Post inflammatory hyperpigmentation (PIH).

It’s a common and well known fact that after having an injury on the skin, as caused by a burn, cut, infection as acne, or any other cause, once the healing has ended, the skin frequently appears to be of a darker colour. This takes place more frequently in darker skin also. This is called post inflammatory hyperpigmentation (PIH).

It happens after a significant Sun exposure on skin that is being repaired from any kind of injury.

PIH is a consequence or reaction from the cells of the skin that are responsible for the protection from the ultraviolet radiation coming from the Sun, or from other sources of light with a similar wavelength. Those cells are called melanocytes and they produce a pigment called melanin.

When melanocytes are stimulated by that type of light, they will produce more melanin as a normal reaction to protect the skin, and that will lead to darker skin colour. 

Although the exact mechanism of PIH is unknown, the increase in melanocyte activity is stimulated by a number of inflammatory mediators as prostanoids, cytokines, chemokines, and other inflammatory mediators as well as reactive oxygen species that are released during the inflammatory process (leukotrienes such as LT-C4 and LT-D4, prostaglandins E2 and D2, thromboxane-2, interleukin IL-1, IL-6, tumor necrosis factor TNF-α, epidermal growth factor, and reactive oxygen species such as nitric oxide) (1, 2).

Obviously, the Sun is the main source of ultraviolet light in the environment, so avoiding Sun exposure is the first precaution to take in order to prevent postinflammatory hyperpigmentation. Hence the importance of using sun-protection creams, at all times, and specially during the healing process of any facial injury (sunblocker, 50+).

But not only the Sun might be responsible and must be avoided, because the use of sun beds and exposure to similar sources of ultraviolet light will produce similar effects.

Considering that during Winter time the Sun exposure in the Northern Hemisphere is weaker, it is then the best moment to start the treatment of these cases, with specific creams, peelings or other procedures, because the risk of having  any Sun exposure is much lower (except if you are going on sunny holidays during that time). 

References

1.     Davis EC, Callender VD. Postinflammatory hyperpigmentation: a review of the epidemiology, clinical features, and treatment options in skin of color. J Clin Aesthet Dermatol. 2010 Jul;3(7):20-31. PMID: 20725554; PMCID: PMC2921758.

2.     Robert A Schwartz, Dirk M Elston. Postinflammatory Hyperpigmentation. https://emedicine.medscape.com/article/1069191-overview