[Misc] If skin renews every 2-4 weeks, how come PIE and PIH can last for months?

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I think someone asked this in the past but I cannot find the thread. I am really curious about why, if skin renews every 2-4 weeks, red/pink/brown marks can sometimes last so much longer than that. Where is that redness originating from?
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Submitted by Sleepyrabbitz