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.Ionizing UV therefore does not penetrate Earth's atmosphere to a significant degree, and is sometimes referred to as vacuum ultraviolet.Although present in space, this part of the UV spectrum is not of biological importance, because it does not reach living organisms on Earth.Illustration of the relative abilities of three different types of ionizing radiation to penetrate solid matter.Typical alpha particles (α) are stopped by a sheet of paper, while beta particles (β) are stopped by an aluminum plate.
While the part of the ultraviolet spectrum that penetrates the Earth's atmosphere is non-ionizing, this radiation does far more damage to many molecules in biological systems than can be accounted for by heating effects, sunburn being a well-known example.
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.
The intensity of all types of radiation from a point source follows an inverse-square law in relation to the distance from its source.
Like any ideal law, the inverse-square law approximates a measured radiation intensity to the extent that the source approximates a geometric point.
Radiation with sufficiently high energy can ionize atoms; that is to say it can knock electrons off atoms and create ions.