Sunscreen's effectiveness, even water resistant sunscreen, has an ephemeral "skin-life". You will only receive the full protection advertised on the bottle if you apply it as often as needed. According the Skin Cancer Foundation that amount is equal to about a shot glass full every two hours. This equates to one quarter to a half of an eight bottle during a day at the beach. The US government agency the National Institute for Health (NIH) recommends nine half teaspoons to be dispersed as follows:
- Face and neck: 1 half-teaspoon portion
- Arms and shoulders: 1 half-teaspoon portion to each arm
- Torso: 1 half-teaspoon portion each to front and back
- Legs and top of feet: 2 half-teaspoon portions to each leg
Obviously common sense rules. Here are a few general guidelines that should help you on your way to adequate sun protection.
Choose water-resistant brands. Even if you don't plan on swimming, if you are planning to enjoy activities in the sun, it is likely you are going to sweat.
Choose a minimum SPF value of 15. If you burn easily or are going to experience harsh sun conditions such as a day basking beside reflective water, sun exposure between 10:00 AM and 2:00 PM, or reflective snow conditions you will want to choose a higher SPF value.
Look for a sunscreen not tanning oil. Tanning oils work to increase exposure to the sun not block the harmful rays.
Reapply sunscreen as a matter of point every two hours. Apply more often if you have participated in any strenuous activities, gone swimming, or toweled off frequently.
Wear protective clothing such as a hat, sunglasses and outer garments, especially during peak sun exposure hours, will go a long way in protecting the skin from the harmful effects of the sun.
Choose sunscreens that are labeled as broad-spectrum as these will protect you to some extent from both UVB and UVA rays.