This post is the 4th post in the for the love of the comb series.
Cause: Your hair produces sparks when it comes in contact with synthetic products. What? Right, your hair produces electricity. The reason is because the hair has the ability to become charged with static electricity, and since keratin is not a good conductor of electricity, friction between your hair and synthetic products gives your hair an electric charge (aka triboelectric). Exchange of electric charges in your hair increases when your hair cuticle is damaged and decreases when the hair is either wet or damp.
Hair picture by xaotherion