OSHA I believe specifies the in plant emissions standards that employees are exposed. The current in plant standards for CO is 15 ppm on a Time Weight Average basis. In plant emissions levels have more to do with the facilities air exchange system vs emissions from lift trucks, welding operations, etc. Obviously, ICE lifts are a contributor to in plant CO levels. But theoretically, you can have the cleanest burning lift truck engine made but operating where the air exchange system is poor you can exceed OSHA limits.
Lift truck tail pipe testers are used to verify the emission levels are in compliance with EPA Tier I, II, III etc levels.