But just as you’re about to doze off, the boss says it’s time to wrap up and the attendees jump out of their chairs. Someone yanks the door open, and the cool, fresh air washes over you, and suddenly you’re back to yourself again.

That stuffy, sweltering, smothering feeling? It’s not just in your head, a fact underscored this week when an astronomer took a carbon dioxide monitor to an academic conference where 100 people were crammed into a stuffy lecture hall. The monitor’s output tells a wonderful little story about what happens to the air quality in a room when you stick a bunch of exhaling humans into it.

Our story begins at 9:05 a.m. Tuesday, when astronomer Adam Ginsburg of the National Radio Astronomy Observatory took his seat for a session during the “Linking the Milky Way and Nearby Galaxies” conference in Helsinki. Ginsburg said in an email that he takes a portable carbon dioxide monitor “everywhere,” particularly, “crowded meeting rooms.”

Ginsburg flipped on the monitor. Ambient indoor air tends to contain about 800 parts per million (ppm) of carbon dioxide. But carbon dioxide levels rise rapidly in poorly ventilated rooms, because exhaled air is about 4 percent carbon dioxide by volume. In the lecture hall in Helsinki, the monitor showed the concentration quickly reaching 1,000 ppm — the threshold at which a room starts feeling stuffy for most people, according to the American Society of Heating, Refrigeration and Air-Conditioning Engineers.

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As the talks went on, the carbon dioxide level continued to rise. At 1,500 ppm, Ginsburg characterized the room as “noticeably stuffy.” By 10:20 a.m., the reading had spiked past 1,700 ppm.

Then two things happened: Attendees took a 30-minute coffee break at 10:50 a.m., and organizers used that time to open the windows. The fresh air combined with the departure of dozens of humans exhaling carbon dioxide sent carbon dioxide levels plunging, all the way down below 600 ppm, within minutes. When the session resumed, the windows remained open and carbon dioxide stayed within the 1,000-to-1,200-ppm range.

Recent research has shown that there’s a lot more at stake in these situations than stuffiness, discomfort and the creeping aroma of whatever your colleagues ate for lunch. Too much carbon dioxide, for instance, is correlated with an increased feeling of drowsiness in office settings.

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Then, there’s the finding that carbon dioxide may quite literally make you dumber: “There is substantial evidence that performance on challenging tests of decision-making and challenging flight simulations is worsened by [carbon dioxide] concentrations as low as 1,000 ppm,” as researchers at the Lawrence Berkeley National Laboratory sum up in their review of the literature.

Other research on air quality in schools and offices shows that levels routinely surpass that threshold. A 2002 study of two schools in Texas, for instance, found that 88 percent of surveyed classrooms had peak carbon dioxide levels above 1,000 ppm. Levels exceeded 3,000 ppm in 21 percent of classrooms.