I have met the idea in conversation on multiple occasions that human hair is dangerous to eat in moderate to large quantities as it will build up in the digestive tract causing a blockage in the digestive tract. Is there any evidence that this is the case? According to this answer on answers.com keratin is not digestible by humans but surely that isn't the same as potentially causing a blockage as keratin is just a fibrous protein structure which I would have thought would not have been on a scale to cause a blockage.
Yes, this is true:
Trichobezoar, a hair ball in the proximal gastrointestinal tract, is a rare condition almost exclusively seen in young females. Human hair is resistant to digestion as well as peristalsis due to its smooth surface. Therefore it accumulates between the mucosal folds of the stomach. Over a period of time, continuous ingestion of hair leads to the impaction of hair together with mucus and food, causing the formation of a trichobezoar. [...] In addition, incidentally, parts of the tail can break off and migrate to the small intestine, causing intestinal obstruction [...] When not recognized, the trichobezoar continues to grow in size and weight due to the continued ingestion of hair. This increases the risk of severe complications, such as gastric mucosal erosion, ulceration and even perforation of the stomach or the small intestine. In addition, intussusception, obstructive jaundice, protein-losing enteropathy, pancreatitis and even death have been reported as complications of (unrecognized) trichobezoar in the literature.