You dont ever expose someone to it intentionally, not until SCBA is in play, and only with saud gear being used.
We had safety people sampling and retrieval gear ready every entry I ever made. I’ve been in well pits, utility tunnels, boiler mud drums, etc. it’s not anything to take lightly. Your reaction reflex if you see a coworker go down is get in there and help. In oxygen deficiency conditions it’ll kill you and the other guy quicker than a fast train. Safety has to be first. IMO. Regards.