Every hair on the human skin grows out of a pore or canal, called a follicle. This follicle contains both the hair and a sebaceous gland, or oil gland. When functioning properly, the sebaceous gland helps keep the skin and hair in good condition, and helps the body get rid of old skin cells. When it's not working correctly, however, the sebaceous gland may cause oiliness, skin problems and even hair loss.
Sebaceous glands produce an oily, waxy substance called sebum. They start working a few hours after birth, and work for the rest of a person's life. According to the National Institutes of Health, secretions from the sebaceous gland keep hair and skin from drying out, help the skin maintain its barrier against bacteria and even kill microbes on the skin's surface. Animals with defective sebaceous glands progressively lose their hair.
Regular brushing and massage may help people with overactive scalp sebum production and the resulting oily hair and acne. Brushing moves the sebum down the hair follicle to the dryer ends, away from the too-oily scalp area. People who practice this technique do not use the brush to detangle. Instead, they detangle their hair with a comb, then brush the hair with a boar's bristle or similar natural bristle brush. This provides scalp stimulation and a reduction in oil build-up. Brushing to redistribute oil from the sebaceous glands is an important part of hair care for many people with extremely long hair.
Overactive sebaceous glands in the hair follicle may lead to acne. The earlier this problem starts, the more likely it is to be serious. The National Institutes of Health note that increased excretion of sebum is a major factor in the majority of acne cases. Because of this, some drugs being developed for acne treatment focus on reducing sebum production, rather than simply treating the symptoms of inflammation and swelling.
An over or underactive sebaceous gland may lead to hair loss. According to DS Laboratories, plugs of sebum in the hair canal can prevent hair from emerging. Inflammatory alopecia (hair loss) conditions, such as lupus and scleroderma, tend to destroy the sebaceous glands early on, creating scar tissue and encouraging the hair to fall out.
So-called "bad hair days" in women often occur in relation to menstruation. Some women have theorised that their bad hair days are caused by greater sebum excretion during their periods. However, according to the National Institutes of Health, there is no discernible difference in sebum levels during or after menstruation. Bad hair days are more frequent in women who wash their hair less frequently, however.