That might be because glycerine can become the toxic acrolein at 290 degrees C, Crown.  There's been a lot of speculation on whether our devices get that hot and acrolein would be very irritating if we were.  It applies to all glycerine anyway.
http://en.wikipedia.org/wiki/Acrolein
	
		
			
		
		
	
			
			http://en.wikipedia.org/wiki/Acrolein