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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (5 marks)

Alpha particles are highly ionising and can damage living cells and DNA. They cannot penetrate far through air or tissue from an external source, but when radon‑222 is inhaled the alpha source is inside the body, especially the lungs, so the radiation directly irradiates internal tissue.

Inside a poorly ventilated building radon‑222 accumulates because it cannot disperse. The concentration of the radioactive gas therefore rises, increasing the level of irradiation and the risk of internal contamination.

Outdoors the radon gas disperses rapidly into the atmosphere. The concentration (and therefore the activity) reaching a person is much lower. Moreover, radon‑222 has a short half‑life of about 4 days, so any activity that does disperse decays quickly, meaning the source does not remain a long‑term risk.

Examiner tips

  • Mention the high ionising power of alpha particles and the need for internal exposure to be hazardous.
  • Explain that indoor accumulation increases concentration and risk, while outdoor dispersion and the short half‑life reduce long‑term exposure.
  • Use the exact terminology: ‘highly ionising’, ‘internal irradiation’, ‘concentration’, ‘short half‑life’.
  • Keep the answer concise and directly linked to the mark scheme points.

Common mistakes

  • Confusing external alpha exposure with internal exposure – students often say alpha particles can penetrate the body, which is incorrect.
  • Failing to mention the short half‑life of radon‑222 as a reason for reduced long‑term risk.
  • Using vague terms like ‘radioactive gas’ without explaining ionising damage or concentration effects.

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 in this question

half-life

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