Why Rising CO2 Levels Cause Brain Fog

By the middle of a long meeting in a packed room, we've probably felt like the conversation was harder to follow and our answers got a little slower. It could just be that the meeting is boring or the caffeine is wearing off, but the room itself can play a major role too.

The standard instinct is to call it stuffiness, meaning a sense that the air has gone stale and we just need a window open. That’s actually pretty accurate, but it suggests the experience is a comfort problem only rather than a cognitive one too. The carbon dioxide we're all exhaling into that room is actually affecting our thinking too well before the air feels noticeably bad.

A Gas We Assumed Was Harmless at These Levels

Carbon dioxide has always been the background gas of indoor spaces, mostly used as a rough proxy for ventilation, in HVAC terms. If CO2 readings climbed, it meant not enough fresh air was coming in to dilute everything else in the room. The assumption for decades was that CO2 itself, at the concentrations we encounter in ordinary indoor life, was basically inert. It's not toxic in the way carbon monoxide is, and we breathe out roughly 40,000 parts per million of it with every exhale and suffer no harm.

That assumption has been tested directly, and it doesn't fully hold. A 2012 study put people in a sealed chamber and raised CO2, with every other air component held constant, to levels common in ordinary offices and classrooms. Decision-making performance dropped for those in the room, and at certain levels, peoples’ initiative and the ability to use new information fell substantially. 

Another study measuring real office workers across buildings with different ventilation rates found cognitive performance roughly 60 to 100 percent higher in the better-ventilated, lower-CO2 locations. We think better with fresh air and absolutely suffer the consequences of brain fog when it starts to get stale.

Why CO2 Buildup Causes Brain Fog

The mechanism behind carbon dioxide and brain fog isn't fully understood, but the piece we do understand runs through blood flow, not oxygen deprivation. CO2 is a strong regulator of the blood vessels supplying the brain, and even modest increases in blood CO2 cause those vessels to widen. That widening pulls more blood toward the brain, which sounds like it should help rather than hurt, but under sustained, mildly elevated CO2 in an unventilated room, it seems to shift our brain's internal chemistry in a way that causes our thinking to become cloudier.

Across studies that dive into this CO2 phenomenon, the tasks that suffer most consistently are the complex ones such as strategic thinking, searching for new information, and planning several steps ahead. Most other things we do aren’t affected. That's why a stuffy room rarely feels like a major issue. We don't lose the ability to read an email or nod along, but we do lose some of the sharper edge we'd want for the harder tasks. It’s the kind of shift that’s easy to misattribute to being tired, bored, or simply bad at the topic at hand.

Where We Encounter This

Elevated CO2 isn't rare. It shows up wherever people gather in a space that isn't moving enough fresh air through it like a packed conference room an hour into a negotiation, a lecture hall by the second half of class, or a car on a long drive with the windows up and the AC off.

Measurements in ordinary classrooms and meeting rooms regularly find concentrations well above optimal levels. These are exactly the settings where we tend to ask the most of our thinking. There's an irony in that the classroom, boardroom, and office are built around the assumption that everyone in the room is operating at full capacity, yet it can be that exact room that’s preventing us from thinking clearly. The air itself may be working against the very thing the space was designed for.

What’s Still Unknown

With all of this said, the research still has real gaps. A comprehensive review of dozens of experimental studies found the effects of CO2 on cognition to be real but inconsistent. The effects seem more pronounced when researchers reduced overall ventilation than when they injected pure CO2 into otherwise clean air. The impacts are also much more noticeable on complex decision-making than for simpler speed or accuracy tasks. 

That inconsistency doesn't mean the impacts aren’t real though, it just means that CO2 is probably one variable among many, tangled up with other factors that aren't yet well understood. Some of us likely notice the effects more than others too, highlightly the individuality to the effects as well.

The next time a room starts to feel thick and the conversation starts to drag, it might be worth wondering less about who's losing focus and more about what's actually building up in the air everyone's sharing. A cracked window or  a break to step outside can start to look less like comfort measures and more like a necessary part of the work itself.

References

1. Satish, U., et al. (2012). Is CO2 an indoor pollutant? Environmental Health Perspectives, 120(12), 1671-1677. https://escholarship.org/uc/item/222631x2

2. Allen, J. G., et al. (2016). Associations of cognitive function scores with carbon dioxide, ventilation, and VOC exposures in office workers. EHP, 124(6), 805-812. https://dash.harvard.edu/entities/publication/73120379-1584-6bd4-e053-0100007fdf3b

3. Du, B., et al. (2020). Indoor CO2 concentrations and cognitive function: A critical review. Indoor Air, 30(6), 1067-1082. https://scispace.com/papers/indoor-co2-concentrations-and-cognitive-function-a-critical-10325jvlxa

4. Battisti-Charbonney, A., et al. (2011). The cerebrovascular response to carbon dioxide in humans. J Physiol, 589(12), 3039-3048. https://pmc.ncbi.nlm.nih.gov/articles/PMC3139085/

5. Zhou, H., et al. (2026). Multimodal evidence of how indoor CO2 concentration impairs office task efficiency. Buildings, 16(11), 2210. https://doi.org/10.3390/buildings16112210