UBC researchers are currently studying CO2 gas euthanasia of laboratory rodents, they have so far found that higher levels of CO2 cause pain while lower levels cause dyspnea.
Imagine that.. after millions upon millions of rats and other small animals have been killed by CO2 gas, someone finally said.. hey, I think this might not be humane, let's study this!
Hopefully their findings go in favour of a better death for their helpless subjects.