A chemical element with atomic number 86. Radon is a radioactive, colourless and odourless gas. All radon isotopes are short-lived alpha emitters; with radon-222 having the longest half-life (3.82 days). Radon is produced by the gradual decay of uranium or thorium, which may be present in the bedrock. The chemically inert radon can then easily seep from the rock through cracks into buildings, where it can accumulate. To prevent high radon concentrations, anti-radon barriers are used to isolate the building from the bedrock. Radioactivity from radon accounts for more than half of the total radiation dose from the natural background.