What's the Story with Radon?

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When it rains, when a low-pressure system passes by, and when water soaks the soil, it's not a great day to sample for radon. Those conditions force radon out of pore spaces and into basements.

It could be worse, though – if I were writing this in, say, November or December instead of May, then the ground would be contracting, forcing radon out of pore spaces, and radon levels would be even higher. A rainy day – especially when the soil is cooling off, will give us higher radon measurements.

If I could choose a time to do a radon sample, I’d prefer a dry day (ideally a dry week or two) with a high-pressure cell parked over my city – maybe sometime in April or May. The high-pressure air will help keep the radon locked in the soil, there’s no rain to fill the pores (forcing radon out), and the warming soil will be expanding and holding the radon better.

That’s one of the problems with radon testing – if the weather is cold and rainy, my basement will fail the test; do it again six months later on a warm, dry day, and the same basement will pass with flying colors. Neither test will give a good idea of what the typical radon levels are in the basement – one will be too optimistic, the other pessimistic. 

Even a two-week sample is only a snapshot in time; to get a good idea about typical radon levels, you need to do several one or two-week tests or, better, do a test that lasts a few months. The more and more prolonged the tests, the more likely it is that you’ll know what your typical radon levels are, and that’s what’s important. 

I’d be reluctant to decide to install an expensive radon mitigation system based on just one test, especially when testing is so much cheaper than mitigation. But maybe I should back up a little bit and talk about what radon is and its source.

What is Radon?

We need to start with uranium – particularly U-238, which comprises more than 99% of all the uranium on Earth.

When U-238 decays, it emits an alpha particle – two protons and two neutrons – and turns into thorium – specifically, Th-234. That decays, emitting another beta particle. Ultimately the uranium-238 decay series has over a dozen steps – each radioactive – before finally ending as stable lead. Of those steps, only radon is a gas. But radon is more than just a gas – it’s a “noble” gas, denser than air, and the great majority of what we measure in our basements.

Being a noble gas means that radon is not chemically active; it will not be locked in the soil due to chemical reactions with the soil minerals. When the radon forms, it can collect in pore spaces and – eventually – make its way into your home.

Being denser than air means that it tends to collect in basements, crawl spaces, or in the first floors of homes that lack a basement or crawl space. If you spend a lot of time in the basement, then your radon exposure is likely a bit higher than if you don’t. 

Radon emits alpha radiation which, when it strikes living cells, can do a lot of damage. Alpha radiation is 20 times as likely to cause cancer as are comparable amounts of beta or gamma radiation. Each radon atom produces three more alpha-emitting progeny before ending up as stable lead. A single radon atom can produce a total of four alpha particles. 

But as long as alpha radiation remains outside your body, it can’t hurt you – alpha particles can’t even penetrate through the thin layer of dead skin cells to reach the living cells beneath. But since radon is a gas, if it’s in the air, we can breathe it in – that’s why people worry. But because radon is heavier than air, it tends to sink to the lowest part of a building – unless you spend a lot of time in the basement, you won’t be exposed to much of it. In a five-story apartment building, most of the tenants will be living in virtually radon-free apartments (that includes me since I live on the second floor). 

How much radon does it take to be harmful? 

When I was selling my home, I was told to do a radon test and that as long as the results were lower than four pCi/Liter, pico-curies per liter, then everything was OK – but any higher readings might require spending time and money on trying to reduce levels to under 4. [1] The question is whether or not this is enough radioactivity to hurt me – do I really need to spend, say, $5,000 trying to reduce four pCi to 3 or 2?

According to the EPA, you should, if possible, try to reduce radon levels to less than two pCi/liter. For anything higher than four pCi/L, the EPA guidance simply says, “fix your home.” On the other hand, the EPA also states that 

  • the risk of living in a home with radon concentrations at four pCi/L is about the same as the risk of dying in a car accident, 
  • the risk of living in a home with radon concentrations of 20 pCi/L is 35 times as high as the risk of dying from drowning. 

That sounds pretty serious – there are about 4000 drowning deaths annually in the US, so if everyone were living in a home with 20 pCi/L of radon, then we’d expect to see 140,000 radon-related deaths every year. That’s a lot.

But how much time do you spend in your basement? When I was a homeowner, I didn’t spend much time in the basement – I was down there to do laundry, to get food out of the freezer, doing work in the small workshop area, and that sort of thing. All in all, I spent less than 10% of my time in the basement. And unless I was in the basement, I wasn’t exposed to the stuff. 

It really didn’t matter if radon concentrations were 20 pCi/L because I spent so little time there. While the risk of living in my basement might have been comparable to the threat from driving, the actual risk is lower because we have to consider the amount of time we spend in that atmosphere. Most people don’t understand this; what they know is that they’re supposed to keep their radon less than four pCi/liter.

Is the radon in my basement going to hurt me?

There are very few people whose homes have enough radon to be genuinely harmful. The reason is that radon concentrations change with the weather and the season, and most of us just don’t spend much time in our basements. 

Do I need to spend money to reduce the radon levels in my basement?

The only answer that makes sense is, “it depends.” 

If I’m living in a home, then I’m concerned about whether or not the radon is a danger to my family and me – and most of the time, it isn’t. If I’m not trying to sell a home, I’m not likely to worry much about radon concentrations higher than four pCi/L because I don’t spend much time in the basement. 

If I’m trying to sell a home, I need to do what I can to sell the house – I’ll try to reduce the radon concentrations to less than four pCi/L. If a relatively short radon test shows that levels are, say, five or six pCi/L, I’ll do a longer test in case weather was causing the problems. If they’re still high, I’d be more likely to try blocking the easy paths for radon to enter a basement.

  • painting the basement walls (to stop radon seeping through cinder block)
  • painting the floor with epoxy paint (to stop infiltration through the slab)
  • covering the sump and caulking or sealing any cracks in the walls or floor 

These are all reasonably effective, cheap, and quick fixes. 

The only way I’d get more involved (and spend thousands of dollars to install a sub-slab ventilation system) would be if these easy measures weren’t able to get radon levels low enough to sell my home. And even then, I’d be looking at the cost of installing a radon mitigation system versus the impact on the estimated sale price of my house. 

Is radon a health risk? 

The way the EPA calculates things, it is. The EPA feels that radon is responsible for 21,000 lung cancer deaths annually. I have to say that, as much as I respect the EPA, it’s hard for me to agree with them on this one – if the EPA is correct, then radon is responsible for about 12% of American lung cancer fatalities each year. That number is based on what seems to be inflated estimates as to how much time most Americans spend in their basements and the number of Americans living in homes with basements. 

About 20 years ago, I attended a scientific conference in Tehran, Iran. As part of the conference, many of us took a trip to Ramsar, a city on the Caspian Sea with the highest natural radiation levels of any populated place in the world. Among other things, we measured radon concentrations in a well-ventilated room in one home in the city – we counted 35 pCi/L. Since the windows were open with a breeze fluttering the curtains, we can assume that the great outdoors had a similarly high level of radon, which (to me) was astonishing – I’m more accustomed to outdoor radon concentrations in the vicinity of less than one pCi/L. 

When we did our study, we found that the population of Ramsar has the same lung cancer rates as any other area in Iran. Clearly, these radon concentrations are not hurting the people in Ramsar. To me, the EPA numbers are worst-case estimates.

Is radon a health risk? As with so many other substances, the correct answer is “it depends.” In this case, indeed, with any case of radiation exposure, it depends on is the total radiation dose a person receives. For radon, that depends on two primary factors

  • how much radon is in the air one is breathing 
  • how much time do you spend breathing that air 

Most of us spend too little time in our basements for a radon concentration of 4 pCi/L to be a huge risk.


[1] Since I work with radiation for a living, I know that a pico curie is a millionth of a millionth of a curie – and four pCi is comparable to the amount of radioactivity I’d expect to find in a shovel-full of dirt I dig up in the back yard.