From: "The Secret Way", by Paul Brunton:

p.113,114:

...It is proper here, to issue an ernest warning against the injudicious practice of the Indian Yoga- Systems of Breathing as they have been published.

The Yoga- system of breath-control may be followed safely, without any danger if under guidance and protection by a teacher, is very dangerous without a teacher.  A follower of an Indian Yogi once told me: "The Old Masters, who knew the effects of the  different ways of breathing, tell us that by this breathing we may acquire godly powers, but that we also might also be ruined by insanity, incurable diseases and sudden death. So you will understand that, though the reward is many times greater, the danger should not be underestimated. Our system does need practice for different ends; some of them are practically harmless, others may induce heavy damage if practiced incorrectly"...

p.115:

...Do breathe through both nostrils: every European or American student that follows the yoga-system whereby the nostrils are used in turn, is exposing his physical and mental health to grave danger - let him not venture into this.At the very least he will risk expanded lungs. Such artificial, unnatural breathing exercises are most often done with the intent of obtaining spiritual powers; they have nothing at all to do with the natural control of breath that we are advocating here, which is meant to get control over the feaverish rush of thoughts and to make our breathing as peaceful as that of a baby in its mother's lap...