How Microgravity Influences Accumulation and Inhalation of CO2 in the Spaceflight Breathing Environment

Abstract CO₂ bubbles and pockets have plagued astronauts for many decades since the inception of space exploration, with recurrent reports from astronauts of headaches as the main symptom. The first CO₂ monitors were deployed in the International Space Station (ISS) in 2001, and have reported average CO₂ concentrations of ~ 0.5% (or 5000 ppm) in air. Despite the enhancement in performance of CO₂ removal systems that extract CO₂ from ISS cabins to the external environment, CO₂ concentrations that are at least ~ 0.3% have prevailed, causing CO₂-related symptoms and subclinical changes to physiology. In order to explain the abnormal concentrations of CO₂ in the breathing environment of the astronauts, the authors suggest that there is a build-up of CO₂ that is influenced by the lack of gravity, also termed microgravity, in low Earth orbit (LEO). This suggestion is motivated by the effect of gravity on buoyancy-driven convection that can facilitate the advection of exhaled CO₂ from the breathing envelope of a respirator, preventing its build-up in the breathing envelope. The processes that define the behavior of CO₂ in the breathing environment of a respirator in astronaut-like conditions in the ISS are revealed through simulations of a computational fluid dynamics (CFD) model of a human respirator that exercises breathing over several breath cycles. This modeling effort is unique among research efforts that have studied CO₂ accumulation in the ISS crew habitat, as it accounts for all the main factors that influence the dynamics of CO₂ – the transient nature of breathing, changes in thermodynamic variables in the environment, and, particularly, changes in CO₂ dynamics over several breath cycles. While simulations of several breath cycles in normal gravity show no tendency for CO₂ to accumulate in the breathing environment, accumulation of CO₂ is evident in microgravity, with the inhalation of CO₂ at average concentrations higher than 0.1% (1000 ppm) in air as a possible outcome in some breath cycles. Breathing in such an environment in a prolonged period of time lasting several days inevitably leads to the build-up of CO₂-related illnesses.