Carbon monoxide is very bad for human respiration as it binds hard to the oxygen transporting protein in your blod. (Carbon dioxide do not bind) This mean that low concentrations of carbon monooxide can be lethal. For orchids there should be no problem at low concentration.
Remember that Carbon monoxide is produced in a combustion engine that runs bad, normaly they produce carbon dioxide if they work properly! The machine do probably run on natural gas and not carbon monoxide. (Never heard of an engine that runs on carbon monoxide)