One of the most important things to look for during a home inspection is the presence or absence of carbon monoxide (CO) detectors. Along with smoke detectors, they are the safety equipment in your home most likely to save your life.
Carbon monoxide is an odorless, colorless gas produced during combustion or burning of fossil fuels and wood. Any home with fuel burning appliances, whether they be natural gas, fuel oil, propane or wood should have one or more CO detectors. Only homes with no combustion appliances at all are free from the need of a CO detector. That includes heat, hot water, oven/stove and fireplaces.
Carbon monoxide binds to red blood cells and prevents oxygen from being absorbed. Exposure results in headache, nausea, fatigue and eventually death. If you have any symptoms of carbon monoxide exposure, move immediately to fresh air.
CO detectors are available in a variety of formats, from a simple model that plugs into an outlet, to a hardwired combination smoke/CO detector with wireless interconnect and battery backup. At the minimum I recommend an AC powered detector with battery backup. There should be a detector outside each sleeping area and one on every floor that has a combustion appliance. As part of the home inspection I test CO and smoke detectors, and recommend that you do so monthly. CO detectors currently on the market have a lifespan of 10 years before they need replacement. Typically there is a sticker with the manufacture date on the back of the unit.
There is a tendency to treat CO and smoke detectors as a nuisance, since sometimes they are prone to false alarms. CO detectors should be taken seriously, since CO cannot be detected with your senses, unlike smoke and fire. Under no circumstances should a CO detector be disabled. For more information on carbon monoxide, visit the National Fire Prevention Association.
Note that Baltimore County now requires 1 CO detector per floor for their Rental Registration Inspection.