Carbon Monoxide is a odorless but deadly gas. It is important to note that CO has an elimination half-life and it varies under different conditions.
When evaluating a patient, we can calculate backwards to determine the COHb level at time of exposure in an acute event.
Carbon Monoxide Half-Life:
Average elimination on room air: 5-6 hours
100% Oxygen: 70-130 minutes
100% Oxygen under hyperbaric conditions at 3 ATA: 23 minutes
There is NO need to recheck COHb level again after initial level because it will be lower- (except in the case of Methylene Chloride exposure).
Thom, Stephen R. "Carbon Monoxide Pathophysiology and Treatment." Physiology and Medicine of Hyperbaric Oxygen Therapy. Ed. Tom S. Newman and Stephen R. Thom, Saunders Elsevier, Philadelphia, PA 2008. Pp: 321-348.