An uneven skin tone, called hyperpigmentation, is irregular darkening of the skin.
The cause of uneven skin tone is the overproduction of melanin, the pigment responsible for skin and hair color. The resulting dark patches and spots are typically perceived as "aged skin."
There are several factor that can lead to uneven skin tone. A few of them are:
Sun exposure: Brown spots and uneven skin tone develop as a result of too much melanin being produced to help protect your skin from UV light.
Pollution: Particles and gasses from traffic-related air pollution are able to penetrate the skin and lead to formation of brown spots, particularly on the face.