WARNING: Graphic.
This scene was taken from "One Nation Under Dog."
This, is most states, is what happens when a shelter is too full and must dispose of dogs. This isn't something that was filmed decades ago and no longer exists. This hasn't changed. This, is probably going on as you read and watch this clip.
Overpopulation of pets is because of one: people don't spay and neuter their pets, and so they reproduce and have litters with nowhere to go, and two: because people keep buying pets from pet stores.
Dogs, cats, ferrets, and hundreds of other species sold in pet stores primarily come from breeding mills, in which cases, the mothers are usually in a small, open-bottomed cage so that their waste will fall through, as they stay in that cage all day, breathing it in. Escape attempts often result in injury as the animals desperately try to chew or bite their way out.
Don't buy from pet stores. Adopt from your local shelter. And don't ever drop your dog off there, because this is where most of them end up, or put to sleep. In today's economy in the United States, people just can't afford to own pets at the time.
The reasons people give up their pets are endless; however, they are inexcusable. You're literally throwing away their life when you dump them off. You may think, "he'll get adopted, he/she's a good boy/girl!" And in some cases, your pet may honestly be adopted. But chances are slim as they only have a short period of time.
And, if the shelter is full of strays with no room, they can't just put one down and make room for your pet. Your pet will be automatically put to sleep.
Not all states use these horrid gas chambers. And they can be stopped through petitions and bills.
Please, spay and neuter, adopt-don't-shop, and especially don't drop your dog off because they have a medical condition or is old. They have a slim-to-no chance of getting adopted.