Irradiation: ionizing radiation, health effects, and medical uses (updated)

Irradiation describes exposure to electromagnetic or particle radiation. Ionizing radiation ionizes atoms and can damage DNA and cells, with rapidly dividing tissues most vulnerable. Natural background sources (notably radon) and medical imaging are the major contributors to human exposure; nuclear testing added fallout historically. Controlled irradiation treats cancer by destroying tumor cells, while the older use of the word also names a visual illusion where bright objects appear larger on dark backgrounds. A historical claim linking fallout to infant mortality requires verification.

What is irradiation?

In physics and medicine, irradiation refers to exposure to radiation - electromagnetic waves (infrared, ultraviolet, X-rays, gamma rays) or particles (alpha, beta, electrons, protons, neutrons). Many of these are ionizing radiations: as they pass through matter they remove electrons from atoms and molecules, creating charged ions. That ionization underlies both the biological harm and the clinical uses of radiation.

Types and penetrating power

Different radiations vary in energy and penetration. Alpha particles deposit energy over short distances and are stopped by skin; beta particles and electrons penetrate further; gamma rays, X-rays, and neutrons travel much farther and can pass through the body. Penetration influences which tissues receive dose and how much biological damage can occur.

How irradiation affects living tissue

Ionizing radiation damages cells primarily by breaking chemical bonds and creating reactive molecules that harm DNA and other cellular components. Damage ranges from temporary dysfunction to cell death. It can also cause permanent changes (mutations) in DNA, which in some cases lead to cancer.

Actively dividing cells are more sensitive to radiation. That is why the bone marrow, cells of the lymphatic system, and reproductive (germ) cells are particularly vulnerable. Rapidly dividing tissues (for example in infants and children) generally show greater sensitivity than many adult tissues.

Health effects are usually described as deterministic (acute, dose-dependent effects such as radiation sickness) and stochastic (random, probabilistic effects such as cancer and hereditary mutations). The probability of stochastic effects increases with dose; their severity does not.

Sources of exposure

People receive radiation from natural sources (cosmic rays, terrestrial radioisotopes, and especially radon gas) and from human activities. Medical imaging and therapy are the most significant controlled sources worldwide: diagnostic X-rays and, in particular, CT scans have increased medical exposure in recent decades and now account for the largest share of man-made population exposure in many countries.

Nuclear weapons testing and releases from reactors contributed radioactive fallout to the environment in the 20th century, increasing global levels of certain isotopes (for example cesium-137 and strontium-90). Any incremental environmental contamination can add to lifetime exposure and theoretical risk; the historic literature includes contested analyses linking fallout to population health outcomes .

Radiation in medicine: risk and benefit

Clinically controlled irradiation treats cancer by delivering high doses that destroy tumor cells. The same DNA-damaging mechanism that can cause cancer also allows radiation therapy to cure or control it. Modern treatment planning and imaging reduce dose to healthy tissues while maximizing tumor control.

A different "irradiation": a visual illusion

Irradiation is also an older term for an optical illusion: bright objects on a dark background often appear larger than identical dark objects on a light background. Scientists attribute this to lateral interactions in the retina and early visual pathways, which can spread excitation and change perceived size.

1. Check historical epidemiological claims linking radioactive fallout to infant mortality in United States studies and clarify context and consensus.
2. If quantifying contributions to population exposure, verify latest UNSCEAR or national radiation protection reports for current percentages of natural vs medical vs other sources.