Background radiation is low-level radiation that is around us all of the time. Everyone is exposed to it, wherever they live and whatever they do.
Some of it comes from natural sources, such as radioactive substances in rocks and cosmic rays that arrive from space. The rest comes from man-made sources, such as the fallout from nuclear weapons testing and from nuclear accidents.
The amount of background radiation a person receives, called the radiation dose, is not the same for everyone. It can depend on location, because some areas have more radioactive rock than others. It can also depend on occupation, for example people who work in nuclear power stations or fly at high altitude, where there are more cosmic rays, may receive a higher dose.
Radiation dose is measured in sieverts (Sv). A sievert is a large unit, so doses are often given in millisieverts (mSv), where 1000 mSv = 1 Sv.