Monday, October 12, 2009

How does carbon monoxide differ from carbon dioxide, and how do detectors work?

How does carbon monoxide differ from carbon dioxide, and how do carbon monoxide detectors work?

If you burn carbon or things that contain carbon (such as wood, oil, and natural gas) in the presence of enough oxygen, you get carbon dioxide - a molecule made of one carbon atom attached to two oxygen atoms.
If there isn’t enough oxygen (when air flow is impeded, say, by a dirty burner or an inefficient engine), you get some carbon monoxide - a molecule made of a carbon atom and just one oxygen atom.
Think of it as a product of incomplete burning. Indeed, one sign that a flame is likely to produce carbon monoxide is that it’s sooty, or orange or yellow, due to the presence of incandescent bits of unburned carbon. A cleanly burning flame producing little carbon monoxide should be pale and bluish.
The big danger of carbon monoxide is that it binds tightly to hemoglobin and prevents oxygen from being attached to it and transported through the blood to the brain and other organs. Blood, as you probably learned in high school biology, also carries carbon dioxide, but not in ways that prevent oxygen from being transported.
Carbon monoxide is odorless, so if you’re breathing it, the only warnings you get are sleepiness, headache, and nausea, by which time you might be quite badly poisoned. You can easily lose consciousness before you realize you’re in danger. If things haven’t gone too far, fresh air will provide enough oxygen to eventually displace the carbon monoxide, and in this way you can recover.
Some detectors are based on chemicals - sometimes analogs of hemoglobin - that change color when they react with carbon monoxide, and this color change can be used to set off an alarm or be seen directly.
Another approach is based on a sort of battery called a fuel cell, in which a reaction between carbon monoxide and oxygen (in a way, finishing off the incomplete combustion that produced the carbon monoxide) produces an electrical signal. Finally, there are devices in which a heated piece of tin dioxide changes its electrical resistance in the presence of carbon monoxide.
Each detector has advantages and disadvantages, depending on how sensitive and fast it has to be, how much electricity it takes, and how long it will last before it needs replacing.
Carbon monoxide poisoning is a very real risk, so be sure to have properly working detectors in your home.
Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.