Protecting Workers from Ultraviolet Radiation - icnirp

Protecting Workers from Ultraviolet Radiation
Eumelanin (black melanin) is produced by melanocytes upon alpha-MSH adsorption on MC1R
gene product (receptor) at the surface of the melanocytes. Phaeomelanin (red melanin) is
produced in melanocytes bearing MC1R variants. Phaeomelanin absorb UV photons and
produce reactive oxygen species (ROS) which are phototoxic. Eumelanin (stable free radical)
absorb UV photons and scavenge free radicals and are photoprotective. All individuals produce
eumelanin and phaeomelanin in different ratios according to genetic makeup and as a
consequence present large differences in solar sensitivity and skin cancer incidence. In addition,
darker skin types have more efficient DNA repair than the skin phototypes I and II (Sheehan et al
2002).
Skin adaptation from frequent UVR exposure is not only characterized by the obvious effect of
skin darkening (“tanning” or “melanogenesis”), but also by skin thickening (Table 2).
Thickening of the outermost layers of the skin (epidermis and stratum corneum) takes place as an
adaptation to UVB-related damage. This can be a 3 to 5-fold thickening of the stratum corneum
within one to seven weeks after several exposures to UVB, and returns to normal about one to
two months after ceasing exposure. This thickening after sun exposure leads to a significant
increase in UV protection by a factor of five or greater, and in lightly pigmented skin types,
thickening is probably more important than tanning in providing protection. The thickening of
the skin after prolonged tanning protects sensitive cells (basal keratinocytes, melanocytes) by
absorbing UVB radiation before they reach the basal layer of the epidermis. After some
shedding (peeling) of the stratum corneum, the basal layer can be directly stimulated by UVB
and thus the thickening or protective processes recur and reach a steady state. However, in
darkly pigmented individuals it is likely that skin pigmentation is the most important means of
protection against UVR.
Tanning becomes noticeable within a day or two after sun exposure, gradually increasing for
several days and persisting for a week. Although a tanned skin does confer a degree of
protection, this seems to be no more than a factor of two to three in the absence of skin
thickening. The wavelengths of the radiation that induce tanning are very similar to those of
radiation producing erythema. Table1 describes the range of skin types and sensitivity to UVR
effects. Subjects with sun-reactive, melano-compromised (skin types I and II) are poor tanners
compared to those with melano-competent (skin types III and IV) who tan well. Melanogenesis
can be stimulated in individuals who tan well with solar UV doses that were considerably below
the erythemal doses in the UVA region.
Table 1. Classification of skin types based on their susceptibility to sunburn in sunlight and their
ability to tan. Modified after Fitzpatrick and Bolognia, 1995
Skin
phototype
Sun sensitivity Sunburn susceptibility* Tanning achieved Classes of individuals
I Very sensitive Always sunburn: < 2 SED No tan Melano-compromised
II Moderately sensitive High: 2 – 3 SED Light tan Melano-compromised
III Moderately insensitive Moderate: 3 - 5 SED Medium tan Melano-competent
IV Insensitive Low: 5-7 SED Dark tan Melano-competent
V Insensitive Very low: 7-10 SED Natural brown skin Melano-protected
VI Insensitive Extremely low: > 10 SED Natural black skin Melano-protected
*The ranges of SEDs are not prescriptive but only indicative
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