Different theories like, overactive oil glands, clogged oil glands, stress, diet, hormonal imbalance, poor hygiene, and possibly a fungus such as P. Ovale may be da caue. According to the Mayo Clinic, the fungus is often present in dandruff but may not cause it. The dandruff may, in fact, be causing the fungus.
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I am in Mumbai for over 3 months now and surprisingly I find dandruff on my skull has increased. This has something to do with the air pollution, I think. Also doing QNA for long hours causes it as we use our brainy stuff to answer many questions taxing our brain.