OAKLAND, Calif. -- You may think you're doing the right thing by slathering sunscreen on yourself and your children before heading outdoors, but a new study shows many of the most popular brands of sunscreen may not live up to their protection promises.

Unusually hot weather around the bay area has drawn big crowds to local beaches and public swimming pools this summer. Most people you talk to are serious about sunscreen.

"I have a seven-year-old and a five-year-old, and every time I come to the beach I put sunscreen on them," said Michael Fields as he lounged at his local beach in Alameda.

But what do you really know about your sunscreen? A public interest group reviewed about a thousand different sunscreens and says many -- especially the most popular brands -- fall well short of recommended levels of protection.

"We found four out of five of them either provide inadequate UV protection or contained ingredients that we're concerned about,” explained Environmental Working Group spokeswoman Rebecca Sutton.

Current FDA rules don't require testing for UVA protection. UVA is a part of sunshine that research indicates may cause skin cancer (along with UVB). This might possibly explain why skin cancer rates have not decreased despite a billion dollars a year in sunscreen sales.

Activists are also concerned about sunscreen ingredients such as oxybenzone.

"Each dose you are going to apply to your skin is going to penetrate into your body and possibly cause hormone disruptions or other health concerns," said Sutton.

Ingredients zinc oxide and titanium dioxide, which do not penetrate the skin, and a new one called mexoryl were found in the sunscreens that earned the best ratings. But consumers need to look at product labels carefully to stay informed when making purchases.

Manufacturers say they comply with FDA rules and the FDA has promised since 2007 to tighten labeling regulations on sunscreens. So far, no target date has been set for more stringent sunscreen rules, leaving it up to consumers to take the proper precautions.