Radon radiation detectors. These devices are designed to be used inside houses to detect the radiation emitted by the radioactive gas radon. Radon occurs naturally, and is found in small quantities in the atmosphere. It is produced by the decay of radioactive isotopes in the Earth's crust. In some locations, radon gas rising from the ground can build up to harmful levels. The plastic polymer PADC is used to detect the radiation. The detectors are left in a house for a period of time (the one at left is a 3-month model, the one at right is a 2-week model), after which the marks are examined to calculate the amount of radon that is present. These detectors are from the UK's Health Protection Agency.