Researchers at Lawrence Berkeley National Laboratory sat test subjects in a sealed room and gradually raised the concentration of carbon dioxide in the air. Starting from the 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 to what the researchers classified as dysfunctional. Around the same time, the team measured an actual conference room in use — the reading came back at 2,143 ppm.

There's no record of what decisions were made in that room. But by the study's benchmarks, the judgment of everyone sitting in it may well have been impaired — and none of them would have had any idea.

In a Closed Room, CO2 Levels Shift Within an Hour

Pack several people into a conference room with the windows shut, and CO2 levels will cross 1,000 ppm within an hour. Keep going for two hours or more without any fresh air, and readings in the 2,000s aren't unusual. Every breath adds carbon dioxide to the room, and it accumulates faster the more people are crammed into the space relative to its volume, and the longer the meeting runs.

The problem is that the body's signals here are ambiguous. Past 1,000 ppm, what people actually feel is drowsiness, a dip in focus, a mild fogginess. They chalk it up to staying up late to prepare, or a boring agenda, or the natural slump after lunch. Almost no one thinks to blame the air.

A Harvard study found that rising CO2 levels produce especially sharp declines in strategic thinking, planning, and the ability to use information under pressure — precisely the capacities that high-stakes meetings demand most. Reviewing the terms of a new contract, deciding how to allocate a budget, negotiating with a client: these are exactly the moments that can end up overlapping with the point in a meeting when CO2 is at its highest.

South Korea's office environment has several structural features that make this worse. Many office buildings in central Seoul limit the intake of outside air to cut energy costs. The study cafés and shared offices that multiplied after the pandemic — pay-by-the-hour study spaces popular with students and freelancers — were built to maximize rentable square footage, often with minimal ventilation systems. And home offices, where people now spend far more of the day, tend to get aired out less often unless someone makes a deliberate habit of it. Sit alone in a closed room concentrating for four hours, and the numbers climb quietly, and fast.

Not Everyone Buys the Numbers

Some researchers question whether CO2 actually causes the cognitive decline. A number of follow-up experiments failed to reproduce the Lawrence Berkeley and Harvard results under identical conditions. Critics have argued that the real culprit may not be CO2 itself but the other pollutants that tend to rise alongside it — volatile organic compounds, or byproducts of human metabolism given off in an unventilated room.

That pushback deserves to be taken seriously. Given the gap between lab conditions and real offices, individual variation in sensitivity, and how hard it is to control for confounders like meeting topic or prior fatigue, a clean formula — CO2 goes up, judgment goes down by a fixed amount — is probably an overstatement. The claim that CO2 concentration alone explains decision quality, in particular, still lacks sufficient replication to stand on its own.

Still, this pushback doesn't really land on "so don't worry about it." Whether CO2 is the direct cause or just travels with the real culprit, study after study confirms the same underlying fact: air quality degrades in a closed space where people have been breathing for a long time. Even if the cause can't be pinned precisely on CO2, that doesn't change the fact that ventilation improves the air as a whole.

Another objection: isn't this common knowledge already? Nobody doubts that fresh air is good for you. But knowing something in the abstract is different from changing your behavior after seeing an actual number. I doubt many people would still leave the window shut after learning their conference room measured 2,143 ppm — a figure that falls squarely in the range researchers classify as cognitively impairing. A concrete number lowers the threshold for actually doing something.

Designing the Environment You Work In

Most efforts to boost productivity are aimed at the person: focus harder, prepare better, ask smarter questions. But the people who are genuinely good at their jobs tend to attend to something else first — the design of their environment. What tools they use, where they work, when in the day they schedule the tasks that demand real judgment. Effort only pays off once those things are already in place. There's a reason the advice to check your working environment before changing your working habits keeps coming back around.

A CO2 monitor is a cheap addition to that design. Small sensors, available for around 30,000 won (roughly $22), display real-time readings accurate to within ±50 ppm. Keep one in your workspace and you can decide when to open the window based on a number instead of how tired or foggy you feel. If you'd rather skip the device, a timer works too — cracking a window for five minutes every hour is enough to bring CO2 levels back down into a meaningful range.

For solo entrepreneurs and remote workers, a good starting point is simply figuring out when in the day levels tend to climb. Ventilation often happens naturally in the morning, but if you close the door in the afternoon and stay heads-down for hours, the numbers rise quietly. If the time you spend reviewing an important proposal or weighing contract terms happens to overlap with a stale afternoon that hasn't been aired out, it's worth considering that the air — not the content — may be what's shaping your judgment.

Teams that work out of small conference rooms can knock CO2 levels down by several hundred ppm just by building a five-minute airing-out routine into the start of every meeting. How much difference that makes to decision quality will vary from team to team. But it costs nothing to try, and if it moves the needle at all, it moves it in the direction of better cognitive function after ventilation.

It's natural to look to methodology or tools first when trying to improve how you work. But I want to ask one question before any of 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 a different result depending on the air in the room — that's a condition that needs sorting out before methodology even enters the picture.

What's clouding your judgment might not be the complexity of the agenda — it might be the closed window. Check that possibility for yourself just once, with an actual number, and "open the window" moves straight to the top of your to-do list before the next important meeting.