Chemical, or partially regenerative, methods for providing breathing oxygen by the regeneration of metabolic products such as water vapor and carbon dioxide include the thermal decomposition of water and CO₂, photolysis and radiolysis of water, electrolysis of fused carbonates and aqueous solutions, and the chemical reduction of CO₂ with H₂, followed by electrolysis of the water formed. Chemical regenerative systems have been developed to remove excess carbon dioxide and water vapor from the atmosphere. Nonbiological regenerative systems are time limited by the amount of food, water, and oxygen that can be carried or recovered. These physical-chemical processes show great potential, but they also present many difficulties, including requirements for extremely high temperatures and considerable amounts of power, the formation of highly toxic materials, and high susceptibility to inactivation. None of the presently studied nonbiological processes can function as completely as a bioregenerative system. All these nonbiological systems have unrealistic supply requirements and produce unusable wastes. Consequently, for long planetary missions the bioregenerative systems, though also beset with problems, are potentially far superior to their physical and chemical counterparts. Table VIII shows average daily metabolic data for a 70-kg astronaut. A man breathes about 10 cubic feet of air per minute, or 400 000 liters, daily. The expired air contains about 4 percent carbon dioxide. Man normally breathes air containing 0.03 percent CO₂, but can withstand comfortably about 1.5 percent CO₂. Anything in excess of 1.5 percent will produce labored breathing, headaches, and, if greatly exceeded, death. A man exhales about 1.1 pounds of water per day and this, in addition to water from perspiration and other sources, must be removed from the air. Table VIII.—_Average Daily Metabolic Data for a 70-kg, 25-Year-Old Astronaut With Normal Spacecrew Activity_ [From [ref.173]]