While an individual cell is made of trillions of atoms, only a small fraction of those will be ionized at low to moderate radiation powers.

This is an important distinction due to the large difference in harmfulness to living organisms.

A common source of ionizing radiation is radioactive materials that emit α, β, or γ radiation, consisting of helium nuclei, electrons or positrons, and photons, respectively.

Radiation with sufficiently high energy can ionize atoms; that is to say it can knock electrons off atoms and create ions.

Ionization occurs when an electron is stripped (or "knocked out") from an electron shell of the atom, which leaves the atom with a net positive charge.

If the source of the ionizing radiation is a radioactive material or a nuclear process such as fission or fusion, there is particle radiation to consider.

Particle radiation is subatomic particles accelerated to relativistic speeds by nuclear reactions.

However, calculating the exact risk and chance of cancer forming in cells caused by ionizing radiation is still not well understood and currently estimates are loosely determined by population based on data from the atomic bombing in Japan and from reactor accident follow-up, such as with the Chernobyl disaster.

The International Commission on Radiological Protection states that "The Commission is aware of uncertainties and lack of precision of the models and parameter values", "Collective effective dose is not intended as a tool for epidemiological risk assessment, and it is inappropriate to use it in risk projections" and "in particular, the calculation of the number of cancer deaths based on collective effective doses from trivial individual doses should be avoided.") in particular.

Gamma radiation (γ) is damped when it penetrates lead.

Note caveats in the text about this simplified diagram.

Radiation is often categorized as either ionizing or non-ionizing depending on the energy of the radiated particles.