Even with hundreds of SPFs out there, the lot can be boiled down to two categories: chemical sunscreens and physical sunscreens.

But what formula is best for sensitive skin -- or overall? In the video above, NYC-based dermatologist Dr. David Colbert, who treats Rosie Huntington-Whiteley, boils down the differences between the two, plus how often you need to reapply.

PHOTOS: Sunscreens celebs swear by

In terms of what the stars are using, Selma Blair chooses a physical formula for her and her son Arthur, 10 months. "I'm green!" the actress has explained to Us. Meanwhile, Jennifer Lopez uses L'Oreal Paris Sublime Sun SPF 50+ on both her and her "little coconuts," twins Max and Emme. In the pic above, Diane Kruger douses herself with Neutrogena's aerosol formula.

VIDEO: Find out if there's a difference between a face and body sunscreen

Tell Us: What is your favorite sunscreen?

This article originally appeared on Usmagazine.com: Summer Skin Fix: Physical vs. Chemical Sunscreen