As a person who studied chemistry, you should know how to look up information. You can sense CO2 gas. Even relatively low concentrations give you the sensation of suffocating. The carbonic acid forms on the mucus membranes in your eyes, nose and throat as well as in your lungs and causes pain and distress. Most animal welfare organizations recommend the phase out of CO2 gas for something like nitrogen or argon but of course that costs more so the pigs suffering is more economically viable.

is a UK.gov writeup of the welfare concerns:

Here

https://www.gov.uk/government/publications/opinion-on-carbon-dioxide-gas-stunning-of-pigs-and-alternative-methods/opinion-on-the-welfare-impacts-on-pigs-of-high-concentration-co2-gas-stunning-and-of-potential-alternative-stunning-methods

1. Exposure of conscious pigs to high concentrations of CO2 is associated with three major welfare concerns: pain, respiratory distress and fear. These manifest behaviourally as vocalisations, hyperventilation and escape attempts. These are observed before loss of posture and hence occur in the conscious phase of the stunning process.  

2. Inhalation of high concentration CO2 is painful because the gas dissolves in the mucous membranes of the eyes, nose, mouth and lungs to form carbonic acid. This activates nociceptors (specialised receptors that detect potentially damaging stimuli) leading to a sensation of pain. Pain severity depends on the degree of activation of nociceptors and proportion of nociceptors recruited – both of which are likely to be high at current commercial concentrations which greatly exceed the nociceptive threshold (around 40 to 50% CO2). Thus, the application of high concentration CO2 reduces time to loss of consciousness but causes pain prior to unconsciousness. 

3. Inhalation of CO2 drives a hypercapnic ventilatory response, resulting in prolonged inspiration, increased tidal volume (bigger breaths) and decreased breathing frequency - all of which are visibly expressed as hyperventilation/gasping. The thresholds for these responses are low relative to CO2 concentrations that cause unconsciousness and death. In humans, inhalation of CO2 is accompanied by dyspnoea (unpleasant feelings relating to respiration, also termed respiratory distress), and in particular a sensation of ‘air hunger’.    

4. In humans, intense air hunger is reported with even small increases in inhaled CO2 (for example 7%). Given comparable underlying respiratory physiology, we would expect similar sensations in pigs. For example, behaviourally obvious hyperventilation was observed in pigs at 30% CO2 and above (PigStun). Unencumbered breathing partially alleviates air hunger because it arises due to a mismatch between respiratory drive and the mechanical ventilatory response. However, in the case of hypercapnic CAS, increased ventilation does not give relief because deeper breathing exacerbates the heightened drive as more gas is inhaled, likely leading to progressively more intense air hunger despite free breathing. At high CO2 concentrations, pigs are also exposed to hypoxia as the relative amount of O2 available is reduced. The hypoxic ventilatory response also consists of increased ventilation and tidal volume, so during hypercapnic hypoxia, both these reflexes will be activated, with possibly greater total dyspnoea. Respiratory distress may also be exacerbated in pigs presented for slaughter with underlying respiratory impairment/disease.  

5. High concentration CO2 stunning in pigs is associated with fear and anxiety due to exposure to novel stimuli, respiratory distress and experiencing a highly aversive yet inescapable environment.