I have been remiss in not following up on my bleg of Feb. 19 about whether, with modern catalytic converters “scrubbing” the carbon monoxide out of automobile exhaust, it is still possible to die in a closed garage with your engine running.

Many readers helped with this, some at great length. Thanks to all. The short answer is: Yes, it can still happen, though less reliably than before these converters came in.

Main points:

  • CO is very toxic. Wikipedia says that 3200 ppm (i.e. 0.32 percent) will kill you in 30 minutes. California standards, for example, allow 5000 ppm (0.5 percent) CO in exhaust, so it can even happen in California.
  • “CO is reduced but not eliminated by a catalytic converter … according to this website by 94 percent. …”
  • Lawnmowers or gasoline powered weedwhackers have no catalytic converters, so even with their lower-power engines they pump out more CO.
  • The actual way that your catalytic converter “scrubs” CO from your exhaust is by changing it into CO2. This requires, duh, oxygen. If the local supply of oxygen gets low, the conversion doesn’t happen. (Note: CO is actually poisonous. CO2 is, so far as the human metabolism is concerned, inert and harmless. Of course, if the oxygen in your air supply is displaced by CO2 — or any other functionally inert substance — you will asphyxiate for lack of oxygen.)
  • Though my initial query was just motivated by curiosity, there is an implicit warning here, illustrated by the following sad story from a friend: “We had a [contractor] in the house, nice local guy … [quite recently]. His daughter, who had some kind of seizure disorder, passed out in their closed garage with the car running — and she died of CO poisoning. This was … certainly since the introduction of catalytic converters.”

More information here, here, here, and here, though that last one fingers asphyxiation, not poisoning.