(WCCO) As much as we enjoy the outdoors, parents know we have to protect our kids and our skin from the sun. That often means slapping on the lotion with the highest SPF. That stands for sun protection factor, but many of us don't use it, which is why skin cancer cases are on the rise.

"I see lots of sun damage," said Dr. Charles Crutchfield, a dermatologist in Eagan, Minn. "Anytime you get darkening of the skin you're promoting skin cancer, aging and all the things you don't want."

The sun exposure that matters most is that we get as kids.

"I have a 4- and 6-year-old and they spend basically the entire day outside," said one woman at a Minneapolis swimming pool.

That's why nearly 80 percent of our sun exposure occurs before age 21.

"The cruel hoax is by the time you're smart enough or wise enough to sun protect it's almost too late," said Crutchfield.

So what do we need to know about sunscreen?

"I wear 35," said one Twin Cities man.

That number refers to the sun protection factor and how long the sunscreen will protect us beyond our skin’s built in protection. For example, if it normally takes 10 minutes for our skin to burn without sun block, an SPF 15 would theoretically protect us 15 times longer or 150 minutes. However, the SPF number only refers to one type of the sun's rays.

"The SPF is how much it blocks the ultraviolet B rays," said Crutchfield. "B stands for burn."

However, we also need to block the sun's UVA rays.

"You can think of 'A' as in aging, wrinkles, damage to skin, skin cancer," he said. "You need protection from both of those. That's actually probably as important, if not more important, than the SPF number."

The FDA is pushing to add the UVA rating to sunscreen bottles. New rules about that should be out in the next few weeks. Here are some other sunscreen tips:

• Regardless of what the label says, sunscreens won't protect us all day. We should reapply every one to two hours.

• There's really no such thing as waterproof sunscreen. The FDA doesn't even recognize that term.

• It takes 20 minutes for sunscreen to become active on our skin, so we need to put it on before we go outside.

• Sunscreen does expire. The chemicals break down and lose their protective ability, so replace any sunscreen you have had for more than a year.