Chemical sunblocks protect skin thanks to sun-protective ingredients contained in the emulsion. Some people have allergies to components of chemical sunblocks (PABA is one common culprit). Waterproof formulas tend to be the most sensitizing.
Exposure Time: When you know that you will be outside for more than a few minutes, use an SPF 15 or higher. The truth is, you will still get some color with high protection. If you will be on the beach, playing golf or tennis, or gardening, try to remember to apply your sunblock at home at least one half hour before beginning your activity to allow the product to absorb. Apply a generous amount of sun protection. (The SPF number is based on a surprisingly generous slathering of sunblock.)
Be careful to avoid the eye area when applying sunblock, especially when using waterproof formulas, which can sting and make eyes tear. Reapply sun protection after swimming (especially if formula is not waterproof) and after more than two hours in the sun.