CausesBy Mayo Clinic staff
Hair structure and direction of growth play a role in ingrown hairs. A curved hair follicle, which produces tightly curled hair, is believed to encourage the hair to re-enter the skin once the hair is cut and starts to grow back. Shaving creates sharp edges in this type of hair, especially if the hair is dry when shaved. When the shaved hair starts to grow out, it curls back to re-enter the skin (extrafollicular penetration).
When you pull your skin taut during shaving, the newly cut hair draws back into the skin, causing it to re-enter the skin without first growing out (transfollicular penetration). Using a double-edged razor also causes hair to re-enter the skin — the first blade pulls the hair out and the second blade cuts it, which allows the hair to retract. Transfollicular penetration also occurs with tweezing, which leaves a hair fragment under the skin surface.
When a hair penetrates your skin, your skin reacts as it would to a foreign body — it becomes inflamed.
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