Gray (unit) - Effect On The Body

Effect On The Body

For effects of whole-body radiation, see Acute radiation syndrome.

The gray measures the absorbed energy of radiation. The biological effects vary by the type and energy of the radiation and the organism and tissues involved. The sievert, which has the same dimensions as the gray is a measure of the potential for damage to human tissue. It is related to the gray by the relationship

where H is the equivalent absorbed dose (measured in sieverts), D the absorbed dose (measured in grays) and Q the quality factor, a dimensionless quantity whose value is dependent on the type of radiation in question, ranging from 1 in the case of gamma particles and X-rays to 20 in the case of neutrons and alpha particles.

A whole-body exposure to 5 or more gray of high-energy radiation at one time usually leads to death within 14 days. This dosage represents 375 joules for a 75 kg adult (equivalent to the chemical energy in 20 mg of sugar). Since gray are such large amounts of radiation, medical use of radiation is typically measured in milligray (mGy).

As experienced from follow-up after radiation therapy, epilation may occur on any hair-bearing skin with doses above 1 Gy. It occurs only within the radiation field/s. Hair loss may be permanent with a single dose of 10 Gy, but if the dose is fractionated permanent hair loss may not occur until dose exceeds 45 Gy. The salivary glands and tear glands have a radiation tolerance of about 30 Gy in 2 Gy fractions, a dose which is exceeded by most radical head and neck cancer treatments, potentially causing dryness. Dry mouth (xerostomia) and dry eyes (xerophthalmia) can become irritating long-term problems and severely reduce the patient's quality of life. Similarly, sweat glands in treated skin (such as the armpit) tend to stop working, and the naturally moist vaginal mucosa is often dry following pelvic irradiation.

A dose of 8 Gy or more to the ovaries generally causes permanent female infertility.