How many TV suicides involve the legendary hose from the tailpipe to the passenger compartment in a closed garage? Perhaps more than happen in real life, although I'd have to do actual research to find out. Regardless, and regardless of the Prius' extraordinarily low emissions, there will be some Carbon Monoxide (CO). CO is a "colorless, odorless gas" that's lethal above a certain percentage concentration. Does the Prius emit enough to kill you? Or to worry you? Can it be ignored or must it be considered?
Power failures come in all sizes. The PriUPS could be in operation for a few seconds, not even long enough to start up the gasoline engine. Or the grid could be down for a day or more, and the vehicular emissions, whatever they might be, will have time to equilibrate with the garage environment (whatever that might be). Will you die the instant you go in the garage? Or will the environment be benign?
The short answer is, I don't know. If you have a tiny garage and no ventilation, there may well be a problem. A "three car gar" with windows and doors everywhere, probably not. Even so, you will want to find out or take steps to prevent a problem. Things you can do:
I spent a while searching eBay for a CO monitor that had a computer output instead of an alarm. The closest I could find was a Fluke model that had a voltage output proportional to CO concentration, but the specification said that it would shut itself off after a brief period. If you know of a solution (that's what products are called now) I'd love to hear about it.
New info - 30 July 2005 - I found almost the right thing. MicroDaq makes a small, relatively inexpensive CO monitor that will log concentration over a period of minutes to months depending on sampling rate. Great for research but not as an alarm.
Still to be done...
This page only discusses the issues. I haven't actually determined the seriousness of the CO problem, or made any measurements. It's been pointed out to me that oxygen depletion is also a potential problem. While I'm skeptical that this would be an issue in a normal garage, it can't be denied that internal combustion engines use oxygen, and in a garage with very poor ventilation there would be a decrease in the O2 percentage in the ambient air. However it takes only a minute percentage of CO to be a danger; small changes on oxygen partial pressure aren't significant. If I come across any actual facts to quantify either hazard I'll add them here.
New Info - June 2007
I found (and responded to) this article in Science News. It's not about the Prius, but rather discusses the dangers of CO being emitted from small portable generators. The main point of the article is that these generators can even be dangerous when used outdoors! The article ends "Future generator technologies might reduce carbon monoxide emissions." The PriUPS sounds like a "future generator technology" to me.
New Test Info Update
And fact I have come across: I found this little CO sensor on eBay at a price I could afford - $10.00 I had originally thought it was one that could be connected to a computer - note the long sensor cable for remote reading. But I misunderstood or misread the data. Even so, for that price I could be a sport, so I kept it and tried it out. After running the Prius in the garage for a few minutes with the sensor by the tailpipe, the reading remained at a comfortable zero. I do not realistically think the Prius is a carbon monoxide danger.
This does not mean you can ignore the "problem." Especially in a small, unventilated garage, the oxygen depletion question is not settled, the Prius could be misadjusted, or the CO meter could be defective.
Request: If you happen to have a Prius and a CO meter, see if you can verify my results.
A New Suggestion - March 2010
New Jersey, New York, Massachusetts, and Illinois, to name only the first four that popped up in a search, have laws that require carbon monoxide detectors for new home construction much as smoke detectors have been required for a longer time. Given the large quantity and hence cheap prices of these devices, there is no reason not to install one in your garage. It would also be a good idea to install them where you presently have smoke detectors. (Dual CO/Smoke detectors are available.) Although I'm skeptical that CO from a PriUPS would be a problem in anything other than a small, closed garage, my skepticism won't save your life if I'm wrong. A CO detector is cheap insurance.
Updated 17 March 20010