In actual use, a sunscreen may not provide as much protection as expected from its SPF rating, largely because people do not apply enough. Most people apply about half as much as the amount used for SPF tests. Full body protection for an adult requires nine portions of one-half teaspoon each, or a total of about an ounce. People with albinism often develop a blotchy sunburn from spots missed with sunscreen lotions. Since sunburn develops two to four hours after the sun exposure, and sunburn may peak in intensity as long as 24 hours after sun exposure, it is important to apply the sunscreen systematically.
People often miss the tops of ears, and the backs of arms and legs.
Bicyclists need to protect the low back area, where shirts often ride up. . It helps to apply the sunscreen one-half hour before going into the sun, since some of the screening chemicals bind to the skin. Remember that rubbing with towels or friction from clothing such as between the legs can remove protection. The sweatproof sunscreens are designed to prevent eye stinging that occurs with most sunscreens when perspiration carries sunscreen into the eyes. The Coppertone product seems to avoid this problem, but rated at SPF 15, it may not provide as much protection as some other products. .