Radon is a gas which naturally occurs in certain rocks, such as granite, and some building materials. It is colourless, odourless and almost chemically inert. In the open air, radon is diluted to very low levels and poses no harm to health, however it can be trapped in houses and build up to dangerous levels.

Relevant radon links:

Radiation: product and services on GOV.UK

UKradon on the Public Health England website.