How do you choose a sunscreen? Maybe you have a favorite brand, but chances are you look for a specific Sun Protection Factor.

Here's the rub: That SPF number doesn't give you enough information.

The Environmental Working Group, an advocacy group, looked at the ingredient labels on about 1,000 sunscreens.

"We concluded that nearly four out of every five sunscreens are either ineffective in their ability to protect you from the sun or contain ingredients that are hazardous in and of themselves," said Sean Gray, a senior analyst with the Environmental Working Group.

"We found that they do a great job of protecting you from sunburn, but they do a terrible job, in most cases, of protecting you from long-term effects - sun aging and sun cancers," he said.

So what do you do? It's important to use sunscreen to protect your skin.

When shopping for sunscreen, you should look for the following ingredients: parsol 1789, avobenzone, mexoryl, zinc oxide or titanium dioxide. They screen out those UVA rays.

The Environmental Working Group says you should avoid products with oxybenzone, and skip the sprays because you can inhale the sunscreen mist.

The group has a list of more than 100 products it recommends. Check out the product you use or find a good one a the group's Skin Deep Cosmetic Safety Database.