Summertime is upon us, and the sun is shining.  Heat waves are sending people out to the pool (or the beach, if you live near one).  But as you ramp up your outdoor activities, experts have a warning about how easy it is to misuse sunscreen.

FOX News Radio's Lilian Woo reports:

You may not be getting as much protection as you think you are when you reach for your sunscreen or sun block.  The SPF number, meant to indicate how many times more exposure a user can handle without getting sunburned, but dermatologist Amy Forman Taub says it's possible...

(Forman Taub) "You're getting about 1/3-1/2 of the SPF that's actually on the label."

The reason: Testers coat the skin much more generously than the average user does.  Experts consider one ounce - about enough to fill a shot glass - the amount needed to cover exposed areas of the body.  Most people use only 1/4 or 1/2 of that.

And if you go swimming or are sweating a lot, you have to do it all over again.  In any case, reapply every two hours.

Lilian Woo, FOX News Radio.