The hard, outer layer that covers and protects the teeth. Dental enamel is the hardest substance in the human body. Enamel protects the less durable, underlying parts of the teeth: the dentin, pulp chamber, root canals, and nerves. Eroded enamel allows decay-causing substances to penetrate the more vulnerable inner parts of the tooth; this leads to more severe decay and possible damage to the root canals and the nerves. The fluoride found in toothpaste helps to keep the enamel more resistant to decay.