I believe canned air is co2 as well. And yes is does suffocate the animal, and can kill. If delivered into the chamber properly(very slowly), it's a humane way to kill feeder rats & mice. If not delivered properly, it can cause an acidic reaction in the throat of the animal and cause much suffering before death, sort of like drowning.
google "Guidelines for Performing Euthanasia via C02" if interested.
At my sessions I noticed it the other way around Try to wait a minute. The carbon dioxide concentrates in particular places at the bottom of your box. Its heavier than air.One thing that was maddeningly consistent was medium and small specimens took more gas to knock out, while the larger 'pedes were relatively easy.