According to the calendar, summer is winding down. But that doesn’t mean it’s time to put the sunscreen away. The sun can burn and damage skin any time of the year.

But how do you know which sunscreen to use and what do those SPF numbers really mean?

Sun protection factor, or SPF, numbers were introduced in 1962 to measure a sunscreen’s effect against ultraviolet B rays, or UVB. But there are two categories of UV light — UVA and UVB. The UVB rays cause sunburn, and UVA creates more long-term, damaging effects on the skin, such as premature aging. But both UVB and UVA rays may increase the risk of skin cancer.

Ultraviolet A (UVA) protection is not measured by SPF numbers and until recently was not available in sunscreens. But today there are several “broad spectrum” sunscreens that protect from both UVA and UVB radiation.

One key to how much protection a sunscreen may provide is in the SPF number, which represents the length of time you can stay out in the sun without burning, multiplied by the SPF number. So if you normally begin to burn in 10 minutes, you could theoretically have 150 minutes of sun protection with a sunscreen that has a SPF of 15.

Note the word “theoretically.” That’s because sunscreen SPFs are not one-size-fits all. One variable is how much you apply.

“To protect your body, you need to use at least 2 tablespoons of sunscreen, and not many of us use that much in every application,” says Jennie Washburn, licensed esthetician and owner of Lawrence’s SkinCare by Jennie.

And as for frequency of application, Washburn concurs with other experts that sunscreen should be applied every 15 to 20 minutes to ensure the best protection.

“It doesn’t matter whether you use a spray or lotion,” she says. “That’s a personal preference. But I believe that everything you put on your face should have a SPF in it. At least one layer of your makeup should have a SPF of at least 20. Couple that with a mineral powder, and you have complete protection.”

This powder uses titanium dioxide and zinc oxide, naturally occurring earth minerals which function as natural sunscreens. Washburn stresses the importance of carefully reading the label on any product you put on your face and looking for natural ingredients, as well as chemicals that you may not want to put on your face. That’s because chemicals, such as Parsol 1789, which block both UVA and UVB rays, are causing concern with environmentalists because they’ve been found in water, groundwater and soil, and potential long-term effects of exposure to these chemicals is not yet known.

“The bottom line is, don’t put anything on your body that you wouldn’t put in your body,” Washburn says.