Risks to People

Exposure to UV leads to either suntan or sunburn, and to skin cancer. UV-A creates suntan, which dries the skin, leading eventually to a texture like used teabags. UV-B is more dangerous. Without suncream, the outer skin (the epidermis) is penetrated by UV-B, which causes dilation of blood vessels in the underlying tissues, resulting in a redness called erythema. Radiation around 0.3 pm wavelength is especially effective. Fair-skinned people absorb more UV-B than those with a naturally dark complexion, and therefore have to be especially careful.

A dose is the product of intensity times the exposure duration, and the dose required for erythema is one unit of sunburning tendency, the equivalent sunburn unit (ESU). More precisely, an ESU is the minimum dose causing erythema with an overhead Sun, a clear sky and a specified amount of atmospheric ozone. It amounts to 200 joules/m2 of solar radiation at a wavelength of 0.297 pm. This typically involves only 12 minutes exposure for a Caucasian skin. Exposure to 5 ESU produces painful sunburn, whilst 10 ESU causes blisters and a consequent risk of infection.

The annual variation of dosage at places in Papua New Guinea and Australia is shown in Figure 2.16. With typical summertime values around 25 ESU, Cloncurry (at 21°S in Queensland) is clearly a relatively dangerous place. The hazard is reduced at Goroka (6°S) by equatorial cloudiness (Chapter 8).

When the Sun is 50 degrees above the horizon, the UV is only half what it is when the Sun is overhead, and there is little danger from UV if the Sun is below 30 degrees. In general, about two-thirds of a day's dose occurs between 10 a.m. and 2 p.m., so that is the time to seek shade.

The most serious outcome of undue exposure to UV is skin cancer, an abnormal, cumulative and irreversible growth of cells of the skin. The incidence of skin cancer is three times as much at Cloncurry as at Brisbane, 7 degrees further south, though the amount of UV is only 30 per cent more. About 1 per cent of Australia's population acquires skin cancers each year,

° J FMAMJ JASONDJ FMAMJ J

Figure 2.16 The annual variation of the sunburning power of sunlight with normal amounts of cloud. ESU is 'equivalent sunburn unit'. These are figures for a horizontal surface, not for the skin on the body of someone moving about.

° J FMAMJ JASONDJ FMAMJ J

Figure 2.16 The annual variation of the sunburning power of sunlight with normal amounts of cloud. ESU is 'equivalent sunburn unit'. These are figures for a horizontal surface, not for the skin on the body of someone moving about.

including over 7,000 people with melanomas, a type which can become lethal.

There has been a worldwide increase of skin cancers, e.g. an 82 per cent increase in Scotland between 1979-1989. The rise is chiefly due to the popularity of suntanning. The problem will worsen if the annual hole in the ozone layer over the poles (Section 1.4) continues to extend towards more inhabited latitudes, and it is unfortunate that the ozone hole occurs in a season of relatively high radiation intensities in the southern hemisphere. Harmful UV is increased by about 1.3 per cent if there is 1 per cent less stratospheric ozone. Measurements at Invercargill (at 46°S in New Zealand) have shown an increase of UV by 6 per cent between 1981 and 1990.

How To Prevent Skin Cancer

How To Prevent Skin Cancer

Complete Guide to Preventing Skin Cancer. We all know enough to fear the name, just as we do the words tumor and malignant. But apart from that, most of us know very little at all about cancer, especially skin cancer in itself. If I were to ask you to tell me about skin cancer right now, what would you say? Apart from the fact that its a cancer on the skin, that is.

Get My Free Ebook


Post a comment