A number of reasons cause hair on the head to become smelly, including poor hygiene, environmental factors, hair care products and medical conditions such as dandruff, acne on the scalp and seborrheic dermatitis. There are also claims of a condition called "smelly hair syndrome" in which an individual's head emanates foul odor without any obvious cause. These claims, however, are yet to be taken seriously by the medical community.
Each strand of hair grows out of the skin through a follicle that also secretes an oily substance called sebum. This substance is the body's natural hair and skin conditioner that helps keep the skin and hair healthy by waterproofing and keeping both moisturized.
Sebum, however, also traps dirt, grime and bacteria, which when left on the skin or hair for a long time can accumulate and contribute in making the hair smell. Frequent washing and shampooing the hair at least twice a week can help keep the hair from smelling bad.
Using hair care products with strong fragrances may help mask natural hair odor, but it can also make the hair smell. Shifting to unscented products may work well for some individuals. Using sulfur and triclosan-based soaps on the hair and scalp may help those with bad hair smell, but it is good idea to consult with a medical professional before using any of these products.