Probability of Causation
Dr. Zoomie, my brother thinks that his cancer is due to radiation he was exposed to at work at a uranium enrichment plant and he’s filed a claim. If his claim is granted then the government will pay his medical bills and give him some compensation, but he’s got to be able to show that his cancer was “more likely than not” caused by his radiation exposure at work. Who makes this determination and how to they do it?
If your brother worked at a uranium enrichment plant then he’s probably looking at the EEOICPA (Energy Employees Occupational Illness Compensation Program Act); and it covers anyone who worked for the Department of Energy, Atomic Energy commission, and Manhattan Project who has cancer, beryllium disease, or silicosis as a result of work they did for one of these agencies or a contractor. The key part is that the claimants need to be able to show that there’s more than a 50% chance that their illness was due to exposure to radiation, beryllium, and/or silica as part of their job. Let me stick with radiation since that’s what I know best (plus, I worked on this project a few decades ago). Here how it works.
Let’s start off with the big picture – roughly speaking, a whole-body radiation exposure of 100 rem (1 Sv) gives a person about a 5% chance of developing a fatal cancer over the rest of their lifetime. So if your brother received a radiation dose of, say, 25 rem to his whole body then he’d have a 1.25% (5% divided by 4) chance that it will give him a fatal cancer. For a cancer to be “as likely as not” to have been caused by a radiation exposure, using this rule of thumb, a person would need to be exposed to a dose of 1001 rem over several years or even decades, which would give just over a 50% probability that the cancer was caused by radiation exposure.
So if your brother can show that he received over 1001 rem working at the enrichment plant then he ought to be in good shape. He’s probably not going to be able to do that – since the early 1990s the annual limit for radiation exposure has been 5 rem; your brother would need to have worked over two centuries to reach this dose, and I’m thinking that he’s not quite that old. In fact, nobody’s that old, and we’ve got to wonder if the EEOICPA might have been written simply to make sure that nobody is ever compensated. Well, it’s not – because things are a bit more difficult than just totting up the lifetime radiation exposure and multiplying by 5%.
See, what’s really important is the amount of radiation of exposure to the part of the body that turned cancerous. Part of the reason is that different types of tissue are more (or less) sensitive to radiation than others – it takes less radiation exposure to damage the radiation-sensitive cells in the bone marrow and it takes more radiation exposure to damage the radiation-resistant cells in our brains. So radiation exposure to the whole body is nice to know, but radiation exposure to the exact site where the cancer formed is essential. This requires a bit of calculation, even if he was wearing radiation dosimetry. And here, maybe an example would help.
Say your brother was working at a place where they milled or extruded uranium. He’d have been exposed to a low dose of radiation from the uranium itself – but not much because uranium isn’t highly radioactive. But thin slivers of uranium and uranium dust will spontaneously combust, putting uranium oxide smoke into the atmosphere; if your brother breathed in some of this smoke, giving him radiation exposure to his lungs. So if he had lung cancer, we’d need to find a way to calculate how much radiation his lungs were exposed to in order to determine the odds that his lung cancer might have been caused by inhaling uranium. But what if he didn’t have lung cancer?
Well…uranium that’s in the lungs will dissolve into the body’s fluids, which will transport it around the body. Some of the uranium will settle in the bones, exposing the cells of the bone and (maybe) the bone marrow to radiation. Uranium also collects in the kidneys, forming nodules if there’s enough of an intake – this can pose a toxicity issue as well as the radiation exposure, but it’s the radiation dose that’s of interest. And if we have information from past air sampling, we can use the airborne uranium concentrations to determine how much smoke might have been breathed in and the dose the lungs would have received. Further calculations can help us calculate how much uranium entered the bloodstream, how much of that collected in the bones and kidney, and to determine the resulting radiation dose to these organs. And with that information and knowledge of the sensitivity of these organs to radiation exposure, we can determine the odds that a cancer was caused by occupational radiation exposure.
There are a lot of factors to consider – the size of the particles in the lungs, their solubility, their chemistry, But we can make reasonable guesses about all of these factors – and more – to come up with the most likely radiation exposure to all of the organs to which the uranium might have traveled once it got into the bloodstream and before it was excreted from the body. If there’s other information available – air sample results, radionuclide concentrations measured in urine samples, and so forth – these can be added to the mix. And these calculations are made, year by year, for every year your brother worked with radioactivity.
From here, knowing how sensitive the various organs are to the effects of radiation, we can calculate the probability that a particular cancer in a particular organ might have been caused by occupational radiation exposure. This part of it was calculated using a program called IREP (Interactive RadioEpidemiological Program) that took all of this information, including the radiation sensitivity of various organs, to determine the likelihood that a particular cancer might have been cause by the worker’s particular history of radiation exposure. If, after entering all of the person’s information and running it through IREP, the calculation showed that the probability of causation was higher than 50%, the conclusion was that the person’s cancer was “as likely as not” caused by occupational radiation exposure.
The process itself could vary from fairly simple to very complex, depending on the particulars of any individual person. People with a long work history, who inhaled or ingested radioactivity, who had a lot of work-related x-rays, who lacked reliable dosimeter readings, or any of a number of other complicating factors – these cases could take a few to several days to calculate. And others were fairly straightforward and could be determined in only a few hours. And everything in between.
Finally, I would caution you to not try to use the IREP program yourself. At the time I was doing this I had over 30 years of experience working in radiation safety and it took a week of training and another few weeks of doing calculations under the supervision of my boss before I was more or less competent to go through this process. And even then, I made mistakes in my first cases. It’s a complex process and, unless someone knows what they’re doing, the answers they get from the software are likely to be wrong. So my recommendation would be for your brother to let his claim go through the process – the people entering the data and doing calculations know what they’re doing, they’re well-trained and well-supervised, and they’re told to do the best job they can. Your brother’s case will be in good hands.