It's more that you'll spend the entire rest of the conversation explaining what a CO2 monitor is, what 'good' and 'bad' readings are, why you want to know, etc.
At the point they realise you're talking about Covid, they suddenly go all vague and distant, because no-one *wants* to get the point, they're living perfectly happily in denial. I try and delay that moment by talking about "stuffy" rooms, in the hopes that some info will seep into their brains before they lock the door.