According to MSN Healthy Living, most hairs that are swallowed just pass through the digestive tract and are eliminated in the feces. Hair isn't digestible due to its structure.
Individuals with trichotillomania or trichophagia that swallow large amounts of hair can develop hairballs in their digestive tracts, reports the Mayo Clinic. These hairballs can cause weight loss, vomiting and intestinal obstruction.
According to Chemistry World, hair is made out of keratin, a very stable fibrous protein. Most proteins are readily digested, but due to its structure ,keratin cannot be digested by most organisms. The protein structures in keratin are held together by disulfide bridges. Only strong reducing agents are capable of breaking apart the disulfide bridges. The chemicals used for changing the texture of hair by hair stylists are an example of reducing agents that can break apart these bridges. Unless the disulfide bonds are broken, the protein cannot be digested by mammals or practically any other life forms.
Certain species of bacteria are capable of digesting keratin, as demonstrated in a study published in the Journal of Bacteriology. Some species of insects carry keratin-digesting bacteria in their digestive tracts. The larvae of these insects are considered to be serious pests by humans because they eat clothing made out of keratin fibers. The Australian Pest Controller Association reports that there are 30 species of moths and 15 species of beetles known to consume keratin as larvae.