To better understand why acne causes your Post Inflammatory Hyperpigmentation (PIH) let's have a closer look at the source.
Let’s kick off with a quick explanation on how pimples are formed (remember, it is all about the source).
All skin follicles contain an acne bacteria called Propionibacterium acnes (P. acnes). P. acnes forms part of your normal skin flora and is found at the bottom of your follicles. Like any bacteria, P. acnes constantly divides (making new bacteria) but is also constantly killed off by oxygen. (Yeah for oxygen!).
The problem starts when your follicles become blocked or “clogged”. Clogging is where a build-up of dead skin cells and sticky, solidified sebum blocks the follicle opening. Clogged follicles prevent oxygen from reaching to the bottom creating an ideal environment for P. acnes to multiply unchecked!
Once P.acnes is out of control, the follicle very quickly becomes inflamed with dead cells, sebum, and bacteria. This forms a pimple.
In an attempt to fight the irritation, the skin's inflammatory response rushes white blood cells (killer cells) causing it to fill up with pus (these are your white pimples or pustules). If the inflammation continues, the follicle wall ruptures and the inflammation spreads under the skin surface creating a firm, red nodule which is larger than a pimple.
Another part of the immune response is the triggering of melanin production.
Once the follicle is "rescued" the inflammation will start to heal but the "extra" melanin that has been deposited will remain as dark marks on your skin. The dark marks (lesions) range from light brown to black in colour. The marks will become darker if exposed to sunlight (UV rays).
This is the story of how acne or pimples trigger (post inflammatory) hyperpigmentation.