A client is confused and has a severe headache after using a generator in an attached garage. Pulse oximetry reads 99%. Which action should the nurse take first?

The exposure history and neurologic symptoms suggest carbon monoxide poisoning. Conventional pulse oximetry can appear normal because it cannot reliably distinguish oxyhemoglobin from carboxyhemoglobin, so 100% oxygen should begin promptly while testing continues.

Quick answer
Start 100% oxygen while the evaluation continues. A normal conventional pulse oximetry value does not exclude carbon monoxide poisoning.

Why this is correct
Carbon monoxide impairs oxygen delivery and cellular use, especially in the brain and heart. High-concentration oxygen speeds carbon monoxide elimination, so treatment should not wait for the carboxyhemoglobin result when the history and symptoms are concerning.

Easy analogy or mental picture
The pulse oximeter is like a counter that sees occupied seats but cannot tell whether the correct passengers are sitting in them. The comparison explains the misleading reading, but it does not replace carboxyhemoglobin testing or assessment of neurologic and cardiac injury.

Memory hook
Generator exposure plus headache or confusion means oxygen now; a normal pulse oximeter can mislead.

NCLEX decision rule
When carbon monoxide exposure is plausible and symptoms are present, choose immediate 100% oxygen and continued diagnostic assessment. Do not wait for confirmation or reduce oxygen because the conventional saturation display looks normal.

Why the other choices are wrong
Waiting prolongs tissue exposure. Low-flow oxygen under-treats suspected poisoning, and room-air observation ignores an active neurologic emergency.