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Effects

All of these units are meaningless until one has some idea of how bad one of them is for you. Here are some rules of thumb that may be off by factors of two from one case to the next:

• Instant Death: It takes a monumental radiation dose to kill outright, typically something like 5000 R (50 Grays) "whole-body" - i.e. half a million ergs of energy deposited in every gram of your body. This amount of energy wipes out your central nervous system (CNS) immediately when delivered all at once. Needless to say, only the military mind makes a strong distinction between this and the next level down.
• Overnight Death: Approximately 900 R (9 Grays) whole-body will accomplish the same thing as 50 Grays but it takes about a day.
• Ugly Death: A somewhat lower dose, around 500 R (5 Grays) causes severe "radiation sickness" (i.e. nausea, hair loss, skin lesions, etc.) as the body's short-lived cells fail to provide new generations to replace their normal mortality ("cell reproductive death"). It is not this trauma which usually kills, however, but the complications that arise from a lack of resistance to infection, due in turn to the lack of new generations of white blood cells. If you survive the initial radiation sickness and avoid infection, you will probably recover completely in the short term; but you are very likely to develop cancer (especially leukemia) in later years (usually some 10-20 years later!) and your offspring, if any, will have a high probability of genetic mutations.
• Sub-Acute Exposures: From a whole-body dose of around 100 R (1 Gray) delivered in less than about a week, you are unlikely to notice any immediate severe symptoms. However, you are likely to develop leukemia in 10-30 years, and there is a significant chance of genetic mutations in your offspring. A whole-body exposure of 5 R delivered over 1 year was believed in 1970 to represent 1.8 "doubling doses" -- i.e. it was thought to multiply your odds of developing cancer by a factor of 2.8 if maintained year after year. At that time it was also the legal exposure limit for radiation workers in the U.S.A., set by the Atomic Energy Commission (AEC) there. Presumably quite a few people received this exposure for a few years, although it is unusual for more than a small fraction of workers to receive the maximum allowed exposure. For perspective, it is noteworthy that a series of spinal X-rays is apt to give an exposure of 1-4 R locally, and that an afternoon on Wreck Beach in midsummer often produces a painful sunburn that represents 10-20 R to the skin; the resultant burn is a bona fide radiation burn and is just as dangerous as any other kind! In fact, the overwhelming majority of all radiation-induced cancer fatalities on Earth can be attributed directly to far ultraviolet from our favourite nuclear fusion power plant in the sky: the Sun.
• Marginal Exposures: The average exposure from natural sources of radiation is on the order of 300 mR per year. As of 1979 this was also the Canadian legal limit for public exposure from artificial sources. Whether an extra 300 mR makes a significant difference epidemiologically in the incidence of cancer depends almost entirely on what one considers significant; however, it is a fact that the statistical difference between populations that have received such an exposure "artificially" and those who have not is smaller than the statistical differences between populations with different eating habits, who live in different regions, who have different types of jobs, etc. This is partly because of the wide variety in the amount and type of natural radiation exposure.

Before we go on to discuss sources of radiation, it is important to note that different organs or body parts have dramatically different resistance to radiation. The hands, in particular, are able to withstand radiation doses that would kill if the whole body were subjected to them! The lens of the eye and the gonads are considered to be the most vulnerable and should be protected first.