Gassing in animal shelters

This is currently a topic that causes controversy in debates concerning animal rights and the suffering of animals during euthanasia. However, I introduce it here to show that death is caused by lack of oxygen not by poisoning. An article from 2005 states:

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.

I am grateful to posters at this AHF thread for the heads-up.