This document gives:

• a description of the composition of the Earth's atmosphere;
• a description of the physiology of human respiration;
• a survey of the current biomedical literature on the effects of carbon dioxide and oxygen on human physiology;
• examples of environmental circumstances where the partial pressure of oxygen or carbon dioxide can vary from that found at sea level.

This document identifies oxygen and carbon dioxide concentration limit values and the length of time within which they would not be expected to impose physiological distress. To adequately illustrate the effects on human physiology, this document addresses both high altitude exposures where low partial pressures are encountered and underwater diving, which involves conditions with high partial pressures. The use of respirators and various work rates during which RPD can be worn are also included.