Today's News & Observer details efforts to get NC pet shelters to switch from gas chambers to lethal injections for killing animals:
[D]ozens of North Carolina shelters  still use gas chambers to kill unwanted dogs and cats. The practice is falling out of favor nationally, and some animal activists are lobbying the state to stop it.
As I understand it, though, the main objection to the gas chamber is that it's the gas chamber. One activist says "it's a pet holocaust." Both methods are painless and effective, though injection is slightly quicker—20 seconds of consciousness after injections, 30-60 after the gas is turned on.
The main substantive objection seems to be that dogs and cats howl and scratch in the gas chamber until they lose consciousness. Vets say they're not in pain, but anxious about being in a dark place. That sounds like a good reason not to build new gas chambers (long-term costs of the two methods are comparable), but not a good reason to make county governments pay to switch. Reducing animal suffering is a valid use for state funds, but as the federal government bows out of state and local spending, I find it difficult to imagine that reducing a dog or cat's lifetime anxiety by 30-60 seconds should be a top priority.
I have two cats—Jack & Grace—and a puppy named Daisy. I just thought I'd get my pet credentials out there before the comments start. Let me suggest that if we're going to start spending more state and local money on animal welfare, we should start with better enforcement of existing cruelty laws, maybe strengthening those laws, and creating effective spay/neuter programs so that we don't have to euthanize 200,000+ dogs and cats a year.