12 things to know when choosing and using sunscreen this summer from the experts at the Harvard Medical School, Harvard Medical Publications, HEALTHbeat:

1. Get rid of old sunscreen. Sunscreen has a shelf life of 3 years, but you don't know when a product first landed on a shelf. Check the expiration date, if there is one — but be mindful that the expiration date is only good if the sunscreen hasn't been exposed to any extreme heat.


2. SPF tells you how much protection the sunscreen offers. Higher SPF values (up to 50) provide greater sunburn protection.

3. Stick with SPF 50 or gear up with protective clothing. You don't need to buy a product with an SPF higher than 50. You are better off buying a hat — or clothing that deflects UV rays.

4. Sunscreen is not waterproof. The FDA has banned the term "waterproof." It's now been replaced by "water-resistant." Labels on water-resistant sunscreens must clearly state how long they provide protection after water exposure or sweating. Depending on how often you swim, you can choose one that needs to be reapplied every 40 or 80 minutes.

5. Broad Spectrum. The FDA has also banned the term "sunblock." Instead, look for labels that state "broad spectrum." A broad spectrum sunscreen must pass tests proving that it truly protects against both ultraviolet A (UVA) and ultraviolet B (UVB) rays.

6. UVA vs UVB. When sunlight hits your skin, it is absorbing both UVA and UVB rays. UVB rays are the main cause of sunburn. UVA rays can prematurely age and wrinkle skin. Both contribute to skin cancer. That's why you always want a broad spectrum product.

7.    Use an ounce of sunscreen per large child or adult. That's about as much as would fill your hand — or a shot glass. You really want to cover all exposed skin well. Reapply at least once every 2 hours, even if your child doesn't sweat or go in the water.

8. Keep the Bug Spray Separate. Don't buy a product that combines a sunscreen and an insect repellent. Sunscreen should be reapplied frequently, but insect repellents should not. Buy separate products and apply them separately.

9. Choose lotions over sprays. While sprays seem easier, it's harder to be sure that you are covering all the skin well. Also, there are concerns that breathing in the spray (which is easy to do while spraying it) could be harmful.

10.  Don't buy sunscreen powders. These go on your scalp. They're easy to inhale, so avoid them.

11.  Apply sunscreen before you go out in the sun. That way, you go into the sun protected. Plus, you don't have to worry about children being squirmy or running away from you. If you have small children, apply the sunscreen before you put on their clothes or bathing suits. Then you'll be sure you don't miss any spots.

12. Safety. The safety of some sunscreen ingredients is controversial. These include oxybenzone, retinoids and nanoparticles. The Environmental Working Group has a list of 184 sunscreens that meet their safety criteria. You can choose one of those to be on the safe side. However, it's important to know that the American Academy of Dermatology does not believe that those ingredients pose a health risk. What does pose a health risk is too much sun exposure. Burns during childhood can especially raise the risk of skin cancer later in life. So the most important thing is to buy that sunscreen — and use it often and well.