Gravity and Human Respiration: Biophysical Limitations in Mass Transport and Exchange in Space

Abstract A major requirement for humans is a breathable atmosphere. In microgravity, despite environmental life support systems regulating air exchange, astronauts complain about air quality, with elevated CO2-levels resulting in detrimental health and performance effects. We extend extant accounts of human respiration to include the role of gravity and buoyancy. Using computational fluid dynamics, we demonstrate that the absence of biothermal convection in microgravity reduces airflow around the human body. This impairs gas exchange by creating an environmental breathing deadspace in front of the face, leading to significant CO2-rebreathing, with implications for astronaut health and countermeasures. In 1g, increasing ambient air temperature can also reduce efficient respiratory exchange, resulting in breathing conditions equivalent to those in microgravity, with implications for treating respiratory disease on Earth.