It's the Dose That Makes the Poison: The Importance of Numbers

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Sometimes people claim that nuclear radiation is uniquely scary because you can't see it, smell it, or detect it with any of the human senses, as though "ordinary hazards" were not so sneaky. We're even told that a single gamma ray can kill us--a statement that affronts both science and common sense. Like all such claims, we need to examine this one in light of real-world experience, not by the exchange of uncheckable rumors.

Let's look first at other, non-nuclear, hazards that surround us daily. How about germs that fill the air when people around us sneeze, cough, or just breathe? How good are we at detecting germs? From birth to death, we are continuously immersed in germs, but healthy bodies protect us from harm. We don't need to see them. Similarly, we are continuously irradiated by cosmic rays and the natural radiation of the soil, water, air, and the flesh of our bodies. Life first arose on a prehistoric Earth many times more radioactive than today's.

We minimize our chances of getting disease not by trying to develop germ-free bodies (which would not be compatible with life) but by trying to maintain a healthy immune system. Similarly, the natural radiation levels we encounter just by living are not a danger, and that presumably is why humans have developed no organs to detect radiation. It is not normally a danger. Of course, there are levels of germs and levels of radiation that are harmful, but we don't normally encounter them in the natural world.

Aha! But how about the unnatural world of nuclear reactors? I won't tackle the theological issue of whether we can properly exclude humankind and its products from "the natural world" to identify things warranting extra concern. Let's just look at the numbers. The proto-scientist Paracelsus said in 1540 that nothing is poison but the dose makes it so. That is the basis for vaccination. That is why ancient king-poisoners knew they could safely swallow a small mouthful of poisoned food, which in larger amounts could kill. It's a fundamental principle of biology that assaults that don't kill you make you stronger.

So the relevant numbers here are those revealing that the average natural radiation level where people live is hundreds of times the radiation levels permitted for exposure to the public from nuclear reactors and other sources of human-made radiation. Natural radiation levels on this radioactive Earth vary hundreds-fold from one place to another. (And indeed the measured fact is, the higher the background radiation the lower the cancer rate.) If we had no radiation whatsoever from nuclear power plants, we could hardly measure the difference in human exposure, because the nuclear contribution to our total radiation dose is trivial.

A further irony is that our dose from increased use of medical irradiation in life-saving procedures is now commensurate with high natural backgrounds, further invalidating concern over the negligible doses from nuclear facilities.

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The facts just stated show how we should respond to occasional reports of leakage or other incidental exposure of radioactivity. The phrase "exposed to radiation" tells us nothing about hazard. It's the dose that makes the hazard (if any). The anti-nuclear activist Sheldon Novick correctly noted that "nuclear waste" is no more hazardous than many other industrial wastes, so if we hear of a spill of radioactivity, we should judge its hazard as we would a spill of oil or any other biologically harmful chemical, asking: What's the toxic dose?

While we're talking numbers, let me take a moment to talk units. The unit of radioactivity used to be the Curie, which was about one gram of radium. A gram is one twenty-eighth of an ounce, so that's a reasonable unit. When the metric system came in, the unit of radioactivity became the Becquerel, named after another radiation research pioneer. The Becquerel (Bq) is a single atomic disintegration per second. There are 37,000,000,000 Bq in a curie. So levels of radioactivity that are entirely harmless are now measured in millions or billions of Bq, instead of a few milliCuries.

What does all this mean in the real world? One example is the case of tritium, which is an isotope of hydrogen that usually shows up as water. It is used widely to light exit signs without electricity. It doesn't stay in the body long, and its radiation is low-energy. So its biological impact if ingested is low, and the U.S. permissible limit in drinking water is 20,000 picoCuries (millionths of a millionth of a Curie) per liter, which is estimated to give the body a 4 mrem dose. This is about 1% of the average dose we receive from natural radiation background.

Recently, some tritium was detected in the soil near the Vermont Yankee nuclear plant, leading quickly to demands that the nuclear industry be shut down. Previous minor leaks elsewhere led to industry promises to make all plants "zero leakage." I've always avoided, as a matter of principle, promising there will be no more accidents. One cannot be sure to deliver on such promises. What we should promise is that we will ensure that accidents will not cause significant harm to people or the environment. And that is the situation here. If we look at the potential dose (i.e. toxicity) from such a leak, we find that it is considerably less than from numerous small leaks of oil and other chemicals that occur from time to time.
In these more hazardous cases of non-radioactive spills, we devote a modest effort to cleaning up the area, leaving earth or concrete permanently stained and contaminated to the degree that, if you tried to eat it, you might suffer some deleterious effects. And we rightly conclude, that's a reasonable level of response. (Maybe we should post OSHA warning signs: DO NOT EAT THE DIRT.) There is no realistic justification for requiring that all the ground be decontaminated to hospital or "clean room" standards.

Radioactivity is not unnatural, nor uniquely hazardous per se. Radioactivity does not multiply like germs; instead it decreases in toxicity. We should stop viewing radiation as demonic and recognize its place in the natural world, and its special role in healing. Even after large spills of oil (and there are many), we don't demand that the oil industry be shut down. Let us base our judgments of danger on measured toxicity, not some panic-driven demand for a radiation-free planet.

Ted Rockwell is a chemical engineer, a former advisor to the Princeton engineering department and to Navy's nuclear program, an author of both fiction and non-fiction, and co-founder of engineering firm MPR Associates.