if I remember correctly, casinos pump in oxygen because it makes people more likely to stay gambling at the slots. keeping co2 low may be a side effect, or maybe they're scrubbing that too to keep people feeling more like gambling
They also need extremely good ventilation and/or filtration to deal with cigarette smoke, at least in places where casinos allow smoking on the casino floor. And any system that can mitigate smoke is also going to have some capacity for mitigating airborne disease transmission, as far as I'm aware?
