Ionising radiation
Radiation energetic enough to knock electrons off atoms and damage living cells; its effect on the body is measured in sieverts.
Ionising radiation carries enough energy to knock electrons out of atoms, leaving electrically charged ions behind. Gamma rays, X-rays, alpha and beta particles, neutrons and cosmic rays all do this. In living tissue that can break chemical bonds and damage DNA. Visible light, radio waves and microwaves do not carry enough energy to do it.
We are all exposed to a little all the time, from cosmic rays, from radioactive elements in rocks and soil, from radon gas and even from our own bodies. Natural background radiation gives a typical American about 3.1 millisieverts a year, and medical scans and other human-made sources add roughly the same again.
Harm depends on the radiation dose, measured in sieverts. There is no firm evidence of increased cancer from doses below about 100 millisieverts. Above about 500 millisieverts, enough cells are killed to cause acute radiation syndrome, or radiation sickness. A whole-body dose of 3.5 to 5 sieverts received over minutes or hours is expected to kill about half of those exposed within 30 days.
Three simple rules reduce exposure: keep time near a source short, keep your distance, since the dose rate from a small source falls with the square of the distance, and put shielding in between.
A common misconception is that exposure and contamination are the same. Exposure means being struck by radiation; contamination means radioactive material itself is on or in you, irradiating you until it is removed.
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