Researchers at Lawrence Berkeley National Laboratory seated test subjects in a sealed room and raised carbon dioxide levels in stages. Starting from an outdoor baseline of 400 ppm, once levels climbed to 1,000 ppm, performance dropped on six of nine decision-making metrics. At 2,500 ppm, seven metrics fell into what the researchers classified as dysfunctional territory. When the same team measured an actual conference room during the same period, the CO2 level came in at 2,143 ppm.

There's no record of what decisions were made in that room. But it's possible that the judgment of everyone sitting there was, by the study's benchmarks, already in impaired territory — and none of them had any idea.

Close the Door, and CO2 Climbs Within the Hour

With several people seated in a conference room with the windows shut, CO2 levels climb past 1,000 ppm within an hour. Keep going for two hours or more without ventilation, and readings in the 2,000s are not unusual. The carbon dioxide people exhale with every breath accumulates faster the more people are packed into a given volume of space, and the longer the meeting runs.

The trouble is that the signals this shift sends the body are vague. Above 1,000 ppm, what people notice is drowsiness, a dip in focus, a mild fog. They chalk it up to staying up late to prepare, or a boring agenda, or the natural slump after lunch. Almost no one suspects the air.

A Harvard study found that rising CO2 levels produced especially pronounced declines in strategic judgment, planning, and the ability to use information under pressure — which, as it happens, are exactly the capacities that important meetings demand most. Reviewing the terms of a new contract, deciding how to allocate a budget, negotiating with a client — these can land squarely in the hours when CO2 is at its highest.

Korea's office environments have several structural features that make this problem more pronounced. Many buildings in central Seoul are run to minimize outside-air intake as a way of cutting energy costs. The study cafes and shared offices that proliferated after the pandemic often minimized floor space to protect margins, and their ventilation systems tend to be rudimentary. Remote workers now spend far more hours a day at their desks, and without a deliberate habit of opening a window, ventilation frequency, if anything, tends to fall. Spend four hours alone, focused, behind a closed door, and the numbers climb quietly — and fast.

Not Everyone Buys the Numbers

Some researchers question the causal link between CO2 and cognitive decline. Several follow-up studies failed to reproduce the Lawrence Berkeley and Harvard results under identical conditions. Critics argue that the real culprit may not be CO2 itself but other pollutants that rise alongside it — volatile organic compounds or human metabolic byproducts, for instance.

This objection is not one to wave away. Given the gap between lab conditions and real offices, differences in individual sensitivity, and how hard it is to control for confounders like meeting topic or prior fatigue, the tidy formula — "CO2 goes up, judgment goes down by a fixed amount" — may be an overstatement. In particular, the claim that CO2 concentration alone explains decision quality still lacks robust replication.

Still, this pushback rarely lands at "so don't bother." Whether CO2 itself is the direct cause or simply travels with the pollutants that are, multiple studies agree on one thing: air quality in a sealed room deteriorates as people breathe in it for extended periods. Even if the cause can't be pinned precisely on CO2, ventilation still improves the air as a whole — that fact doesn't change.

There's also the objection that "we already know this." No one is unaware that fresh air is good for you. But knowing something and changing your behavior after seeing a number are two different things. I don't know how many people, having confirmed with their own eyes that their conference room hit 2,143 ppm — a figure that falls squarely in the range researchers classify as cognitively impairing — would still leave the window shut. A concrete number has a way of lowering the threshold for action.

Designing the Room You Work In

Most attempts to improve productivity are aimed at the person: focus harder, prepare better, ask sharper questions. But the people who are actually good at their jobs tend to start somewhere else — with the environment itself. What tools to use, where to work, when in the day to schedule work that demands judgment. Get these right first, and effort actually pays off. There's a reason the advice to examine your working environment before overhauling your working method keeps coming back.

A CO2 monitor is a relatively cheap addition to that kind of design thinking. Small CO2 sensors, available for around 30,000 won, display real-time readings accurate to within about ±50 ppm. Keep one in your workspace, and you can decide when to open a window based on a number rather than fatigue or a hunch. Without a monitor, a timer works too: opening a window for five minutes every hour is enough to keep CO2 in a meaningful range.

For solo entrepreneurs or remote workers, the place to start is figuring out which hours of the day the numbers climb. Ventilation often happens naturally in the morning, but keep the door shut through an afternoon of focused work, and the readings creep up quietly. If the hours you spend reviewing an important proposal or weighing contract terms overlap with an under-ventilated afternoon stretch, it's worth considering that the air, not the content, may be shaping your judgment.

Teams working out of small conference rooms can cut CO2 levels by several hundred ppm just by building a five-minute airing-out into the routine before a meeting starts. How much difference that makes to decision quality will vary by team. But it costs nothing to try, and if it does make a difference, the direction only runs one way: cognitive performance improves after ventilation.

It's natural to look to methods or tools first when trying to improve how you work. I'd like to ask one question before that: when did you last open the window in the room you're working in right now? The possibility that the same person, using the same method, gets different results depending on the air in the room is a condition that arguably needs sorting out before methodology even enters the picture.

What clouds judgment may not be the complexity of the agenda — it may be the closed window. Check that possibility with your own numbers once, and "open the window" ends up at the top of the to-do list before your next important meeting.