people receive in their homes. That’s where the linear no-threshold model comes in. Both reports discussed that model extensively. They also discussed challenges to the linear no-threshold model and why they rejected them. The EPA report states:

The BEIR VI committee adopted the linear no-threshold assumption based on our current understanding of the mechanisms of radon-induced lung cancer, but recognized that this understanding is incomplete and that therefore the evidence for this assumption is not conclusive.

In other words, if scientists find enough evidence to contradict the linear no-threshold model, they’d have to revise their use of it. Maybe it’s not linear. Maybe there is a threshold. The current status, though, is that the model is valid.

The reports go into great detail describing the model, the data, the history, and the uncertainties. Then the EPA report states definitively: “There is overwhelming evidence that exposure to radon and its decay products can lead to lung cancer.” We know it’s true for miners with high exposures. Scientists have a high level of confidence that it’s true in homes with lower exposures.

Yeah, they’re saying radon really does cause lung cancer.

What’s your risk level?

One thing that’s clear from the studies done is that smokers are at much greater risk than nonsmokers. Actually, they distinguish the two groups as “ever smokers” and “never smokers.” If you smoked for ten years and then quit, your susceptibility to radon-induced lung cancer is higher than if you had never smoked. So what we know is that your chances of getting cancer from radon exposure at home depends on:

  • Your smoking history
  • The indoor radon level
  • The amount of time you’re exposed

If your home has an elevated radon level but you’re rarely there, your risk is lower. A person who spends most of their time at home in a house that has a lower radon level may have a higher total dose and be at greater risk.

My take

I’ve looked at the EPA and BEIR VI documents, spending more time with the EPA report. I don’t have the time to digest even the EPA report completely, but I’ve read enough to have confidence in their conclusions. The only real debate is whether or not the linear no threshold model applies. And if not, what threshold should we use?

Is it ironclad that exposure to 4 picocuries per liter of radon in your home will lead to a certain number of cancers? No. Every scientific measurement and result is uncertain. Uncertainty is part of science. Models are part of science, too. But the basic facts are well understood.

  • We breathe in radon and its progeny.
  • Radon and its progeny emit alpha particles.
  • Alpha particles kill and mutate many cells.
  • The radiation health science community mostly supports linear extrapolation of dose and cancer risk with no threshold.

So, does radon really cause lung cancer? Yes, the data on miners’ exposure makes that clear. The real question is what happens at lower exposures. Without a better alternative, the linear no-threshold model seems the best way to extrapolate.

But you have to consider the risk assessment, too. According to the EPA, nonsmokers exposed to 4 picoCuries per liter over a lifetime will result in 7 out of 1000 getting lung cancer. They compare that to the risk of dying in a car crash. For smokers, that number jumps to 62 out 1000.

The bottom line is that it takes a long exposure time for low levels of radon. And it’s far worse if you’re a smoker. Having seen both of my parents die of lung cancer, though, I’m going to do what I can to avoid it. I’ve never been a smoker, and I’m doing what I can to reduce the radon level in my house.


EPA Assessment of Risks from Radon in Homes

Biological Effects of Ionizing Radiation (BEIR) VI Report: “The Health Effects of Exposure to Indoor Radon.”

EPA radon page


Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a popular book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to our newsletter and follow him on LinkedIn.


  1. Expert Member


    Thanks for this. It's a really useful, comprehensive overview of a topic that has troubled me.

    1. Expert Member
      Michael Maines | | #2

      I agree; I'll be directing clients and contractors to this article if they express any doubt about the need for radon mitigation systems.

  2. norm_farwell | | #3

    Thanks for this.

    Radon’s pretty weird stuff. According to what I’ve heard, measurable levels can vary widely over time, so the typical radon test snapshot doesn’t necessarily give a home owner useful info. (The company Air Things sells low cost data logging monitors that seems pretty useful:

    I saw a presentation years ago by a radon expert who said that radon levels were unaffected by the air tightness of a building. This was both a bit of a relief, since I was doing a lot of energy retrofits and air sealing , and also kind of baffling. It’s a gas, and abatement involves exhausting it, so how could ventilation not improve things? I assume the answer involves the decay process and the fact that if you get the gas out before it decays then you have little exposure, whereas if it decays in the house, then there’s no effective dilution at that point.

    Anyhow here’s an interesting question I’ve been puzzling over. In a house with a known radon issue in the basement, does an ERV with sealed ducts located in that basement increase the exposure level in the floors above? Ducts are flexible fabric, not metal. Seems like alpha particles would be blocked by the duct material ? Or is it more complicated?

    1. Expert Member
    2. Patrick_OSullivan | | #9

      > I saw a presentation years ago by a radon expert who said that radon levels were unaffected by the air tightness of a building.

      I find this assertion to be both dubious on its face and at odds with my own anecdotal, yet empirical, observations.

      I have a monitor in my basement office. I moved into this basement office towards the end of a major construction period, so there's some very specific phases I can highlight and the radon observations for each.

      Phase 1: Some relatively large sources of natural ventilation directly into the basement: a couple future sump pump pipes to the outside, but with hoses through them instead of direct plumbing, a window cracked for a cable to pass through, and some 4" pipe sleeves not completely sealed.
      Phase 2: Sump pumps fully plumbed (therefore no leakage). Window closed. All pipe sleeves/conduit fully packed with duct seal.
      Phase 3: Phase 2 + ERV in office installed and on.

      The typical readings during these phases were as follows:

      Phase 1: 1-2 pCi/L
      Phase 2: 2-4 pCi/L with occasional spikes above 4
      Phase 3: 0-2 pCi/L, often on the lower end of the spectrum

      The differences after these changes in envelope were so stark and consistent, I almost couldn't believe it. In fact, after transitioning from phase 1 to phase 2, I thought "oh, I really need to get this ERV going..." and sure enough, it did the trick.

      None of these readings are off the charts. Despite the spikes, during phase 2, the long term average stayed below the 4 pCi/L threshold at which the EPA suggests intervention is required. Nonetheless, I'm quite happy with the ERV solution. It's simple when planned for accordingly, and I'm convinced enough it makes a difference.

      1. DennisWood | | #10

        Patrick, I would agree 100% with your observations, consistent with my own. Given the case of an older home with heavy retrofits, ventilation rates definitely correlate with radon levels, particularly in a tight home and particularly in the basement.

        The highest rates measured in our home was during a weekend this summer when no one was home, and the automation system shut down the HRV completely. I've modified that code since so this does not happen unless CO2, radon, and VOC levels are all very low. Radon levels above 200 bq/m3 will force the HRV system into it's max vent mode.

    3. here_to_learn_more | | #13


      You wrote "I saw a presentation years ago by a radon expert who said that radon levels were unaffected by the air tightness of a building." I suspect there were certain interpretations to this statement. Air tightness helps in certain aspects, for example, I had open crawl spaces with high levels of 13 pCi/L (much higher than EPA's desired <4) throughout the basement and 8 pCi/L on the first floor. This was with a radon fan! By sealing and encapsulating the crawl spaces and extending the fan to pull from under these crawl spaces now covered with crawl space barrier (plastic), this greatly reduced the radon levels to around 1 pCi/L. So, sealing and creating better air tightness helps “IF” you utilize it the air tightness of the envelope to create slight negative pressure under those spaces (say under the entire basement). On the other hand, the radon fan went out in this house for a day and the levels rose back to 13 pCi/L. So, in some sense air tightness alone may not prevent the leakage of radon into the home.

      I agree with comments that radon tends to be highest in the basement, but that does not mean it won’t be high in other parts of the home. My understanding is that radon comes from the ground and works its way into basements through cracks and the like. (So, air sealing should help, but may not be a cure all.)

      Now, many may be uncertain about the dangers of specifically radon. But, I was sold on fixing this radon system to lower the radon levels and other benefits. I prefer to think of this as “radon and moisture mitigation”. The moisture levels in this basement are so much lower now, since it is better sealed and there is a negative pressure under the crawl space floors and parts of basement slabs. In addition, there is some thought that other harmful earth gases could be concentrated in the basement leaks as well. These are removed through the mitigation system creating this slight negative pressure under basement floors.

      Lastly, if interested I suggest looking into “optimal mitigation”, which is a phrase that I have seen thrown around for a mitigation system designed to only have ideally a couple Pascals negative pressure under the slab throughout the year. Energy enthusiasts that hang out on this site would be all for this, since otherwise a super powerful oversized radon fan would pull conditioned air causing an energy penalty. Moreover, that supersized radon fan might create big pressure differences in a super tight house.

  3. norm_farwell | | #5

    Thanks Malcolm. I can see how balanced pressure (or even positive pressurization) from a ventilation system would be helpful counteracting natural negative pressure which would tend to pull soil gas into a building.

    1. Expert Member
      MALCOLM TAYLOR | | #7


      Both studies seem to describe ventilation (both as a method of dilution, and inducing positive pressure) as a poor substitute for the primary strategy of diverting under-slab air outside, but better than nothing.

  4. DennisWood | | #6

    Allison, great job as usual :-)

    Norm, we're using a few AirThings Wave Plus AQ sensors (measure VOC, C02, radon) in our home. They provide feedback to the automation system which also controls ventilation rates based on C02, VOC, or radon levels. There are 3 AirThings units, one in the basement, 1st floor and 2nd floor. The home is old construction, heavily retrofitted. There is no dedicated radon mitigation system, but the HRV main stale air return is in the basement stairwell. There is a basement sump, with a conventional (not sealed) lid. A few observations after four months or so of data collection:

    1. Radon levels in the basement change often, and quickly (see attached image). You can see visually why an average is taken over a longer period. The images shows the last month's data. Levels on the 2nd floor are much lower, about 80% lower than the basement, and remain fairly consistent at those levels. Research suggests that mitigation should be correlated with exposure time in a given location, so for example a basement office (with daily exposure) vs just using a basement for storage would change your mitigation priority.

    2. Summer with open windows definitely lowers levels everywhere in the house.

    3. Our HVAC fan (an EC motor setup) runs nightly at around 400 CFM to increase air circulation to upstairs bedrooms and we do not see much in the way of elevated radon upstairs when this fan is running.

    4. Increasing ERV/HRV ventilation rates in response to elevated radon does make a difference, but the AirThings sensor API only provides the 24 hour radon average to automation, so adjusting ventilation rates to hourly radon changes is not possible currently. In other words, you can view the hourly sampled radon levels from each sensor via the AirThings web dashboard, but the web data available for automation (the API, radonShortTermAvg) is the 24 hour average.

    Hope that helps.

    1. norm_farwell | | #12

      Interesting, thanks Dennis.
      It makes sense to me that fluctuating radon levels might be connected to ambient pressure differences between inside and outside. If so then you could probably lower radon levels by deliberately unbalancing the ventilation so that the hrv is positively pressurizing the building. I will try this when I have the chance.

  5. LLOYD ALTER | | #8

    Just living in Denver for a year is good for 10 mSv. But alpha particles are different, getting right into your lungs. When I practiced as an architect I supervised the decontamination of a building where they painted dials for instruments in WWII. I wore a respirator and Tyvek suit for 6 months and learned to fear those little particles. Now I won’t even have a smoke detector in my house if it isn’t photoelectric rather than ionization, and don’t get me started on granite counters!

  6. gerrha | | #11

    The above article is interesting and provides some useful information on a broad scale. However, from the above discussion, it is also quite clear that radon abatement in a residential structure is not a simple one-size fits all situation.

    Radon is about 8 times heavier than air. Unless stirred up somehow, it wants to hunker down in low spots, which is probably why abatement and basements are always discussed. It has a half-life of about 3.8 days, so a given batch of radon will effectively be gone in a week or two. But, at the same time, chemistry says that all things go from higher concentration to lower concentration. That creates a bit of a tug-of-war with radon. On the one hand, it is so heavy it just wants to lay there in a low spot. On the other hand, it wants to dilute itself into areas of lower concentration.

    Combine these factors with the many types of residential construction and the day-to-day variation of conditions in a given house (AC on/off, furnace on/off, ventilation, windows/doors, fireplace, etc) causes radon abatement to be not straightforward. The current recommendation of installing perforated piping below the house with fans to suck out air is likely the best that can be expected - as long as the fans are on all the time and have the capacity to maintain air pressure under the house lower than in the house.

    There is, however, another point rarely discussed, and this point has to do with water. If you are in radon country and have your own private water well, you likely should be looking very carefully at that water. Water coming from underground will be saturated with whatever gas is down there and that includes radon if it is there, and it will be there, if you are in a radon area. When water comes spraying out of a sink or shower faucet whatever gas it contains will be released straight into the air. Imagine you have spent a lot of money putting in a radon abatement system only to discover you are sucking it in with every breath while showering. Not so good.

    Now, there are a few radon abatement systems designed for private water wells. However, I have spent quite a bit of time looking at those various options, and none of them are inspiring to me.

    Anyway, my two cents...

  7. jeffrey_miller | | #14

    Nice carefully written summary. One sentence is wrong. You write

    ‘So, 2.28 mSv is pretty low on the chart. It’s well below the 100 mSv where scientists can state definitively that you’re likely to get cancer.’

    The second half of this sentence is wrong. 100 mSv per year is the lowest dose where it is clear that the radiation exposure increases cancer risk. .

    Having a detectably higher chance of getting cancer is not the same thing as being likely to get cancer.

    1. GBA Editor
      Allison A. Bailes III, PhD | | #15

      Jeffrey: Thanks for that. I've just updated that sentence to correct it.

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