By Heather Brown

MINNEAPOLIS (WCCO) — Surveys have shown one-third of us don’t know the basics about sunscreen: Does it expire? When should you put it on? How much do you use?

Ahead of what’s likely to be one of the hottest days of the summer Thursday, we’re taking a look at some of your questions about sunscreen.

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How does sunscreen work?

Sunscreen works in two different ways. The first kind is a physical blocker that reflects the sun.. Those sunscreens contain titanium oxide and zinc oxide and are usually found in baby sunscreens or sensitive skin sunscreens.

The second, and more common, kind is a chemical sunscreen that absorb the sun’s rays.

Whichever kind of sunscreen a person uses, dermatologists recommend it have an SPF 30 and say “broad spectrum” to guard against UVA and UVB rays.

How much sunscreen should you use?

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“The amount of sunscreen they recommend is an ounce of sunscreen for your entire body,” says Dr. Liz Farhat, a dermatologist with Allina Health, “So that’s quite a bit of spray.”

Does sunscreen expire?

Yes. Sunscreen manufacturers suggest people throw it out after three years. According to Dr. Farhat, the chemicals within the sunscreen break down after three years.

When should you apply sunscreen?

Sunscreen should be applied 15 minutes before a person goes into the sun to allow the ingredients to bind to the skin. It should be reapplied every two hours and more often if you go into the water.

Should you use the spray?

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“The main thing with the spray is that you have to rub it in,” says Dr. Farhat. “If you use (only) spray, it’s not getting evenly distributed on your skin.”

Heather Brown