Melasma

Melasma is a common hyperpigmentation disorder, which means a patch of skin gets darker than it should. It's acquired, meaning you aren't born with it, and it's benign: it isn't cancer. It shows up as brown-to-gray patches that are symmetric, so they look roughly the same on both sides of the face, mostly in the areas that get sun.

To see why it happens, look at the epidermis, the outer layer of the skin. That's where the melanocytes are: the cells that make melanin, the pigment that gives skin its color. In melasma there usually aren't many more melanocytes than normal. What changes is that they overwork. Picture a paint shop that didn't hire any new painters, but the same painters are now going through far more paint. That extra work is called hyperfunctional melanogenesis. The extra melanin ends up stored in keratinocytes, the cells that make up the epidermis, in its deepest layer and in the layer just above it.

So what pushes them to work this hard? The most consistent factor is light. Being exposed to it chronically, day after day, helps melasma appear and also contributes to relapses, when it comes back after it has gotten better. Hormones play a part too. Pregnancy, and estrogen or progesterone that come from outside the body (both are sex hormones), can make melasma start or get worse.