What are “razor bumps”?

The hair follicles of blacks and many other people of color, such as Puerto Ricans, are elliptical, leading to development of tightly curled hair. After shaving, as the hairs regrow, there is a tendency for the sharp end of the curled hair to curve back into the skin. When the hair pierces the skin, it causes an inflammatory reaction, just as one might see with a splinter. This inflammatory reaction leads to the development of pseudofolliculitis barbae. This condition is not normally seen in men who grow beards, because after attainment of a certain length, usually 3 to 6 mm, the hair does not curve back into the skin. Accordingly, the condition is most common among populations required to be clean-shaven, such as black men in the military. Acne keloidalis nuchae represents a similar condition arising on the occipital scalp and/or nuchal area of those with shaved or very tightly cropped haircuts.

Kelly AP: Pseudofolliculitis barbae and acne keloidalis nuchae, Dermatol Clin 21:645–653, 2003.