First let me say a little about what scientists believe causes acne
and then I'll explain why they might showing up where you are describing and how to prevent their future formation. Scientists believe that there are four major factors that lead to acne: 1) increased production of a protein on the skin called keratin which leads plugging of follicles and the creation of "pimples;" 2) increased production of an oily substance called sebum that helps to plug the follicles; 3) breakage of the pimples which causes inflammation of the skin; 4) and finally a bacteria called p. acnes that lives on the skin and helps to worsen the inflammation. A pimple is one form of acne and called a comedo. It is the result of the plugged follicle containing the excess keratin and sebum. Acne will form only in places with well-developed sebaceous glands such as the face, neck, chest, and back. The distribution of acne that you are describing is likely simply related to where the most glands are. And so it is not likely that dried shampoo is the cause, though it may make things slightly worse by plugging follicles; it is much more likely pure anatomy. Nevertheless, oily hair can contribute to acne and make it worse by plugging follicles and so the type of shampoo you use may make things worse. You should avoid oily products for both your hair and your face, including shampoos that leave an oily residue. Your primary care provider or a dermatologist
can better evaluate your symptoms and discuss the further treatment options for you.