In reading your November 2012 article on oxygen systems I ran across, "The FAA requires using a mask instead of a cannula above FL180."
I've read this many times, but it is not in the FAR Part 91 regarding operations. It only appears in the FARs in Part 23, which applies to aircraft certification, and then requires a mask be provided for systems used above FL180. It says nothing about what a pilot or passenger must use.
There is not a limit of FL180 for passenger or pilot use in operational sections of the FARs.
With a reservoir cannula and pulse oximeter, there is no reason a cannula cannot safely be used above FL180.
Charles Robertson, MD St. Louis, Missouri