Washington State Underground Storage Tank (UST) Site Assessment Certification Practice Test

The breathing range of oxygen for humans is best defined as being between 19.5% to 21% in the atmosphere. This range is significant because it represents the level at which oxygen is sufficient for normal respiratory function. At concentrations lower than 19.5%, individuals may begin to experience symptoms of oxygen deficiency, including dizziness and impaired cognitive function. Meanwhile, levels above 21% are still typically safe, but they may not be necessary for vital function.

Understanding this breathing range is critical, particularly in occupational safety and health, as environments with oxygen concentrations below 19.5% can pose a health risk and require specific safety protocols to ensure worker safety. This is why the answer that identifies this specific percentage range as the breathing range is correct.