I’ve been researching the topic of nuclear radiation, and so I’ve decided to update an older post: What Level of Radiation Exposure is Safe? First, a brief introduction to the topic.
Radiation exposure is measured in two different ways:
1. equivalent dose, using the units rems or sieverts (Sv)
2. absorbed dose, using the units rads or grays (Gy)
The units rads and rems are still common in the U.S. The SI units grays and sieverts are more common in most other nations.
1 gray (Gy) = 100 rads
1 sievert (Sv) = 100 rems
Equivalent dose is simply the amount of ionizing radiation (radiation capable of damaging cells by kicking electrons out of atoms) to which an object or person is exposed. It is an objective measure of radiation level. Absorbed dose starts with the equivalent dose, and then adjusts the measurement based on a “quality factor” that takes into account how damaging that radiation will be to the human body. Most radiation (x-rays, gamma, beta) has a quality factor of one, meaning that 100 rem = 100 rad and 1 sievert = 1 gray. So for many purposes, one can use rems and rads (or sieverts and grays) interchangeably.
However, alpha radiation has a quality factor of 20, meaning that 5 rems of alpha radiation does 20 times the damage of 5 rems of gamma radiation. Thus, 5 rems of alpha radiation is equal to 100 rads. In other words, 5 rems of alpha radiation does as much damage as 100 rems (= 100 rads) of gamma radiation.
Does this make alpha radiation the worst type of radiation? Not exactly. Alpha radiation is a particle composed of two neutrons and two protons (the same as a helium molecule without its electrons). Certain types of radioactive material decay by emitting alpha particles. These particles are stopped by a thin layer of clothing. They can literally be stopped by a sheet of paper. And they cannot penetrate the skin. However, they are dangerous if inhaled, or if emitted by radioactive material elements that are in the body. Essentially, alpha radiation does little harm, despite its 20x quality factor, as long as it is outside the body.
Beta radiation is high-energy electrons. These can do some damage to the skin if exposure is exterior to the body. The main danger, as with alpha radiation, occurs if the radiation is internal.
Gamma radiation penetrates walls, various obstacles, and the body thoroughly. It is the most damaging type of radiation. Gamma rays are not particles, like alpha and beta radiation. They are similar to x-rays, but at a higher frequency.
For x-ray, gamma, and beta radiation, 1 rem is the same as 1 rad, and 1 sievert is the same as 1 gray. For alpha radiation, absorbed interior to the body, multiply by 20 to convert from rem to rad (or sievert to gray). Alpha radiation is very damaging internally. For exterior exposure, gamma radiation is the most harmful
What are the effects of different levels of exposure?
A millisievert (mSv) is one thousandth of a sievert (Sv). A micro-seivert (uSv) is one millionth of a Sv.
1.0 Sv = 100 rems
1.0 mSv = 100 mrems (millirems)
100 mSv = 10 rems
3.1 mSv (0.31 rems) is the average normal background level of radiation per year in the U.S. A CAT scan (3D x-ray) ranges from 5 to 20 mSv in radiation level. So a person might be exposed to as much as 25 mSv in a year (2.5 rems), from medical x-rays and background radiation combined, without the level of exposure being too unusual.
The U.S. Nuclear Regulatory Commission (NRC) limits occupational exposure in the nuclear industry to 50 mSv/year (5 rem/year). If there is a nuclear power plant disaster or fallout from a dirty bomb or a nuclear bomb, you want to keep your exposure to radiation below 50 mSv (5 rems). There is no statistically-verifiable increase in cancer at this level. But researchers believe that any level of radiation exposure increases cancer risk, at least a slight amount.
In the range of 50 to 100 mSv (5 to 10 rems), there are harmful changes to blood chemistry. Then 100 mSv (10 rem) is the threshold for harm to a developing fetus. The range of 100 to 200 mSv (10 to 20 rems) is the threshold for a statistically-significant, but small increase in the risk of cancer. You definitely want your radiation exposure to be well below 100 mSv (10 rem).
The EPA guidelines permit emergency workers to be exposed to a maximum of 100 mSv (10 rems) for the protection of valuable property, and up to 250 mSv (25 rems) for the protection of the health and lives of a large population.
At 500 mSv (50 rems), persons will begin to show signs of radiation sickness. These effects worsen with an increase in radiation exposure, up to 1.5 Sv (150 rems). At about 1.5 Sv (150 rems), some healthy persons may die. But the very young, very old, or chronically ill may die from a level of exposure well under 1.5 Sv.
“Although radiation affects different people in different ways, it is generally believed that humans exposed to about 500 rem [5.0 Sv] of radiation all at once will likely die without medical treatment. Similarly, a single dose of 100 rem [1.0 Sv] may cause a person to experience nausea or skin reddening (although recovery is likely), and about 25 rem [250 mSv] can cause temporary sterility in men. However, if these doses are spread out over time, instead of being delivered all at once, their effects tend to be less severe.” [NRC: High Radiation Doses]
No particular dose of radiation will affect each person in the same way. When a dose of radiation is lethal, except for very high doses, it kills only a percentage of persons exposed to that level of radiation. The LD50 concept is used as a measure of lethality. The LD50 is the dose that will be lethal, in a large population, to 50% of that population. However the lethality is not random. A healthy adult may be able to withstand a higher dose of radiation than an infant, or an elderly person, or the chronically ill. In addition to the LD50, the length of time over which those persons die is also given, e.g. LD 50/60 (50% die within 60 days). In addition, radiation exposure may increase one’s risk of developing cancer later in life, even years later. So a percentage of the survivors at an LD50 level of radiation will die of cancer years later.
Unfortunately, experts do not agree on the level of radiation needed to reach the LD50. Radiation levels for LD50 in different sources range from 2.2 Sv (220 rems) to 5.0 Sv (500 rems).
See my previous articles on general types of preps for a nuclear disaster: