A toothbrush is an oral hygiene instrument used to clean the teeth, gums, and tongue. It consists of a head of tightly clustered bristles, atop of which toothpaste can be applied, mounted on a handle which facilitates the cleaning of hard-to-reach areas of the mouth. They should be used in conjunction with something to clean between the teeth where the bristles of the toothbrush cannot reach - for example floss, tape or interdental brushes. They are available with different bristle textures, sizes, and forms. Most dentists recommend using a soft toothbrush since hard-bristled toothbrushes can damage tooth enamel and irritate the gums. Because many common and effective ingredients in toothpaste are harmful if swallowed in large doses and instead should be spat out, the act of brushing teeth is most often done at a sink within the kitchen or bathroom, where the brush may be rinsed off afterwards to remove any debris remaining and then dried to reduce conditions ideal for germ growth (and, if it is a wooden toothbrush, mold as well). Many toothbrushes have plant based handles, often bamboo. However, numerous others are made of cheap plastic, such brushes constitute a significant source of pollution. Over 1 billion toothbrushes are disposed of into landfills annually in the United States alone. Bristles are commonly made of nylon (which, while not biodegradable, as plastic is, may still be recycled) or bamboo viscose. Before the invention of the toothbrush, a variety of oral hygiene measures had been used.