You will also find this chemical in thrush creams. You would have also noticed zinc pyrithione in many anti-dandruff shampoos. It's an antifungal and antibacterial which helps stop itching and flaking. It is also used in paints and on carpet backings to prevent mildew and algae. Coal tar has been a popular treatment, although it is not very nice stuff.
Coal tar can irritate the skin and even cause dermatitis.
It's also carcinogenic and has been responsible for cancer of the scrotum. Use of coal tar on large sections of the body is not recommended. Some people swear that dissolving half a dozen aspirins in a cup of water and leaving it in your hair for 15 minutes will get rid of the dreaded flakes. There is some merit in this, as the main ingredient in aspirin is salicylic acid. Massaging tea-tree oil into the scalp may also help.