Ingrown hairs occur when a hair that has been removed starts to grow back and curves into the skin, causing inflammation. The following are some of the causes of ingrown hair:

• Hair removal methods: Ingrown hair usually happens after shaving, tweezing, or waxing. Cutting a hair can force it back into its follicle, and dead skin can clog the follicle, causing the hair to grow under the skin or re-enter its follicle or one nearby. The hair then grows inward instead of pushing through the surface of the skin.

• Hair structure and direction of growth: Hair structure and direction of growth play a role in ingrown hair. 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 a sharp edge on hair, making it easier to pierce the skin.

• Pulling your skin when you shave: This action causes hair to draw back into the skin.

• Tweezing: When a hair penetrates your skin, your skin reacts as it would to a foreign body — it becomes irritated.

• Hair type: People with very curly or coarse hair are more likely to get ingrown hairs. Cutting curly or coarse hair can lead to a type of ingrown hair called pseudofolliculitis or "razor bumps" after shaving, tweezing, or waxing hair in the area.

Ingrown hairs can cause tiny, swollen bumps on the skin that may hurt. The area around the ingrown hair can become swollen, red, and itchy. It can become inflamed so that it is painful and looks like a pimple with pus. Ingrown hairs can form cysts (sacs of fluid beneath the skin) that can range from a small painless lump to a large, infected swelling.