Carbon monoxide, primarily as a by-product of combustion, is a colorless, odorless gas that may cause illness or death at sufficient concentrations. NFPA standard 720:2009 calls for CO detection in most buildings including schools, hotels, nursing homes, hospitals and commercial occupancies equipped with fuel-fired appliances or attached garages; not just residences as found in earlier NFPA standards.
The NFPA recommendation for greater use of CO detectors is driven in part by advancements in technology that allow for more reliable, longer life CO detectors at much less expense — many high quality, single station CO detectors i.e. plug into an electrical outlet, sell for around $60; and, some detectors can be bought for much less.