Look for sunblocks with higher SPF (Sun Protection Factor). They will generally protect you better. Sunblock also comes in waterproof.
How much protection you get is determined by the not only the SPF, but also your skin type, time of day/angle of the sun in the sky (mornings and afternoons, the sun's rays have to pass through more of the atmosphere before they reach you), activities you perform (ie, if you swim or not), and how often you apply the sunblock. Make sure you get sunblock/sunscreen, not tanning oil.
Use it on cloudy days too, because the burning rays are not filtered out by the clouds.
← Vampire FAQs