Biophysics by William C. Parke

Author:William C. Parke
Language: eng
Format: epub
ISBN: 9783030441463
Publisher: Springer International Publishing

8.15 Radiation Dosimetry

8.15.1 UV Light Limitations

Since ultraviolet light does not penetrate far below the cutaneous layer of skin, measures of UV radiation effects are often calibrated by the energy deposited per unit area of the skin rather than energy per unit mass used for Equivalent Dose.

By the way, do not let your dentist convince you that UV light on one sunny day is equivalent to the X-ray exposure he/she is about to give you. X-rays are penetrating; UV is stopped by the skin. You are likely to be unaware of the development of a deep tumor, but a melanoma is visible. The issue for getting an X-ray is whether, for a given intensity, the chance of a later cancer is more of a concern than the present treatment you may need. A soldier may decide to cross a field of bullets if his present danger is high, or he decides whether the risk is worth the hoped-for outcome. X-ray photons act as bullets.

One ‘Standard Erythemal Dose of UV’ (SED)15 is a measure of the energy, in joules, deposited over each square meter of skin. One SED is 100 joules per square meter.

The ‘UV Index’ is an adopted value intended to indicate to the public the UV sunburn danger. The raw value is the integrated ground-level solar UV light intensity in each frequency interval (dI∕df) times a skin sensitivity factor called the ‘erythemal action spectrum’. 16 The raw UV index is then normalized to have a value of 10 for a clear sky mid-latitude zero-altitude midday solar exposure. Finally, the index is corrected for the effect of expected clouds, altitude, latitude, air pollutants, surface albedo, and stratospheric ozone layer holes.

Because the radiation effects are dominated by single photon events, over a wide range of intensities, the damaging effects of UV radiation will be proportional to the UV index. For example, a value of 5 would produce the same sunburn in an hour as a value of 10 in half an hour. Exposure of light skin to solar UV with index of 10 will produce a sunburn in about 12 min. If the UVI is 5, sunburn should be expected after 24 min of exposure.

As UV is an ionizing radiation, damage to DNA can be expected. For UVB, damage is mainly caused by the formation of a thymine-thymine cyclobutane dimer mutation.

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