Doug Fakkema, an animal-euthanasia expert, said that, in theory, the gas chamber doesn't sound bad, but in reality it's awful.
"The animal is in a warm or hot box, usually with other animals. They don't know what's going on. The hiss of the gas is going on inside. They get dizzy, and they panic," he said. Fights can break out, and animals' calls can sometimes be heard.
Today most private and city animal shelters euthanize animals with sodium pentobarbital, a controlled substance that is injected into one of a dog or cat's veins. Animals die in seconds, experts say, and without pain or suffering.
Private-practice animal hospitals also use sodium pentobarbital to euthanize sick and old family pets.
The American Humane Association（AHA）, an animal- and child-welfare nonprofit, says that lethal injection is the only acceptable method for putting down dogs and cats.