There are two common ways in which dentine can be exposed; gingival recession and tooth wear. The main cause of DH is gingival recession (receding gums) with exposure of root surfaces, loss of the cementum layer and smear layer. Receding gums can be a sign of long-term trauma from excessive or forceful toothbrushing or abrasive toothpaste (dental abrasion), or a sign of chronic periodontitis (gum disease). A less common cause is acid erosion, which is the loss of hard dental tissues due to acids e.g. related to gastroesophageal reflux disease, bulimia or excessive consumption of acidic foods and drinks. Repeated exposures to a low pH cause the mineral content of the teeth on the outer layer of enamel to dissolve therefore leaving the dentine exposed and leading to hypersensitivity. Other causes include dental bleaching, smoking tobacco (which can lead to recession and therefore sensitivity) cracked teeth and abfraction or grinding of teeth. Evidence of abfraction may be shown by wedge shaped defects that are developed at the cervical region of the teeth known as abfraction lesions. There is no direct relationship between abfraction lesions and diet, periodontal disease or abrasion.