Between these extremes are applications that address specific parameters. The discussions are grouped by the level of intervention consider that a simple hospital mask only serves to filter particles but does not affect O 2 concentration, gas flow, or ambient pressure, whereas a hospital ventilator can take complete control of the human breathing system and dictate timing, flow, volume, and level of oxygen.

The article describes the current understanding of the physics of breathing and the processes for distributing O 2 from the air to the human tissues, organs, and the brain. Herein we discuss all manner of human breathing parameters and how different support systems work. This commentary describes normal breathing functionality and strives to put human breathing into context with the modern technologies used to extend the safety envelope beyond sea-level pressure and O 2 concentration to support life in situations ranging from extreme pressures in diving, to low pressures in aviation. Additionally, breathing protection, supplementation, and therapy appear in the media with respect to firefighters battling wildfires, pilots flying high-performance military aircraft, recreational self-contained underwater breathing apparatus (SCUBA) and professional divers, and medical and laboratory researchers. At the outset of the pandemic, the news media reported on the availability and use of medical ventilators as well as oxygen (O 2) supplementation via less invasive nasal cannulas or masks to help patients recover from pulmonary disease ( Kliff 2020, Ranney et al. Wearing hospital masks or improvised facial coverings was broadly implemented to protect against spreading infectious aerosols ( Pleil et al, 2020a, Eikenberry et al. The topic of human breathing mechanics and support has come to the forefront of public awareness due to the current COVID-19 crisis. The primary purpose of this article is to provide a basic understanding of normal human breathing and the adaptation of breathing in different environments using available technologies. The topics for medical intervention include nasal cannula, continuous positive airway pressure (CPAP), and medical ventilators. Topics for environmental intervention include filter-based and flowing air-supply masks for toxic environments (occupational and laboratory protection), and on-demand gas supply systems for firefighters, self-contained underwater breathing apparatus (SCUBA) divers, and altitude (high performance aircraft, spacecraft) applications. This overview describes the various technologies applied to maintaining a safe breathing environment. On the medical side, mechanisms for assisting breathing include O 2 supplementation at ambient pressure, positive pressure/flow without additional O 2, or a combination of both. On the environmental side there are two conditions to be addressed: toxicity of the surrounding atmosphere and changes in external pressure and O 2 concentration. When more challenging external environmental conditions or pulmonary illnesses exceed the capability for these adaptations, the human system requires technology to maintain sufficient ventilation to preserve life. Longer term adaptations for exercise and altitude include increases in red blood cell counts and higher concentrations of capillaries in muscle tissue. The body automatically adjusts for stress, exercise, altitude, and mild disease by increasing the volume and frequency of breathing. Normal breathing for healthy humans is taken for granted it occurs without conscious effort using ambient (1-atmosphere) pressure with 21% oxygen (O 2) concentration.

Air force pilot breathing techniques