Yet the question still has not been answered I work around older IC Engines without Catalytic converters Even with good ventilation there is still some level of CO in the area Without hyperbaric treatmenthow long does it take low levels of CO bonding to dissipate Ie If one were exposed to an atmosphere of 20 PPM for 3 or 4 hours daily for 3 or 4 days how long should one wait before again exposing ones self to those levels again My work around old engines is a hobby I can take extended breaks doing other things between times that I require them to actually be running for testing My Google searches have not yet turned up a substantial answer Gerald Livingston CST