o2
411 pressure breathing is not required untll 40,000 feet, where it is essential. There are many aircraft that fly at 30,000 ft unpressurised.
This book is well worth reading:
Fundamentals of Aerospace Medicine By Jeffrey R. Davis, Robert Johnson, Jan Stepanek.
Firewall : Nitrogen narcosis is an effect on the brain of gaseous nitrogen that occurs to divers who go below 100 FSW, due to the laws of partial pressures.