But all that knowledge doesn't do us much good if we aren't sure of the best way to harness vitamin C's powers. Historically, the ingredient has been notoriously tricky to stabilize in topical products. Like vampires, vitamin C loses stability and potency when in contact with air, light, and water (the basis of most skin-care products) — which makes it extremely tricky to work into a serum or mask. “Because vitamin C is so unstable in the presence of light, extremes in pH, and temperature, products that contain it are specially formulated,” says New York-based dermatologist Joshua Zeichner , MD. In other words, not all C-based skin care is created equal. Adds Dr. Graf, “The most important part of topical vitamin C is what type of vitamin C it is, since it must be stable in order for it to be of value.”