When animal-shelter employee Rosemary Ficken opened the door to the St. Louis pound's gas chamber one August day in 2003, she couldn't believe her eyes: A reddish brown mutt, standing on top of six dead dogs, was still alive.

　　In the shelter's 64 year history, no dog had ever survived the chamber's noxious fumes.

　　Unwilling to close the door and re-gas the dog, Ficken called Randy Grim, the founder of Stray Rescue of St. Louis. The Missouri organization rescues abused and neglected animals, restores them to health, and places them in new homes.

　　Grim retrieved the big-eared Basenji mix and named him Quentin after California's San Quentin prison.

　　Quentin's life was spared that day, but many others are not so lucky. Nearly four million dogs and cats in the United States are put to death in shelters each year.

　　Carbon monoxide gas chambers—a euthanasia method used since World War II—are routinely used in animal shelters throughout the country, including Rhode Island, Ohio, Texas, and Virginia.

　　The American Veterinary Medical Association—whose euthanasia guidelines are widely followed—considers carbon monoxide gassing an acceptable method when done in a properly manufactured and equipped chamber. Many animal-welfare advocates, though, say the method is inhumane.

　　"It's America's dirty little secret," said Grim, who has written the book Miracle Dog: How Quentin Survived the Gas Chamber to Speak for Animals on Death Row (Alpine Publishing). "If people actually saw the gas chamber working, they would sign a petition tomorrow to ban it." Due to Grim's fundraising and lobbying efforts, the St. Louis gas chamber shut down in January of this year.

　　From start to finish, the process of gassing an animal takes about 25 minutes. One or more animals are placed in an airtight chamber, and a high concentration of bottled carbon monoxide gas is released. Cats and dogs are rendered unconscious within a minute, then eventually die from lack of oxygen.

　　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.

　　Currently 13 states, including California, Florida, and New York, require animal shelters to perform death by injection, according to the AHA.

　　In the rural farming community of Enoch, Utah, the animal shelter's brick gas chamber uses carbon monoxide exhaust from an old pickup truck.

　　The city was heavily criticized for its method by animal welfare organizations in 2002. To put the controversy to rest, the city hired a veterinarian to perform a necropsy on a 50-pound (23-kilogram) dog euthanized in the shelter's gas chamber.

　　His report found that there was no evidence of heat injury to the dog's respiratory tract. No mouth or foot abrasions were found, indicating the dog did not try to escape.

　　Enoch's animal-control officer, Jim Mitchell, said the shelter will soon use bottled carbon monoxide gas, because a newly constructed addition to the shelter is blocking the truck's access to the chamber.

　　The shelter, however, will not switch to sodium pentobarbital, Mitchell said. "Unless you have an actual veterinarian on site to administer and supervise the process, in my mind euthanization by injection is inhumane."

　　Jodi Buckman, director of animal-protection services for the American Humane Association, said training shelter workers on proper euthanasia techniques is important.

　　"We want them to be the most humane people in our communities, because they are taking care of the homeless animals that no one else has taken responsibility for," she said.

　　Currently four states—Colorado, Georgia, Indiana, and Nebraska—require special training for workers who use lethal injection to euthanize animals.

　　Animal welfare advocates say euthanasia rates are on the decline. Experts attribute the decline in large part to aggressive spay and neuter programs initiated by shelters and humane societies.

　　In some parts of the United States, adoptable animals are now even in demand. To cover the shortfall, volunteers drive to areas where a severe overpopulation still exists, then take dogs and cats to cities seeking adoptable pets.