If you've ever had acne, you are probably familiar with the fact that the skin can look irritated or red long after the pimple itself is gone. The technical term for this is post-inflammatory erythema, also known as PIE, and it can be just as frustrating and stubborn to treat as the acne itself. So, why does this happen? Inflammation associated with acne can trigger dilation of blood vessels, giving the skin a lingering red appearance. However, mild and non-inflamed acne like whiteheads and blackheads don't typically cause PIE. Instead, you should look out for acne with pustules and inflamed papules which are more likely to cause it. Want prevention tips? Don't pop, squeeze, or pick at acne, as it can increase the risk of triggering more inflammation and redness.