Scientists think that they've found a link between elevated levels of CO2 (carbon dioxide) and sluggish thinking in people.
We inhale air, which is a mixture of several gases. Our lungs extract the oxygen from that air and what we exhale is heavy in carbon dioxide. This makes plants happy, because they like the CO2 and when they process it, they release oxygen. In fact, this makes everyone happy, and that's good because we all know how much of a bringdown a depressed ficus can be. Not to mention irritable gladiolas.
Well, when there are a lot of people in a room, and they insist on breathing, and the room is one of those new-fangled energy efficient rooms that tries to keep externally-temperatured air on the outside and internally-temperatured air on the inside with tighter and more effective sealing, you get yourself elevated levels of carbon dioxide because it doesn't escape as fast through those effective seals.
Of course, that's just with regular respiration -- the scientists measured the effects in a room where people were working but not very many people were talking. Had the scientists conducted their measurements and experiments in a room where many people talked a lot -- say, the chamber in which any legislative body has met since the day our knuckles left the ground -- they would have probably seen even greater effects.
They wouldn't even have had to conduct their tests to measure cognitive function. Just read the legislation that these bodies produce or listen to some of the speeches that carry the CO2 into the room. Anyone with more than two firing neurons could do that and conclude: "What a bunch of maroons."