Firefighter 1 Practice Exam 2026 – Complete Prep Guide

Oxygen-deficient atmospheres are those with too little oxygen to support normal breathing, especially under physical work. Normal air contains about 20.9% oxygen, so when the level drops to 19.5% or lower, the air is considered oxygen-deficient and requires protective measures such as respiratory protection or evacuation until conditions improve. This 19.5% threshold is the standard cutoff used in safety guidelines, which is why it’s the correct choice. The other values would either still be breathable or represent far more severe shortages, but they do not define the point at which air becomes oxygen-deficient.