Nick Stockton, a reporter for Wired magazine, sent me some questions about radiation risk and radon, and Phil and I replied. I thought our responses might be of general interest so I’m posting them here.

First I wrote:

For homes with high radon levels, radon is a “dangerous, proven harm,” and we recommend remediation. For homes with low radon levels, it might or might not be worth your money to remediate; that’s an individual decision based on your view of the risk and how much you can afford the $2000 or whatever to remediate.

Regarding the question of “If the concerns about the linear no-threshold model for radiation risk are based on valid science, why don’t public health agencies like the EPA take them seriously?” I have no idea what goes on within the EPA, but when it comes to radon remediation, the effects of low dose exposure aren’t so relevant to the decision: if your radon level is low (as it is in most homes in the U.S.) you don’t need to do anything anyway; if your radon level is high, you’ll want to remediate; if you don’t know your radon level but it has a good chance of being high, you should get an accurate measurement and then make your decision.

Low dose risk is inherently difficult to estimate using epidemiological studies. I’ve seen no evidence that risk is not linear at low dose, and there is evidence that areas with high radon levels have elevated levels of lung cancer. When it comes to resource allocation, we recommend that measurement and remediation be done in areas of high average radon levels but not at a national level; see here and here and, for a more technical treatment, here .

Then Phil followed up:

The idea of hormesis [the theory that low doses of radiation can be beneficial to your health] is not quackery. Nor is LNT [the linear no-threshold model of radiation risk].

I will elaborate.

The theory behind LNT isn’t just ‘we have to assume something’, nor ‘everything is linear to first order’. The idea is that twice as much radiation means twice as many cells with damaged DNA, and if each cell with damaged DNA has a certain chance of initiating a cancer, then ceterus paribus you have LNT. That’s not crazy.

The theory behind hormesis is that your body has mechanisms for dealing with cancerous cells, and that perhaps these mechanisms recognize become more active or more effective when there is more damage. That’s not crazy either.

Perhaps exposure to a little bit of radiation isn’t bad for you at all. Perhaps it’s even good for you. Perhaps it’s just barely bad for you, but then when you’re exposed to more, you overwhelm the repair/rejection mechanisms and at some point just a little bit more adds a great deal of risk. This goes for smoking, too: maybe smoking 1/4 cigarette per day woud be good for you. For radiation there are various physiological models and there are enough adjustable parameters to get just about any behavior out of the models I have seen.

Of course what is needed is actual data. Data can be in vitro or in vivo; population-wide or case-control; etc.

There’s fairly persuasive evidence that the dose-response relationship is significantly nonlinear at low doses for “low linear-energy-transfer radiation”, aka low-LET radiation, such as x-rays. I don’t know whether the EPA still uses a LNT model for low-LET radiation.

But for high-LET radiation, including the alpha radiation emitted by radon and most of its decay products of concern, I don’t know much about the dose-response relationship at low does and I’m very skeptical of anyone who says they do know. There are some pretty basic reasons to expect low-LET and high-LET radiation to have very different effects. Perhaps I need to explain just a bit. For a given amount of energy that is deposited in tissue, low-LET radiation causes a small disruption to a lot of cells, whereas high-LET radiation delivers a huge wallop to relatively few cells.

An obvious thing to do is to look at people who have been exposed to high levels of radon and its decay products. As you probably know, it is really radon’s decay products that are dangerous, not radon itself. When we talk about radon risk, we really mean the risk from radon and its decay products.

At high concentrations, such as those found in uranium mines, it is very clear that radiation is dangerous, and that the more you are exposed to the higher your risk of cancer. I don’t think anyone would argue against the assertion that an indoor radon concentration of, say, 20 pCi/L leads to a substantially increased risk of lung cancer. And there are houses with living area concentrations that high, although not many.

A complication is that the radon risk for smokers seems to be much higher than for non-smokers. That is, a smoker exposed to 20 pCi/L for ten hours per day for several years is at much higher risk than a non-smoker with the same level of exposure.

But what about 10, 4, 2, or 1 pCi/L? No one really knows.

One thing people have done (notably Bernard Cohen, who you’ve probably come across) is to look at the average lung cancer rate by county, as a function of the average indoor radon concentration by county. If you do that, you find that low-radon counties actually have lower lung cancer rates than high-radon counties. But: a disproportionate fraction of low-radon counties are in the South, and that’s also where smoking rates are highest. It’s hard to completely control for the effect of smoking in that kind of study, but you can do things like look within individual states or regions (for instance, look at the relationship between average county radon concentrations and average lung cancer rates in just the northeast) and you still find a slight effect of higher radon being associated with lower lung cancer rates. If taken at face value, this would suggest that a living-aread concentration of 1 pCi/L or maybe even 2 pCi/L would be better than 0. But few counties have annual-average living-area radon concentration over about 2 pCi/L, and of course any individual county has a range of radon levels. Plus people move around, both within and between countties, so you don’t know the lifetime exposure of anyone. Putting it all together, even if there aren’t important confounding variables or other issues, these studies would suggest a protective effect at low radon levels but they don’t tell you anything about the risk at 10 pCi/L or 4 pCi/L.

There’s another class of studies, case-control studies, in which people with lung cancer are compared statistically to those without. In this country the objectively best of these looked at women in Iowa. (You may have come across this work, led by Bill Field). Iowa has a lot of farm women who don’t smoke and who have lived in just a few houses for their whole lives. Some of these women contracted lung cancer. The study made radon measurements in these houses, and in the houses of women of similar demographics who didn’t get lung cancer. They find increased risk at 4 pCi/L (even for nonsmokers, as I recall) and they are certainly inconsistent with a protective effect at 4 pCi/L. As I recall — you should check — they also found a positive estimated risk at 2 pCi/L that is consistent with LNT but also statistically consistent with 0 effect.

So, putting it all together, what do we have? I, at least, am convinced that increased exposure leads to increased risk for concentrations above 4 pCi/L. There’s some shaky empirical evidence for a weak protective effect at 2pCi/L compared to 0 pCi/L. In between it’s hard to say. All of the evidence below about 8 or 10 pCi/L is pretty shaky due to low expected risk, methodological problems with the studies, etc.

My informed belief is this: just as I wouldn’t suggest smoking a little bit of tobacco every day in the hope of a hormetic effect, I woudn’t recommend a bit of exposure to high-LET radiation every day. It’s not that it couldn’t possibly be protective, but I wouldn’t bet on it. And I’m pretty sure the EPA’s recommended ‘action level’ of 4 pCi/L is indeed risky compared to lower concentrations, especially for smokers. As a nonsmoker I wouldn’t necessarily remediate if my home were at 4 pCi/L, but I would at least consider it.

For low-LET radiation, I think the scientific evidence weighs against LNT. If public health agencies don’t take LNT seriously for this type of radiation it’s possible that they acknowledge this.

For high-LET radiation, such as alpha particles from radon decay products, there’s more a priori reason to believe LNT would be a good model, and less empirical evidence suggesting that it is a bad model. It might be hard for the agencies to explicitly disavow LNT in these circumstances. At the same time, there’s not compelling evidence in favor of LNT even for this type of radiation, and life is a lot simpler if you don’t take LNT ‘seriously’.