If you want to start with Part 1 of “Radioactivity and Risk,” click HERE. This post is part of a loose series, most of which were regular Wednesday posts that unfolded in the wake of the nuclear accident at Fukushima Daiichi. We include the whole list at the end of this post. Here is “Radioactivity and Risk (Part 2)”:
One of the risks every astronaut faces in orbit and beyond is the exposure to radioactivity. Last Friday, when we were in Florida for the not-launch of Endeavour (which as of today is delayed at least until May 16), we spoke with astronaut Michael Barratt about this particular risk. He’s interested in this topic not only because he spent almost 200 days on the International Space Station, which exposed him to a lot of radiation (from which the rest of us on the ground are better protected by Earth’s atmosphere), but also because he is a medical doctor who has studied the exposures and effects of radiation and written about it in his book Principles of Clinical Medicine for Space Flight. One of the problems he pointed out in understanding the risk astronauts face from radiation is that astronauts are a relatively small population for medical study. Even so, he said that recommended radiation exposure guidelines for astronauts have become more conservative in recent years and now are also weighted for gender, weight, age, and other health criteria.
That’s the tricky thing about exposure to radioactivity: it’s difficult to predict its effect on any specific individual. In fact, most current assessment of the risk of exposure is based on the atomic bomb survivors of Hiroshima and Nagasaki, a much larger population than the astronaut corps, but problematic for extrapolating to ourselves today. Excepting Chernobyl workers, most of us are not exposed to very large blasts of radioactivity. That said, the radiation to which we are exposed today has increased since the late 1940s.
We have seen a dramatic rise in exposure to radioactivity used in medicine, such as CT scans, though also mammograms, dental x-rays, and other diagnostic and treatment procedures. Last year, at ScienceWatch.com, Director of the Center for Radiological Research at Columbia University David Brenner said, “In the US, the average radiation dose to which we are exposed has doubled in the past 30 years. The average dose from natural background sources has not changed, but what has changed is a more than six-fold increase in the average radiation dose from medical imaging.” In 2008, Time noted of CT, or CAT, scans, “some physicians are raising concerns about the safety of such procedures—most notably, an increase in cancer risk. A CT scan packs a mega-dose of radiation—as much as 500 times that of a conventional X-ray.” One study in the article raised additional concern about the 41% of patients undergoing CT scans who had already undergone two or more scans. While the benefit of having a CT scan may more than offset the risk of radiation exposure, Brenner also points out that “at the doses corresponding to a few CT scans there are direct epidemiological data from about 30,000 A-bomb survivors who were on the peripheries of Hiroshima and Nagasaki, and who were exposed in this low-dose range. This low-dose subpopulation has been followed for more than 50 years, and shows a small but statistically-significant increased cancer risk.”
In addition to having more medical tests than in previous decades, we’ve increased air travel. The more you fly—and the higher you fly—the more radiation to which you’re exposed. According to the Environmental Protection Agency, “For a typical cross-country flight in a commercial airplane, you are likely to receive 2 to 5 millirem (mrem) of radiation, less than half the radiation dose you receive from a chest x-ray.”
Of course that doesn’t include that extra, scattered smattering if you fly out of an airport with the new body scanners. Wired reported in March that the Transportation Security Agency (TSA) “mandated [backscatter X-ray machines] as the preferred airport screening method in February 2009” but is reevaluating the more than 500 scanners in at least 78 or so airports “after testing produced dramatically higher-than-expected results.” An earlier article in the Wired series about the backscatter X-ray machines discussed a group of scientists raising concerns and quoted one of them as especially concerned with the increased risk when exposed to x-rays as we get older. In a CNN piece last year, David Brenner said, “If you think of the entire population of, shall we say a billion people per year going through these scanners, it’s very likely that some number of those will develop cancer from the radiation from these scanners[.]”
When it comes to risk, we’re talking probabilities. The risk that one airport scan poses to one person is very, very small. Almost no one will develop cancer just because they take a few jaunts to Europe. But the risk is not zero. And once the group exposed is large enough, then statistics indicate that someone will develop cancer as a result of exposure to radioactivity that is, for each individual, not very risky at all. In addition, as Michael Barratt pointed out in our interview with him, we’re not each equally vulnerable or hearty. What if you’re a weekly business traveler from Denver (the higher above sea level you live, the greater the exposure to cosmic radiation) who has undergone radiation treatment for cancer, whose father died of cancer, and who had a couple of CT scans after a car accident several months ago?
Even for those workers who know they may be exposed to ionizing radiation, the risk is not always clear to them. The average nuclear power plant worker in the United States is exposed to 300 mrem whole body equivalent, in addition to the presumed average of 300 or 360 mrem background radiation to which the average American is exposed, depending on which source you read. According to an article in the American Journal of Public Health, “In the United States, regulatory standards allow workers to be exposed to ionizing radiation that can cause 1 additional cancer fatality per 400 workers per year. Because radiation-dose limits cover only single sources (e.g., a nuclear plant) or exposure classes (workplace, medical, or public) and are defined for average occupational exposure, workers typically do not know their precise cumulative, individual, and relative risks from radiation.” In other words, no individual seems to know how much exposure to radioactivity he or she faces, nor the risk of cancer that exposure poses long term.
If we don’t understand the risk, we can’t manage it very well. If the level of risk is unknown or unclear, it’s difficult to weigh a given risk against the benefits. Clearly, many people don’t think twice about taking a cross-country flight. Maybe that’s because we’ve heard that the average American is more likely to die in a car accident than in a plane crash. In fact, according to NOVA in 2006, the chance of dying in a plane crash is 1 in 11 million, whereas the chance of dying in a car accident is 1 in 5000. But does knowing that keep you from getting into a car? Our perception of risk doesn’t always line up with the facts. And who is this average American anyway? As NOVA states, “[Y]ou are not the average American. Nobody is.”
In addition to today’s post, check out our previous posts in our Radioactivity Series as follows (CLICK on the title):
March 16: Measurement and Scale
March 28: Three Mile Island Anniversary
March 30: Radiation vs. Radioactivity
April 6: Uranium & Plutonium & Fission
April 13: Fission Products & Half-Lives
April 20: Radioactivity Units of Measure
April 26: The Anniversary of Chernobyl
April 27: Nuclear Secrecy