I mean wouldn’t you see it? I use my fireplace all the time in the winter and I would classify the amount of smoke that gets into the room as “negligible”. Especially compared to smoking a cigarette.Would love if this study actually followed up with some measurements and the “why” of actual fireplaces and stove setups.

You can measure. Buy an AQI meter. But I recommend buying an air filter with an AQI meter attached, as when you see what the supposedly negligible amount of smoke is actually doing to your air quality you are going to want the former.