What evidence is there that death by CO2 poisoning is painful to slaughtered animals?

[teo123]

Today, most animals in slaughterhouses are killed by CO2 poisoning. That's thanks to the HSA (Humane Slaughter Association) and similar organizations.

Critics of HSA claim, including but not limited to BiteSizeVegan, claim that killing animals via CO2 is not humane, that being exposed to high levels of CO2 causes the feeling of suffocation and burning of the eyes.

However, one student at my university is developing a low-cost electronical device for detecting elevated levels of CO2 for his bachelor thesis. He says that human beings can't detect lethal levels of CO2, that countless people have been exposed to lethal levels of CO2, mostly in the wine industry, and that many have died that way.

So, who is telling the truth here? I can imagine that both of them are true, that perhaps if you are exposed to air with 2% of CO2 for hours you die without feeling anything, whereas animals in slaughterhouses are exposed to around 70% CO2, which they can and do feel. However, I have no evidence for that.