U+33C6 Square C Over Kg

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Radiation exposure is a measure of the ionization of air due to ionizing radiation from photons. It is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air. As of 2007, "medical radiation exposure" was defined by the International Commission on Radiological Protection as exposure incurred by people as part of their own medical or dental diagnosis or treatment; by persons, other than those occupationally exposed, knowingly, while voluntarily helping in the support and comfort of patients; and by volunteers in a programme of biomedical research involving their exposure. Common medical tests and treatments involving radiation include X-rays, CT scans, mammography, lung ventilation and perfusion scans, bone scans, cardiac perfusion scan, angiography, radiation therapy, and more. Each type of test carries its own amount of radiation exposure. There are two general categories of adverse health effects caused by radiation exposure: deterministic effects and stochastic effects. Deterministic effects (harmful tissue reactions) are due to the killing/malfunction of cells following high doses; and stochastic effects involve either cancer development in exposed individuals caused by mutation of somatic cells, or heritable disease in their offspring from mutation of reproductive (germ) cells.

Absorbed dose is a term used to describe how much energy that radiation deposits in a material. Common measurements for absorbed dose include rad, or radiation absorbed dose, and Gray, or Gy. Dose equivalent calculates the effect of radiation on human tissue. This is done using tissue weighting factor, which takes into account how each tissue in the body has different sensitivity to radiation. The effective dose is the risk of radiation averaged over the entire body. Ionizing radiation is known to cause cancer in humans. We know this from the Life Span Study, which followed survivors of the atomic bombing in Japan during World War 2. Over 100,000 individuals were followed for 50 years. 1 in 10 of the cancers that formed during this time was due to radiation. The study shows a linear dose response for all solid tumors. This means the relationship between dose and human body response is a straight line.

The risk of low dose radiation in medical imaging is unproven. It is difficult to establish risk due to low dose radiation. This is in part because there are other carcinogens in the environment, including smoking, chemicals, and pollutants. A common head CT has an effective dose of 2 mSv. This is comparable to the amount of background radiation a person is exposed to in 1 year. Background radiation is from naturally radioactive materials and cosmic radiation from space. The embryo and fetus are considered highly sensitive to radiation exposure. Complications from radiation exposure include malformation of internal organs, reduction of IQ, and cancer formation. The SI unit of exposure is the coulomb per kilogram (C/kg), which has largely replaced the roentgen (R). One roentgen equals 0.000258 C/kg; an exposure of one coulomb per kilogram is equivalent to 3876 roentgens.