How does anyone know what a “safe” level of radiation is?

Excellent question, especially since many people have been led (incorrectly) to believe that all radiation should be considered dangerous. However, before we can answer the question, we first need to discuss the meaning of the word radiation and how radiation exposure is measured.

In science, the word radiation refers to any kind of energy transmitted by either waves or particles. This means that even ordinary, visible light is a form of radiation, since all forms of light travel through space as electromagnetic waves (that is why light is also called “electromagnetic radiation”; see the Brief Review box in Chapter 6). Clearly, then, not all forms of radiation are dangerous.

For radiation to be considered dangerous, it must be of a type that can cause biological damage; that is, damage to living tissue. Dangerous forms of radiation include ultraviolet light, x-rays, gamma rays, and various subatomic particles , including those called alpha particles (which are helium nuclei) and beta particles (which are electrons).

Different forms of dangerous radiation can cause different levels of biological damage. As a result, scientists have developed a special unit, called the sievert (Sv) , to measure the overall biological effect of an absorbed dosage of radiation, regardless of the particular type of radiation. The larger the radiation dose in sieverts, the greater the potential damage that can it can cause to living tissue. In practice, the doses are usually small enough that they are measured in millisieverts (mSv).

We can now rephrase our original question as: What is a “safe” dosage of radiation in millisieverts (mSv)? Scientists can answer this question by considering the dosages that we receive from the natural radiation around us.

Exposure to natural radiation varies significantly around the world, depending both on local geology and altitude. In most places, the average radiation dosage that people receive from natural sources varies between about 1 and 10 millisieverts per year (mSv/yr), but there are a few places with large populations where the annual dosage is more than 100 mSv/yr. Importantly, careful studies have not found significant health or life span differences based on these differing doses of natural radiation (except in places where radon becomes concentrated indoors). In other words, this evidence suggests that doses up to 100 mSv/yr should be considered safe.

 
So how does this safe dose compare to what you’d get from a nuclear power plant? Measurements show that even if you lived next door to the plant, your annual dosage would be only about 0.01 mSv/yr, which is far less than the average of 1 to 10 mSv/yr that we receive from natural sources. In fact, even if you have a full time job working in nuclear power plant, data show that you’ll generally receive a dose of less than 5 mSv/yr, still less than the natural average. Moreover, studies of people who have spent decades working at nuclear power plants have found no increases in cancer rates or decreases in life span, which brings us back to the conclusion stated in the main text: A properly operating nuclear power plant does not pose any radiation danger either to its workers or to the public.