Posted for "Lancair" <lancair@USTEK.COM>:

 Mark,
 
 The small clear plastic masks that are found typically in light aircraft
 are not pressure masks.  Their seal is quite imperfect and even the
 facial-hair challenged typically have leaks.  They could be used above
FL180 to our effective ceiling of FL290 providing you do not need to
 communicate a lot.  The masks leak more with facial movement so you
 really need to be starting down if you have lost cabin pressure - and if
 non-pressurized ya really shouldn't be there long.
 
 There was a great article a few months ago in which a twin-Comanche
 pilot tried to extend his range by flying with a mask at FL250-270.  He
 could not control his blood oxygen level and finally decided to come
 down, so I guess that he was not completely delirious.  Values in the
 low 80's would get you critical care in an ER, and would get you into an
 ER if flying that way for long.
 
 The fast-donning masks that MH sells are about $5,000 and are required
 at higher flight levels (jet-jocky stuff).  However pressure masks are
 not required below ~FL350.  So says MH.
 
 Robert M. Simon,
 ES-P N301ES