You're heading to the pool or beach, and you stop to pick up some sunscreen.

But knowing what is the best SPF is tough when you have no idea what SPF actually means.

"It stands for sun protection factor. Is simply a ratio of the number of minutes that you can stay outside with the product on before getting minimal redness to the skin."

And Dr. Dawn Davis, a Mayo Clinic dermatologist, says preventing that painful redness is one of the biggest factors in preventing skin cancer.

So how do sunscreen brands calculate an SPF?

"So if you stand outside in a particular location and you're testing the sunscreen and it takes you 10 minutes to develop redness to the skin without the product on, but then you apply the product on a different area of skin and it takes 50 minutes for the skin to show redness, then that is an SPF factor of 50 over 10, which equals 5."

Dr. Davis recommends a minimum of SPF 30, which theoretically means you could stay protected from UV rays 30 times longer than without sunscreen.

For the Mayo Clinic News Network, I'm Ian Roth.