Human Respiration Complications from Limitations of Mass Transport in Microgravity

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 bio-thermal 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. We also show that in 1g, increasing ambient air temperature can also reduce buoyancy required for efficient respiratory exchange, resulting in breathing conditions equivalent to those in microgravity, with implications for treating respiratory disease on Earth. The simulations were conducted in 2D and 3D. 2D direct numerical simulations (DNS) were conducted using high-order spectral element methods (SEM). 2D and 3D RANS simulations were conducted to validate the 2D DNS simulations.