Excerpted from the Hardcover Edition
ASSESSING THE RISKS
How Can I Determine My Risk of Cancer From Radiation, and Why Is There So Much Disagreement Among Experts?
On July 16, 1945, in the Jornada del Muerto (Journey of Death) desert near Alamogordo, New Mexico, the fiery explosion of the Trinity test—the first atomic bomb—generated a light brighter than any ever seen on Earth. As it dimmed, it revealed a mushroom cloud of vaporized water and debris that grew thousands of feet into the air. J. Robert Oppenheimer (1904–1967), who more than anyone else was responsible for building the weapon, wrote afterward that watching the explosion brought to mind two lines from the sacred Hindu scripture the Bhagavad Gita: “If the radiance of a thousand suns were to burst into the sky that would be like the splendor of the Mighty One.” And: “I am become Death, the shatterer of worlds.” (It is perhaps more likely that his first thought was, Wow! Thank God, it worked!)
That shattering burst of energy was also an act of creation: it produced radioactive forms of natural elements that—apart from laboratory work during the bomb’s development—had never before existed on Earth, including cesium-137, iodine-131, and strontium-90. During the months that followed these newly created radionuclides circled the globe and silently entered the bodies of everyone alive. And because some of these radionuclides remain radioactive for hundreds or thousands of years, the children of these people, their children, and all humans from that date until our species ceases to exist will have radionuclides created at the Trinity explosion in their bodies. The same is true for the radionuclides released by the more than 450 atmospheric nuclear weapons tests carried out by the United States, the Soviet Union, Britain, France, and China between 1945 and 1980, and from several nuclear power facility accidents. Of course, the amounts of radionuclides released from each of these sources differ vastly. It is inappropriate to consider atomic weapons and nuclear power facility accidents comparable, because the quantity of radionuclides released varies greatly, because they are not uniformly distributed over the Earth, and because different people have different likelihoods of encountering them.
Some of the radionuclides released by nuclear weapons testing and by nuclear power facility accidents can cause cancer. But some of the same radionuclides are used to diagnose and treat cancers and save lives. What is the balance between the potential harm and benefit posed by radionuclides and by all forms of radiation?
To determine whether this balance favors harm or benefit, it is necessary to know what radiation dose a person has received. This is not as simple as it might seem (in fact it is exceedingly complex, even for radiation experts), so we ask the reader please to bear with the following several pages of technical information, knowing that in the end all you really need to remember is one technical term: millisievert (mSv), named for the Swedish medical physicist Rolf Maximilian Sievert (1896–1966), who did pioneering work on the biological effects of radiation exposure. A sievert (Sv) is a unit of potentially harmful radiation. Each year we generally receive a few thousandths of a sievert, called a millisievert. People in the United States on average receive 6.5 mSv of radiation annually.
Radioactivity is measured by the number of atoms decaying (losing energy by emitting radioactive particles and/or electromagnetic waves) in a certain amount of time. The disappearance of a radionuclide is measured by how long it takes for one-half of its atoms to decay. That can take a long time, as something can be reduced by one-half almost forever, until only one atom remains—and then it decays. But most of the starting radioactivity is gone after about 10 half-lives; only about one-thousandth of the starting radioactivity remains.
These measurements have many names, depending on what you want to quantify. At first it is easy to mistake which unit to use, so one can end up comparing the radioactive equivalent of eels to elephants. It is also easy to mistake amounts: 1 microsievert (a millionth of a sievert) is a thousand times smaller than 1 millisievert (a thousand mSv make 1 Sv), yet several news reports of the Fukushima-Daiichi nuclear power facility accident confused these units.
In estimating how a radiation exposure might affect us, scientists need to consider the amount of radiation we are exposed to; what type of radiation it is; how much of it gets into the various cells, tissues, and organs in our body; and how susceptible these tissues and organs are to radiation-induced damage. Some cells, like bone marrow, skin, and gastrointestinal tract cells, are especially sensitive to damage from radiation. One reason is that they divide frequently—rapidly dividing cells are more sensitive to radiation that damage DNA. For example, a normal person needs to produce about 3 billion red blood cells each day to stay healthy. Other cells, predominantly those that divide infrequently, if ever, like heart, liver, and brain cells, are relatively resistant to radiation-induced damage.
So to determine the amount of radiation in an exposure, we must calculate the quantity of radiation emitted or released from a source, be it a CT scanner, a radiation therapy machine, a nuclear weapon, a failed nuclear power facility, or a radioisotope injected for a PET scan.
But calculating the quantity of radiation is complex. Some diagnostic radiation machines emit electromagnetic waves such as X‑rays or particles such as protons, neutrons, or electrons. Other radiation-related activities, like fissioning uranium-235 or plutonium-239 in a nuclear weapon, emit gamma rays and neutrons. Most fission products emit electrons and gamma rays. The explosion of the Chernobyl nuclear reactor released into the environment more than 200 radionuclides in diverse physical and chemical forms, including radioactive gases such as xenon-133 and iodine-124 and -131, as well as radioactive particles. These gases rapidly disperse into the atmosphere. The radioactive particles also disperse across a very broad area—unless it happens to rain when the radioactive cloud passes over you and particles of cesium-137 and strontium-90 fall to the ground with the raindrops.
Unfortunately, when the radioactive plume from the 1986 Chernobyl accident containing particles with iodine-131 and cesium-137 passed over Scotland, it was raining. Consequently, substantial amounts of these radionuclides landed on grass. The grass was subsequently eaten by grazing animals, especially sheep, and those radionuclides were incorporated into their bodies and secreted in their milk. The iodine-131, with an 8-day half-life, was gone in about three months. But the cesium-137 was concentrated in the meat of the sheep, and with its half-life of 30 years, it stayed around for the lifetime of the sheep. The level of cesium-137 in many of these animals exceeded government safety standards; consequently many sheep were killed and buried, and their meat was quarantined from the market.
For a radioactive substance or radionuclide, like a gram of radium-232 or a gram of cesium-137, we can compute how much radiation it releases by considering the number of spontaneous disintegrations (decays) that occur in the nuclei of the atoms in that gram over a certain time interval, for example, one second. This rate of decay, referred to as the amount of radioactivity in radionuclides, is measured in units called becquerels (Bq), named after the nineteenth-century French physicist Antoine-Henri Becquerel (1852–1908). One becquerel equals one nuclear disintegration per second. Because this is an extremely small quantity, scientists often speak of thousands of becquerels (a kilobecquerel, KBq), millions of becquerels (a megabecquerel, MBq), a million million becquerels (a terabecquerel, TBq), or even a billion billion becquerels (a exabecquerel, EBq). It’s like an expression of speed. If a becquerel is a person walking 1 mile per hour, a kilobecquerel is like the same person walking (or rocketing) 1,000 miles per hour, and so forth. However, the quantity of becquerels a substance contains is not the only consideration for human health. Because different nuclear disintegrations release different electromagnetic waves and particles, the same quantity of becquerels released can have substantially different potential health consequences. Also, not all radioactive substances are equally radioactive. When we compare similar quantities of thorium-230 and uranium-234, for example, the thorium-230 is about 1 million times more radioactive—it has 1 million times more disintegrations per second.
Once we have ascertained the amount of radioactivity, we must determine how much radioactive energy it deposits into something. That “something” can be the air, another substance, or our bodies.
Then we must determine how this radioactivity interacts with humans. This is referred to as dose, which is quite different from emitted radiation. Imagine a gram of cesium-137 inside a lead box. It is releasing radiation in the form of electrons and gamma rays, but no one is being exposed to it, because these radiations cannot penetrate the lead. So the dose of radiation to any person is zero, and hence it has no chance for harm to us. But if you are holding this same gram of cesium-137 in your hand, the same electrons and gamma rays that it emits through the spontaneous decay of its nucleus will interact with the skin, muscle, and nerve cells in that hand. And because gamma rays can travel considerable distances and pass through many substances, other parts of your body will be exposed to radiation, although not uniformly. As these gamma rays pass through your cells, they will deposit some of their energy within each cell they strike. This amount of energy is the radiation dose to the cell.
Another concept in radiation dosimetry is the radiation absorbed dose, which is expressed in units of gray (Gy), after the British physicist Louis Harold Gray (1905–1965). A gray is the amount of energy a dose of radiation deposits in a tissue. We will skip over them other than to say that the quantity of grays absorbed into a tissue or organ (adjusted for some biological factors) can be converted to a number of sieverts, the unit used to estimate risk of harm, like cancer, from radiation exposure.
Finally, determining the effective dose, measured in sieverts, considers two issues. First, not all types of radiation are equally damaging—for example, a dose of neutrons absorbed is much more damaging than the same dose of X‑rays. Second, different cells, tissues, and organs in the body, as we saw, have different sensitivities to radiation damage. Effective dose adjusts for these variables and thereby gives a better estimate of the harmful consequences of a radiation exposure. This, at last, brings us to the end of units of radioactivity and activity. But please try to remember millisieverts, as we will translate everything into them from now on.
Having slogged through so many technicalities, let’s examine how scientists analyze radiation emitted, energy absorbed, and biological damage from that radioactivity, so that we can make the one judgment that really matters: What am I exposed to, and is it bad for me? We’ll use a basketball analogy, for simplicity.
When a player sends a basketball through the hoop, the number of points awarded can vary. A free throw is worth 1 point, a basket shot from inside a circumscribed area is worth 2 points, and a basket shot from outside that area is worth 3. The team’s score in a game is not the number of times its players sent the ball through the hoop but the total of the points awarded from those baskets. It’s the same with measuring radioactivity: the amount you are exposed to is not necessarily the amount that you will absorb, and that is not necessarily directly correlated with the amount of harm. Determining that final harm score means weighing and balancing several factors.
If there were a direct correlation between a specific amount of exposure and the onset of disease, a simple chart would clarify things for you. But the relationship between radiation and disease is not entirely linear.
The conventional approach to determining a person’s cancer risk from a radiation exposure is to compare the range of possible effects from the dose in the scientifically accurate but difficult-to-understand units we’ve detailed. When there is a nuclear or radiation accident, public health authorities often give information in terms of what radiation dose people received (or will receive in the future) and/or how much radioactivity is in something they may encounter, such as food or water. They then compare these doses or amounts of radioactivity to a benchmark, such as the normal background radiation dose, or the dose a nuclear power facility worker receives annually, or the regulatory limit or threshold for radioactivity in food or water.
Such information, given to people who are not radiation scientists or physicians, is likely to be uninformative at best and misleading at worst, and it is at once confusing and simplistic. The implication is that if you receive a dose similar to or less than your normal background dose, or less than a regulatory limit for food or water, you need not worry. For example, if the regulatory limit for radioactivity in milk is 500 Bq per liter and the milk you are drinking contains 350 Bq per liter, you are not at risk.
But things are not so simple. For any radiation dose, the risk of getting cancer also depends on one’s age at the time of exposure, estimated remaining life span, exposure to other cancer-causing agents (like cigarette smoke), concurrent health problems that can be exacerbated by radiation, and other complicated variables. Simply put, the implications for an eighty-year-old exposed to a given dose of radiation are entirely different from those for a three-year-old who receives exactly the same dose.
Assessing risk requires statistical analyses. You cannot rely only on dose to express a person’s risk of getting cancer, because dose is only an intermediate quantity between their radiation exposure and their cancer risk. A more helpful way to link cancer risk to exposure is to specify a person’s lifetime risk of cancer regardless of the cause; specify the additional lifetime risk resulting only from a specific radiation exposure; estimate future cancer risk for persons exposed in the past (or who soon will be exposed) and who are currently free of cancer, radiation related or not; or estimate the likely increase in numbers of cancers in an exposed population such as people evacuated from Fukushima.
When we talk about the dangers of radiation, we are usually referring to ionizing radiations, which can alter the structure of atoms, molecules, and chemicals in our cells and cause cancers. Most data suggest that exposure to nonionizing radiations (except UV), like those from TVs, computer screens, high-voltage electrical transmission wires, and the like, are not harmful. This area is controversial and conclusions may change, but the adverse effects of nonionizing radiations, if any, are unquestionably small compared to the proven harmful effects of ionizing radiations like neutrons and gamma rays. The challenge in considering risk of illness from a new exposure to an ionizing radiation—say, from a radiation accident—is to compare it to voluntary and involuntary cancer and noncancer risks in everyday life, like driving a car, riding a motorcycle, flying in a jet aircraft, or going into a basement containing radon gas. By looking at the whole picture, we can weigh the cancer risk from a radiation exposure and decide whether a past exposure is important or whether a future exposure is acceptable.
Excerpted from Radiation by Robert Peter Gale, M.D., and Eric Lax. Copyright © 2013 by Robert Peter Gale, M.D., and Eric Lax. Excerpted by permission of Vintage, a division of Random House LLC. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.