Asphyxiation can occur at any oxygen level below what percent?

• A. 20 percent
• B. 19.5 percent
• C. 18 percent
• D. 15 percent

Answer: (B)

Oxygen concentration dictates whether air is safe to breathe. Normal outdoor air contains about 20.9% oxygen. Safety standards define an oxygen-deficient atmosphere as 19.5% oxygen or lower because at that level the body's tissues receive insufficient oxygen, leading to impairment, dizziness, confusion, unconsciousness, and a real risk of asphyxiation if exposure continues. In wastewater work and confined-space safety, this threshold is used to trigger monitoring, ventilation, or protective measures. So, asphyxiation can occur at any oxygen level below 19.5 percent. The other options either sit above this hazard threshold or are not the standard safety cutoff used for defining oxygen deficiency.