The other breathing exercise which is dangerous--not physically so much as mentally--is that which prescribes breathing through alternate nostrils so that one nostril is closed by a finger and only the other is used until the changeover is made to the other nostril. This exercise is the one that threatens sanity. I would enforce as a rule that everyone who sets up to teach hatha yoga to others should be compelled to go through a course of at least one year in the anatomy of the body and then in the physiology of the body. The work must have a scientific basis because it encroaches on the medical domain.
-- Notebooks Category 5: The Body > Chapter 6: Breathing Exercises > # 4
-- Perspectives > Chapter 5: The Body > # 26