The Scientist: Chris Adigun, M.D., an assistant professor of dermatology at NYU Langone Medical Center focusing on medical dermatology and nail disorders.
The Answer: If your nails look yellow after you wipe off your latest manicure, it could very well be a stain left over from the polish. The discoloration happens because, although they feel solid, your nails are quite porous, which makes them liable to absorb the pigments in your polish. The iron oxides used in red and pink pigments are the most aggressive agents of yellowing. It’s basically like the polish is rusting your nail, since iron oxide technically is rust.
Fortunately, this is a purely cosmetic—and temporary—problem. The pigment is absorbed in the superficial surface of the nail plate, so it won’t stain your fingers or nail bed, and has no effect on the nail growing in. A slight yellow tint will fade over time. More intense stains may last longer, but even the worst ones will disappear within six months when the fingernail grows out completely.
Still, if yellowing bugs you, a few tips. The longer the polish is on your nail, the more likely it is to stain it, so don’t keep it on forever. Applying a clear base coat underneath your polish may decrease the amount of yellowing by creating a barrier between the pigment and the nail (but that barrier isn’t impenetrable). If you have brittle nails that means they are more porous. Hydrating your brittle nails with moisturizer could help fortify them. But don't follow moisturizing with nail polish remover before painting your nails the way manicurists do, as this will open up brittle nails to further infiltration.
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