For people who do HVAC, a CO monitor is a good idea, but may not cover all bases. If you are entering confined spaces like manholes, sanitary or storm sewers, tanks or other unoccupied spaces, you need to know about more than CO. Is there enough oxygen for you to breathe? Is the atmosphere explosive? Is hydrogen sulphide present? Even an brand-new empty steel tank can be dangerous, since the rusting process depletes oxygen. Any decaying organic matter can produce methane, which is explosive, and hydrogen sulphide, which is pretty sneaky stuff. It stinks of rotten eggs, but in higher concentrations, it will paralyze your olfactory nerve, and you will no longer smell it, but it will kill you. It is also very corrosive. Any flammable chemical vapors also are a problem. Getting a CO monitor is a good move, but it isn't the whole story. I'm sure Larry from OSHA will have some more on this subject.