Radiation units and biological effects
We meet radiation everywhere in nature. Naturally occurring radioactive elements in the ground emit radiation which gives an external dose. Some of these elements enter food chains and can enter the human body, resulting in an internal dose. In addition, there is constant exposure to radiation from space (cosmic radiation). This is collectively described as "background radiation".
A number of units are used to express quantities of radiation absorbed by the body or amounts of radioactivity deposited on the ground or existing in a cloud. These units have been expressed differently over the years and each has its own precise physical description. For the purposes of this booklet, the most recently agreed international units have been employed although their detailed scientific descriptions are not addressed.
The most useful in this context is the unit of effective dose -- the sievert. (The older unit was the rem. 1 sievert (Sv) = 100 rem.) In the present context, the millisievert (mSv) is often employed. That equals 1/l000th (1 x 10- 3) of a sievert. The following notes put the size of this unit in perspective.
The total natural background dose varies a great deal. The average is about 1 to 2 mSv per year, but values several times higher are not uncommon. In some areas, the natural background dose is more than 60 times the global average. It has not been possible to find a relation between natural radiation dose and adverse health effects. Natural radiation background and its variations therefore may serve as an indicator of what doses may be considered to be acceptable.
The naturally occurring radioactive radon gas from the ground is sometimes trapped and concentrated in buildings. Where houses are well insulated, doses from radon in homes may exceed the natural background dose many times. Doses of radon of more than 10 mSv per year are not uncommon, and they are levels at which controls must be exercised.