Most people use sunscreen improperly by not applying enough. They apply only 25% to 50% of the recommended amount. Sunscreen should be applied liberally enough to all sun-exposed areas that it forms a film when initially applied. It takes 20-30 minutes for sunscreen to be absorbed by the skin, so it should be applied at least a half an hour before going out in the sun. Sunscreen should also be the last product applied especially on the face since some sunscreens can break down in the presence of water contained in water-based foundations and moisturizers.
Most instructions on sunscreen labels recommend reapplying sunscreen "frequently", but the definition of "frequently" is vague. A common instruction is to reapply sunscreen after 2-4 hours in the sun. However, one study has shown that reapplying sunscreen 20 to 30 minutes after being in the sun is more effective than waiting 2 hours. It is possible that this time period is more effective because most people do not apply enough sunscreen initially, and this second application approximates the actual amount needed. Sunscreen should also be reapplied after swimming, excessive sweating, or toweling.
Sunscreen should be applied daily. The daily use of a low-SPF sunscreen (15) has been shown to be more effective in preventing skin damage than the intermittent use of a higher SPF sunscreen.
Sunscreen and Insect Repellents
Insect repellents reduce the sunscreen's SPF by up to one-third. When using sunscreen and insect repellent together, a higher SPF should be used and reapplied more often.