Oxygen deficiency is defined as an oxygen level being less than what percentage by volume?

Oxygen deficiency is defined as an oxygen level that falls below 19.5% by volume. This threshold is significant because it denotes a condition in which there may not be enough oxygen available for humans to breathe safely, as the normal atmospheric oxygen concentration is around 21%. Levels below 19.5% can lead to symptoms of hypoxia, which can impair cognitive functions, physical abilities, and overall health, making it crucial to monitor and maintain safe air quality in various environments, especially in occupational and industrial settings.