There is some evidence that the numerical measure of protection indicated on the product pack is generally higher than that achieved in practice. The photoprotection of sunscreens (the SPF) is measured by photo-testing in vivo at internationally agreed levels of thickness of application 2 mg/cm2. To receive the SPF quoted on sunscreen packaging, an individual would need to use 35 ml of sunscreen for total body surface protection. Studies have demonstrated that individuals are more likely to use 0-5-1-5 mg/cm2 and that most users get, in protective terms, the benefit of between one-quarter and one-half of the product.25 Individuals get sunburnt because they use too little sunscreen, spread it unevenly, miss parts of the body surface exposed to the sun and because sunscreen is rubbed or washed off. Thus, individuals' use of a sunscreen makes a difference in how effective sunscreens are in the prevention of sunburn and explains why sunburn still occurs even with higher SPF sunscreens. If individuals want to be supine in the sun for long periods of time (hours) then it is recommended that SPFs of 20-30 or higher are necessary. Sunscreens need to be applied evenly 30 minutes before going out in the sun. They need to be reapplied at regular intervals as much is washed off by swimming and other water sports and by any abrasive action particularly from sand on the beach.25
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