Radon gas (222Rn) from the decay of 238U in the Earth's crust seeps up from the ground and accumulates in buildings. The average concentration of radon gas in the air of a house in Britain has an activity of20 Bequerels m-3. The half-life of 222Rn is 3.8 days and the α-decay energy is 5.5 MeV The average volume of air in the lungs is about 3 litres. We breathe at a rate of about 6 litres per minute. Radon daughters will be present in the air we breathe attached to aerosol particles which are deposited in the lungs.