PIH is an acronym for Post-Inflammatory Hyperpigmentation. Post-inflammatory means "after inflammation." Hyperpigmentation means "increased pigmentation," or a darkening of the skin. So post-inflammatory hyperpigmentation, or PIH, is the discoloration or darkening of the skin that develops after an inflammatory wound.
PIH spots are darker than the surrounding skin tissue, and can vary in color from pink to red, purple, or brown.
In a word: inflammation. PIH is the skin's natural response to inflammatory wound.
Acne is a common cause of PIH. Controlling inflammatory acne breakouts is an important step in reducing PIH development.
And if you pop your pimples or pick at the scabs, you're upping your chance of developing PIH.
If you have skin, you can develop PIH. Certain factors can make you more susceptible to developing PIH, though. Obviously, having inflammatory acne will increase your chances of developing PIH than mild, non-inflammatory breakouts.
Your skin tone also influences your chances of developing PIH. It is more common in medium to dark complexions.
PIH isn't a scar, at least not in the true sense of the word. There is no loss (or gain) of skin tissue. Only the color of the skin is changed. So, luckily, PIH isn't necessarily permanent though in some cases it can be.
PIH can fade on its own. Granted, it does take quite some time (think in terms of months rather than weeks).
There is good news for you: PIH can be treated. Mild discolorations can respond to over-the-counter products containing alpha hydroxy acids, hydroquinone, or N-acetyl glucosamine (NAG) and niacinamide.
But more stubborn cases are best treated by your dermatologist. He may prescribe topical medications or suggest other procedures that will help diminish PIH.