The radiation absorbed dose is how much radiation hits your tissues. Because different tissues absorb radiation differently, human exposure has to be measured using a weighted average over the different tissues of the body. The distinction between absorbed dose (measured in grays) and absorbed dose equivalent (measured in sieverts) is absolutely critical to health and safety.

A person's total annual absorbed exposure is of the order of one millisievert. This comes from cosmic radiation, from radon decay from the rocks underneath us, from medical practices like x-rays, and from flying in jets. Workers in nuclear power plants have a slightly higher exposure. Some people because of how and where they work will get perhaps five millisieverts a year. If you're near a nuclear power plant when an accident happens you might suddenly get a flash dose of ten; a single dose of twenty to fifty millisieverts is survivable but you might start worrying about your fertility. When you're close enough that you get a 100 mSv dose or more is when you're likely to have to worry about radiation sickness and possible death.

The damage done to living tissue by radiation varies with both the type of radiation to which tissue is exposed and the energy associated with the radiation. The conversion factor used to change absorbed dose into dose equivalent is Q, the quality factor, which is derived thus: Those types of radiation which deposit their energy in small areas of living tissue are more likely to cause biological damage than those types of radiation that deposit their energy over a broader area, as the body can repair damage more easily if many cells are slightly damaged than if a few, concentrated cells are extensively damaged. Therefore, types of radiation which deposit their energies in concentrated areas have high quality factors, while types of radiation which deposit their energies in broad areas have low quality factors. This is measured as linear energy transfer, LET, the energy deposited per unit distance traveled by a particle. The quality factors for common types of radiation are:

Particle                   Q
---------------            -----------
X-Ray                       1.00
Gamma                       1.00
Beta                        1.00
Alpha                      20.00
Neutrons (unknown energy)  10.00
Thermal Neutrons            2.00
Fast Neutrons              11.00
High Energy Protons        10.00

Multiplying absorbed dose measured in rads by the quality factor gives the deep dose equivalent, measured in rem. Note that this means that for X-rays, Gamma rays and beta particles, 1 R = 1 rad = 1 rem.

source: chapter 10, part 20, code of federal regulations, 20.1004.

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