Sunscreens may be comprised of chemical blocking agents which absorb UV light or physical blocking agents such as titanium dioxide or a combination of both. The base may be alcohol, a water or oil-based lotion, an ointment or a cream.
The sun protection factor, SPF, is the ratio of the dose of UV needed to produce burning with the sunscreen on the skin to the UV dose needed to produce the same amount of burning without sunscreen on the skin. For example, if it normally takes you 10 minutes to burn then theoretically an SPF of 15 allows you to stay in the sun 1-5 x 10 minutes before getting sunburnt. After this time the skin will have been exposed to as much sun as it can handle, so that this period should not be extended. The Australasian College of Dermatologists in its consensus statement on photoageing and Photodamage states: `The use of sunscreens to extend sun exposure time negates the beneficial effects of these agents' (Photoaging & Photodantage ns a Public Health Concern. The Australasian College of Dermatologists Consensus Statement 1989). It is true that sunscreen often needs to be applied more frequently because of sweating and swimming which wash it away from the skin. While sunscreens labelled with SPF values greater than 15 are available overseas, it can be seen from the following table that there is little additional benefit at the higher SPF values. In fact adverse reactions are more likely with increasing concentrations of sunscreening chemicals.
Sunscreen containing chemical sunscreening agents should be applied 15-20 minutes prior to going out in the sun so that the chemicals can bind with the outer layer of the skin.
Sunscreens not uncommonly cause irritation of the skin, but true allergic reactions are uncommon. Most problems relate to the fact that these products are often applied in hot conditions when sweating is likely. People prone to eczema or with sensitive skin are more likely to develop skin irritation.
Exposure during work can be an important contributing factor in the development of skin cancer. It is appropriate that work duties and leisure pursuits be organised to avoid sun exposure during the middle of the day. Protective measures such as clothing and sunscreens are also very important. Sunscreens should be applied not only to the face but also the back of the hands, back of the neck, ears, lips, particularly the lower lip, and bald scalps. These are the areas where skin cancers most commonly develop.
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