Control of Air Revitalization Using Plants: Results of the Early Human Testing Initiative Phase I Test

The Early Human Testing Initiative (EHTI) Phase I Human Test, performed by the Crew and Thermal Systems Division at Johnson Space Center, demonstrated the ability of a crop of wheat to provide air revitalization for a human test subject for a 15-day period. The test demonstrated three different methods for control of oxygen and carbon dioxide concentrations for the human/plant system and obtained data on trace contaminants generated by both the human and plants during the test and their effects on each other.

The crop was planted in the Variable Pressure Growth Chamber (VPGC) on July 24, 1995 and the test subject entered the adjoining airlock on day 17 of the wheat's growth cycle. The test subject stayed in the chamber for a total of 15 days, 1 hour and 20 minutes. Air was mixed between the plant chamber and airlock to provide oxygen to the test subject and carbon dioxide to the plants by an interchamber ventilation system. Three methods of control of the air revitalization system were demonstrated: 1) use of integrated physicochemical systems to complement biological air revitalization; 2) actively controlling the level of biological air revitalization by modulation of photosynthetic photon flux (light level) to control the rate of photosynthesis; and 3) passively controlling the level of biological air revitalization by limiting the amount of available carbon dioxide to control the rate of photosynthesis. The three methods of control were demonstrated sequentially during the 15-day test.

The test successfully showed that plants could be used to support the air revitalization needs of a human test subject. The three different methods of controlling the human/plant system were successfully demonstrated, verifying that a biological system can be easily controlled to provide specific performance when desired. No negative impacts of the human on the plants or the plants on the human were observed during or after the test due to trace contaminants generated in the closed system, although the plants' generation of ethylene appeared to prevent pollination. This paper will describe the test operation and results.