CO2 hazard threshold in ambient air At approximately what concentration of carbon dioxide in atmospheric air (percent by volume) does an acute danger to human life exist, with a high likelihood of rapid loss of consciousness and death if exposure continues?

Introduction / Context:
Understanding toxicological thresholds for common gases is essential in environmental engineering and industrial safety. Carbon dioxide (CO2), although not toxic in the same way as carbon monoxide, becomes dangerously asphyxiating at high concentrations by displacing oxygen and depressing the respiratory drive. This question asks for the approximate concentration in air at which CO2 poses an acute, life-threatening danger.

Given Data / Assumptions:

• Ambient air baseline CO2 is roughly 0.04% (400 ppm).
• Exposure is short-term, acute (minutes), not chronic.
• No supplemental breathing apparatus; normal atmospheric pressure.

Concept / Approach:
CO2 is an asphyxiant. As concentration rises, the partial pressure of oxygen in the inhaled air falls and CO2 directly stimulates chemoreceptors, causing hyperventilation, headache, and eventually narcosis. Typical guidance values: 1–2% causes mild symptoms, 3–5% pronounced distress, 7–10% severe dizziness and unconsciousness, and near 20% rapid loss of consciousness with a high probability of death without immediate rescue. Therefore, the threshold of “acute danger to human life” is most closely represented by about 20%.

Step-by-Step Solution:

Verification / Alternative check:
Safety datasheets and industrial hygiene references classify 20% CO2 as an atmosphere rapidly fatal to unprotected personnel due to combined oxygen displacement and hypercapnia.

Why Other Options Are Wrong:

• 1%: Generally causes mild symptoms only.
• 3%: Distressing but not typically immediately lethal.
• 7%: Severe effects and loss of consciousness can occur, but the phrase “acute danger to human life (death)” is more definitively associated with ~20%.
• 12%: Highly dangerous but not provided in the original set; 20% better matches the “immediate death” framing.

Common Pitfalls:
Confusing occupational exposure limits (long-term) with acute lethality thresholds; mixing up CO2 (asphyxiant) with CO (chemical hypoxia).

Final Answer:
20