If the enamel is lost or damaged due to decay, chemical erosion or abrasion, the softer dentin is exposed and teeth wear rapidly. In third world countries a lot of the food is unprocessed and contains dirt and other abrasives which can cause wear while chewing. Most wear on teeth in the United States and other developed countries however, occurs due to clenching. Normally the teeth only contact during chewing and swalowing. If you find your teeth contacting at any other time, you are clenching.
Research indicates most people clench to some extent during sleep.
Others clench more frequently or have habits such as biting the fingernails, pipe smoking or gum chewing that contributes to wear. Occupational environment can also influence the amount of wear in someone who clenches. Wear of the teeth can easily be distinguished from erosion because tooth structure is only missing where the teeth contact each other. Below are some examples of severe wear. Note the wear at the arrows on the back side of these upper front teeth.