Scalp acne is less common than acne on the face or back, but it develops in the same way. Skin naturally produces a protective layer of oil known as sebum. Sebum makes its way to the skin's surface through pores, and excess oil, dead skin cells or other substances can clog them. Bacteria multiply in the trapped sebum, forming a pre-pimple, or a microcomedone.
As the microcomedone becomes larger and more visible, it becomes a comedone -- a whitehead or a blackhead. The comedone can become inflamed and red, and it may fill with pus: Then, a zit is born.
So, at its most basic level, any pimple -- including one that pops up on your scalp -- is caused by some sort of blockage in the pore. It's a little harder to pinpoint exactly what causes the blockage. Hormones play a part, as does the quantity of oil your skin produces. People with oily skin seem to be more prone to acne of all types.
Your scalp produces sebum, too, and hair holds that sebum close to the scalp, contributing to the buildup of oil. On top of that, hair products, from conditioners to styling gels, can contain ingredients that can clog pores.
Since throwing out all your hair products probably isn't an option, what can you do to prevent scalp pimples?