Hazards and uses of radioactive emissions and of background radiation

• •Cosmic rays from space
• •Radioactive materials naturally present in rocks and the ground (including the air we breathe, e.g. radon gas)
• •Radioactive material released by nuclear weapons testing/fallout
• •Medical sources, such as X-rays and radiotherapy

Radiation dose is measured in sieverts (Sv) and measures the risk of harm to human tissue from radiation exposure. The level of background radiation, and the dose a person receives, varies depending on location (e.g. more cosmic rays at high altitude, more radon in some rock types) and occupation (e.g. airline crew, nuclear industry workers, or radiographers receive higher doses than average).

The suitability of a radioactive isotope for a particular use depends heavily on its half-life, alongside the type of radiation it emits.

For example, a medical tracer needs a short enough half-life that the patient's radiation dose doesn't remain high for long after the test, but a source used in a smoke alarm benefits from a very long half-life so it doesn't need replacing.

Medical tracers: a gamma-emitting isotope, such as technetium-99m (half-life about 6 hours), is injected into or swallowed by a patient. Because gamma radiation is weakly ionising and highly penetrating, it passes out through the body and can be detected externally without being absorbed by (and damaging) surrounding tissue. Technetium-99m decays into a stable, safely-excreted product, and its half-life is short enough that the patient's radiation dose doesn't stay elevated for long, but long enough to complete the scan.

Cancer treatment (radiotherapy): gamma rays from a radioactive source are directed carefully at a tumour to kill cancerous cells. Because targeting can't be perfectly precise, some healthy tissue surrounding the tumour is also irradiated and damaged — this is why radiotherapy causes side effects, and why treatment plans try to minimise the dose to healthy tissue.