Trouble is it sneaks up on you. Make you feel sleepy, tired, & eventually sick/dead. Not a smart thing they did, but we all make mistakes.
Had a malfunctioning machine years back. In a box van with back door open, side door open, exhaust runs out of the box, & a 2200+ cfm fan blowing. Anywho I was standing in there with ear protection on filling a water transfer tank while the labor was running the tool in the building. After a bit I started feeling weird, jumped out & ralphed. Then it dawned on me that I was experiencing co poisoning. Few minutes of fresh air cleared it up.
Who woulda thunk? Everything was open & I had a powerful fan running. Wasn't 2' from outside. It can happen to anyone, even with precautions. The van has a co detector now, just like at home. Several die each year from cracked furnace burners inside their home, better make sure ya have a detector in the house guys. It'll get you even if your not a dullard. It puts you to sleep, hard to recognize in time.