Oxygen depletion sensors can sometimes detect incomplete combustion, which can lead to dangerous carbon monoxide levels. However, ventless gas heaters are often regarded as potentially dangerous, especially if not used in a safe manner.
When natural gas and propane are combusted optimally, they produce only carbon dioxide, which is considered non-toxic, and water vapor, which poses no health risks. However, small inefficiencies within the combustion chamber, such as not drawing enough air, causes carbon monoxide to form. Oxygen depletion sensors are capable of detecting this type of dangerous combustion, and systems with these systems are safer than older systems that lacked them.
However, these systems cannot detect a number of other potential problems. Gas heaters depend on gas pressure to operate, and faults within the gas tank can cause carbon monoxide formation. In addition, these systems assume the air being used is relatively free of dust and other gases that can interfere with combustion and cause potentially dangerous gases to form.
Some recommend against using ventless gas heaters at all, and a few states even ban their use. As a source of supplemental heating, however, they can be used for a short period of time while posing little health risk. Still, relying solely on the primary heating system in a home is generally recommended when possible, and electric space heaters, while more expensive to run, don't pose the potential health risks of gas units.