External causes include excessive use of hair sprays and gels, improper use
of hair-coloring products, cold weather and dry indoor heating, infrequent shampooing
of the
hair or inadequate rinsing of the scalp and hair.
Dandruff is associated with a tiny fungus called Pityrosporum ovale, or P. ovale for short. This fungus lives on our bodies and scalp all the time, usually without causing a problem. Unfortunately, for some people, it can get out of control -- leading to dandruff.