The risk of damage to body tissue from a dose of radiation depends not only on the size of the dose, but also on the type of body tissue absorbing the radiation. Some tissues of the body are more susceptible to harm from radiation than others. This is taken into account when estimating the dose of radiation a person has effectively received.
Doses of nuclear radiation (and X-rays) are measured in sievert (Sv) - a measure of the amount of possible harm it could do to the body. (Strictly speaking, we should use the term equivalent dose here). The dose is based on:
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science