We all know how important it is to slather on sunscreen before heading outdoors, but did you know that it’s equally — if not more — paramount to apply SPF even when you’re inside? Or that you really should be reapplying about every two hours? Because the hard and fast rules of UV protection are often blurred, we’ve reached out to dermatologists to find out the truth behind sunscreen, when to wear it, and for how long.

1. Sunscreen should be worn daily, no matter your location — indoors and outdoors.

“Completely indoor activities don’t require sunscreen, but many of us discount the sun that we get on a daily basis from just running errands and all the ‘incidental’ sun damage adds up,” explains Elizabeth Tanzi, founder and director of Capital Laser & Skin Care and associate clinical professor, department of dermatology at the George Washington University Medical Center. “That’s why we recommend daily sunscreen application, so you are always protected and don’t have to think about it.”

That means even if you spend most of your day indoors, says Ava Shamban, a dermatologist in Los Angeles: “Indoor UV exposure can occur from ultraviolet that penetrates through glass, which is UVA. UVA is emitted at the same level — all day long — where is UVB, which is blocked by glass peaks mid-day.”

2. For everyday wear, sunscreen should be applied to the face, as well as other areas of concern.

Sunscreen should be worn on all over the face, ears included, recommends New York City-based dermatologist Debra Jaliman. “Cover your entire face with sunscreen, as well as your neck and hands,” she says. “If you’re not sweating a lot, you can just apply it first thing in the morning.”

3. The magic SPF number is 30.

“The American Academy of Dermatology always recommends an SPF of 30, because it is clinically proven to be a sufficient amount of protection to reduce or minimize the adverse effects of sunlight,” says David Colbert, a New York City dermatologist.