On average, 90% Americans shampoo their hair daily. Centuries ago people washed their hair on monthly basis. In 1950s getting hair washed and styled once a week from a salon became customary. An advice column was published in the year 1908 in the New York Times, encouraging people to wash their hair twice a week, and claimed it to be completely harmless. Thanks to them and the aggressive marketing campaigns by the shampoo manufacturers, today people cannot stay without washing their hair frequently. What can be the hazards of frequent washing? We will get back to that later. First it is important to understand what makes your hair look dirty?