Any skin inflammation or injury down to the junction between the superficial and deep level will result in darkened skin. A very common cause is acne.

PIH is more common in the dark skin type of Asian people. Sometimes this slowly fades away if given enough time (as long as 1 year or more), but they can also be permanent.

Different treatment modalities have been tried. These include topical bleaching cream, micro-dermabrasion, chemical peeling, lasers etc. All work to a certain extend, sometimes very good effect, but to clear PIH completely 100% is rather not possible.