Topical influences on hair growth
Areas of increased blood supply can stimulate hair growth, for example, moles and birthmarks with their increased blood supply can become hairy. Medical permission must always be obtained before performing epilation on a mole or birthmark.
The friction caused from a plaster cast rubbing on the skin can cause a temporarily increased blood supply to the area and sometimes stimulate extra hair growth. When the aggravation is removed, the hair growth will reduce.
Tweezing, which tears out the hair, stimulates a better blood supply and stronger hair to grow. Vellus and fine hairs are also frequently removed with tweezing, thus worsening the condition. Waxing of facial vellus hair will have the same effect as tweezing and may distort the follicle.
The ultraviolet light in direct sunlight affects the cuticle in a similar way to bleach, and eventually breaks down the keratin protein of the hair. The result is than the hair is gradually weakened and becomes drier. The effect shows up as light streaks in the hair (sun bleaching). Overexposure to sunlight for prolonged periods can damage hair and stimulate excessive shedding.
Hair dryers and other heated appliances soften the keratin of the hair and if they are too hot, they can actually cause the water in the hair to boil, and tiny bubbles of steam then form inside the softened hair shaft. The hair is thereby weakened, and may break altogether.