Here is a paper on the topic, if you’re interested.

CO2 is an aversive stimulant to pigs, which is why they respond in these ways. CO2 irritates the mucosal lining of the trachea and nostrils, and when combined with natural bodily moisture, carbonic acid can also form on the eyes. CO2 causes acidosis (acidifying blood and tissues) and hypercapnia (excessive levels of CO2 in the blood), which creates a sense of breathlessness, hyperventilation, and “air hunger”.This is a significant animal welfare concern. Ultimately, unconsciousness is initiated by the cerebrospinal fluid of the brain becoming too acidic and the brain ceasing to function. It takes an average of 3–5 min for pigs to die within these CO2 pits.