Environmental engineering safety: in human exposure terms, at approximately what carbon monoxide (CO) concentration in ambient air (ppm) does the risk escalate to a level where death is expected if exposure persists (note: 0.3% by volume equals 3000 ppm)?

Introduction / Context:
Carbon monoxide (CO) is a colorless and odorless gas produced by incomplete combustion. In environmental engineering and occupational hygiene, recognizing the order of magnitude at which CO becomes fatal is essential for designing controls, alarms, and response plans. This item asks for the concentration at which death is expected if exposure persists, using the ppm scale common to air monitoring.

Given Data / Assumptions:

• Ambient air exposure context, not short-duration, high-pressure lab exposure.
• Units are parts per million (ppm).
• Relationship: 0.3% by volume equals 3000 ppm.
• No supplied oxygen enrichment or respiratory protection.

Concept / Approach:
CO binds with hemoglobin to form carboxyhemoglobin (COHb), reducing oxygen delivery to tissues. As concentration and duration rise, COHb increases nonlinearly. Guidance tables associate low hundreds of ppm with serious symptoms over time and thousands of ppm with rapid collapse and death. Thus the highest option given, 3000 ppm, corresponds to a level where fatality is expected in the absence of rapid rescue.

Step-by-Step Solution:

Verification / Alternative check:
Alarm standards and toxicology summaries consistently indicate that several thousand ppm of CO can be rapidly fatal, matching 3000 ppm as a lethal regime.

Why Other Options Are Wrong:
50 ppm: elevated but not generally fatal with brief exposure.
500 ppm: very hazardous, but not inevitably fatal with short exposure.
1000 ppm: extremely dangerous; still lower than typical fatal regime in this framing.

Common Pitfalls:
Confusing short-term ceiling limits with certain-fatality thresholds; ignoring exposure duration effects and victim susceptibility.

Final Answer:
3000 ppm (0.3%)