CAUSES OF DANDRUFF IN YOUR DOG

There can be may causes of dandruff - poor diet, low humidity, and too infrequent grooming to distribute the oils.
A very thin layer of fat and waxes, called sebum, normally prevents the moisture in the skin from escaping into the air. Some animals do not receive enough dietary fat, or cannot absorb the fat, to form a good sebum layer. Diseases that cause an excessive loss of body water in the urine or bowel movement, such as diabetes mellitus, kidney failure, or chronic diarrhea, will cause dryness of the skin and dandruff.
Any chronic irritation of the skin will speed up the skin reproduction rate and cause dandruff.

Flakes from the surface of the skin may not be skin cells. Substances applied to the skin which dry and flake may mimic dandruff. Shampoos, conditioners, and other skin cosmetics should be thoroughly rinsed after application to the skin. If your dog has white skin flakes around the neck, or back, he's probably got dandruff and there are some things that you can do.
Do not use shampoos made for human beings on your pet, especially not baby shampoos or medicated shampoos. A dog's skin has different ph level than human skin, so the chemicals in people shampoo can actually harm your pet's skin.

Make sure that you thoroughly rinse your dog after shampooing. The shampoo residue can cause your dog to itch and scratch. The presence of dandruff can sometimes indicate that the onset of serious health problems such as skin infections, allergies or an invasion of nasty parasites.