We need measures of the amount of radiation a person is exposed to. There are three measures of importance.
Exposure is a measure of the total ionisation produced in air. It is defined as
exposure, with units coulombs per kilogram (C/kg).
charge produced (only one sign is counted)
mass of air.
The absorbed dose is the energy absorbed per kg of tissue. It is defined as
absorbed dose with units joules per kilogram or gray, Gy.
total energy absorbed
mass of body tissue.
The dose equivalent estimates the radiation damage that occurs in tissues. It is defined as
dose equivalent, with units joules per kilogram, J/kg.
The Q factor is different for the different types of radiationand radiation when absorbed by the body does the most damage to tissue, so has the highest Q factor, while radiation does the least damage so has the lowest Q factor.