Makeup brushes are typically used on a daily basis, touching and picking up makeup particles and from the face also picking up dead skin, dirt, and oils. They are little dirt traps that you essentially rub all over your face that could cause, acne, clogged pores, and other breakouts. The best way to prevent all this build up is to regulatory clean your brushes. Regular, as in on a weekly basis.
The best way to prevent dirt build-up, is to clean your brushes on a weekly basis!