Ultraviolet radiation and the skin
An interaction between skin and sunlight is inescapable. The potential for harm depends on the type and length of exposure. Photoageing is a growing problem, because of an increasingly aged population and a rise in the average individual exposure to ultraviolet (UV) radiation.
The electromagnetic radiation spectrum
The sun’s emission of electromagnetic radiation ranges from low-wavelength ionizing cosmic, gamma and X-rays to the non-ionizing UV, visible and infrared higher wavelengths (Fig. 1). The ozone layer absorbs UVC, but UVA and smaller amounts of UVB reach ground level. UV radiation is maximal in the middle of the day (11.00–15.00 h) and is increased by reflection from snow, water and sand. UVA penetrates the epidermis to reach the dermis. UVB is mostly absorbed by the stratum corneum – only 10% reaches the dermis. Most window glass absorbs UV less than 320 nm in wavelength. Artificial UV sources emit in the UVB or UVA spectrum. Sunbeds largely emit UVA.
Effects of light on normal skin
Physiological
UVB promotes the synthesis of vitamin D3 from its precursors in the skin, and UVA and UVB stimulate immediate pigmentation (due to photo-oxidation of melanin precursors), melanogenesis and epidermal thickening as a protective measure against UV damage (p. 7).
Sunburn
If enough UVB is given, erythema always results. The threshold dose of UVB – the minimal erythema dose (MED) – is a guide to an individual’s susceptibility. Excessive UVB exposure results in tingling of the skin, followed 2–12 h later by erythema. The redness is maximal at 24 h and fades over the next 2 or 3 days to leave desquamation and pigmentation. Severe sunburn causes oedema, pain, blistering and systemic upset. The early use of topical steroids may help sunburn; otherwise, a soothing shake lotion (e.g. calamine lotion) is applied. Individuals may be skin typed by their likelihood of burning in the sun (Table 1). Prevention is better than cure, and ‘celts’ with a fair ‘type 1’ skin should not sunbathe and must use a high protection factor sunblock cream on exposed sites (p. 108). Some evidence suggests the sun avoidance message may have gone too far in that it could be causing white populations to become vitamin D deficient.
Table 1 Skin type according to sunburn and suntan histories
| Skin type | Reaction to sun exposure |
|---|---|
| Type 1 | Always burns, never tans |
| Type 2 | Always burns, sometimes tans |
| Type 3 | Sometimes burns, always tans |
| Type 4 | Never burns, always tans |
| Type 5 | Brown skin (e.g. Asian caucasoid) |
| Type 6 | Black skin (e.g. black African) |
