Smoking and Gray Hair
A: Well, technically, there is no research to prove that smoking causes gray hair directly. What is known through studies is that smoking can contribute to premature aging.
Gray hair is formed when the melanocytes in the hair follicles stop producing melanin (pigment). The cessation of melanin production is usually triggered by hereditary encoding as we age, but there is evidence that shows that prolonged illness, periods of stress, and some medical interactions can cause the melanocytes to stop producing prematurely.
As smoking does add stress to the body (chemical dependency is always stressful to the body) and since smoking is known to contribute to premature aging, smoking can be - indirectly - responsible for causing gray hair.
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See also:
Going gray
Why and how hair turns gray
Is smoking bad for your hair?