# Radon-222 is a radioactive gas that can seep into buildings from underground rocks. It is an alpha emitter with a half-life of approximately 4 days. Explain why radon-222 trapped inside a building poses a serious health hazard, and why a radon-222 source left outdoors in the open air would present much less of a long-term risk.

> OCR A-Level Physics B: Advancing Physics (H557) — 6.3 Nuclear and particle physics · Explain · 5 marks

> Radon-222 is a naturally occurring radioactive gas produced by the decay of uranium in rocks. It can accumulate inside poorly ventilated buildings, where it decays by alpha emission. Its half-life is approximately 4 days.

## Mark scheme (5 marks)

1. Alpha radiation is highly ionising, so it can damage living cells / DNA inside the body.
2. Alpha radiation cannot penetrate far through air or tissue from an external source, but when radon is inhaled the alpha source is inside the body / lungs, so the radiation directly irradiates internal tissue.
3. Inside a building, radon-222 accumulates / cannot disperse, increasing the concentration of radioactive gas and therefore the level of irradiation / contamination risk.
4. Outdoors, radon disperses into the atmosphere so the concentration / activity reaching a person is much lower.
5. Radon-222 has a short half-life of 4 days, so outdoors where it disperses, the activity quickly decreases / the source does not remain a long-term risk, unlike an isotope with a long half-life.

## Key terms

- [half-life](https://www.gradenine.co.uk/glossary/half-life)

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