Everybody is exposed to ionising radiation, wherever they are. This ‘background radiation’ is measurable everywhere, and in the UK it’s approximately 2.7 milliSieverts per year (mSv/y). Most of this dose, about 50%, is due to radon gas with the rest coming from other environmental sources. Only a small amount comes from man-made sources and most of this is due to medical exposures such as X-rays.