When oil glands become plugged, acne occurs. Normally, the sebum (oil) flows out of the gland and out a pore to your skin's surface. If this path gets blocked, that is when Acne occurs because the sebum cannot reach your skin's surface. The most acne-prone areas are located on the face, back and chest. Many factors can trigger adult onset acne such as being hereditary, stress, and hormonal factors. Adult women are much more prone to develop adult onset acne.