I came across this article which outlines a controversy regarding a shelter in Garland, TX. The shelter uses a gas chamber which uses carbon monoxide to euthanize its surplus and unadoptable animals. The article includes a description of the procedure as witnessed by the reporters. This procedure is currently considered a humane method of euthanasia in the US.
The controversy comes from the use of such a technique to euthanize animals. Most of us are more familiar with the idea of an intravenous injection of (essentially) an overdose of an anesthetic that is commonly used for this procedure.
From the shelter's perspective, the use of carbon monoxide as opposed to the injection has clear advantages. The euthanasia solution is a controlled substance, requiring special permits to possess and use. The shelter workers that do the injections need special training. From my own personal experience with the procedure, it can be difficult to find usable veins in some animals. And it can be next to impossible to humanely inject the solution intravenously into an aggressive or frightened animal, which many of these dogs and cats obviously are. The carbon monoxide method allows shelters to euthanize in a humane manner without putting personnel at risk for injury and helps minimize the fear and anxiety of the animal. This would be especially useful for feral cats which are always a large number of the adoptable animals in any community and can be next to impossible to catch let alone safely restrain.