Since the advent of nuclear energy and space travel, humans have been exposed to radiation more frequently and it has become a hot topic. There are various different things that people call “radiation”, but the type that damages humans is composed of either electromagnetic waves or atomic nuclei, which do their damage through ionization — knocking the electrons in atoms off their orbits. If a human is exposed to too much of this energy, it can cause damage, sometimes making it impossible for certain cells to continue replacing themselves, which can be fatal. How much a person can receive safely depends on the nature of the radiation and the tissue it encounters, but doses of 6 sieverts or more are nearly always deadly.
Radiation absorbed by tissue is measured in rads, where a rad is a hundredth of a joule per kilogram of tissue. A gray, a hundred times this value, is the more recently used International System of Units (SI) equivalent. Rads and grays only measure the physical intensity of the energy, which does not accurately correlate to “damage done.”
By multiplying the intensity by two variables, Q and N, a more accurate “damage done” unit is derived, the sievert (Sv), which is a hundred times the rem. Both units are used to measure the same thing, and though the latter term is more commonly used, the former is more scientifically proper. The rad converts to the rem, while the gray converts to the sievert.
The variable Q changes based on the nature of the radiation. Photons, a relatively light form of radiation, have a Q of 1, while atomic nuclei, a relatively heavy form, have a Q of 20. The variable N changes based on the relevant tissue and species. Humans are more sensitive than most animals, therefore holding the relatively high N value of 1. A virus may have an N value a hundred or even ten thousand times smaller than this. Because of variations based on these two values, a given gray can do a wide range of actual damage, measured in sieverts.
There is a small amount of ambient background radiation everywhere on earth, which we and all life on this planet are adapted to deal with. This varies significantly based on location, but a good average is 2.4 millisieverts (mSv) per year. In space, the level may be dozens or hundreds of times this value.
Radiation today is mainly released in abrupt exposures, so researchers mostly know about the effects of absorbing a given quantity in a very short time frame. At about 0.5 sievert to 1 sievert, the effects of radiation sickness can be felt. A portion of the red blood cells are temporarily wiped out, and sperm in the testes are deprived of their ability to fertilize an egg until they are recreated. Mild headache and loss of focus temporarily occur.
In exposures ranging from about 1 to 2 sieverts, permanent effects begin. Most people experience mild nausea, sometimes accompanied by vomiting, which lasts for about a day. A feeling of general illness persists for a week or two.
For levels of radiation more intense than this, bad things happen. For every additional sievert past 1, the chance of death within 30 days increases by about 15%, adding to a base rate of around 10%. This means that about 25% of all people die within 30 days of exposure to 2 sieverts, around 40% of people die after exposure to 3 sieverts, and about 55% of people die after exposure to 4 sieverts. At 6 sieverts, the death rate is 90%, which increases quickly to 100%. The primary causes of death are internal bleeding or immune system failure that rapidly gives way to lethal infection. Hair is lost, people are rendered sterile, bone marrow is destroyed, and recovery can take years and may never be complete.