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.
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.
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.
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.
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.
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
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.
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.
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.
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