Carbon dioxide impairs cognitive function through multiple pathways that begin the moment blood CO2 levels rise: it shifts brain pH, alters cerebral blood flow, dampens neural excitability, and may interfere with key neurotransmitter receptors. Controlled studies in office-like settings have found that cognitive test scores can drop substantially at CO2 concentrations commonly found in poorly ventilated rooms. The story is more nuanced than the alarming headlines suggest, though, because not every study agrees on the threshold where problems begin, and short exposures affect people differently than chronic ones.
What Happens Inside Your Brain When CO2 Rises
The brain is exquisitely sensitive to its own acid-base balance. When you inhale air with elevated CO2, more of it dissolves into your blood and crosses into brain tissue, where it reacts with water to produce hydrogen ions. Those hydrogen ions lower pH, making the local environment more acidic. Even small pH shifts have outsized effects on neurons because hydrogen ions act as powerful modulators of how excitable nerve cells are. In general, a more acidic (lower pH) environment suppresses neuronal firing, while a more alkaline environment ramps it up.
1PubMed. Carbonic anhydrases and brain pH in the control of neuronal excitabilityThis suppression of neural excitability shows up on brain-wave recordings. EEG studies comparing people breathing normal air with those breathing CO2-enriched air have found a shift toward slower electrical rhythms in the brain, a pattern associated with drowsiness and reduced alertness.
2PubMed Central. The influence of carbon dioxide on brain activity and metabolism in conscious humansThe brain’s blood vessels respond to rising CO2 as well. Higher CO2 triggers vasodilation, widening blood vessels to increase flow. This is actually a protective reflex: the brain is trying to wash out excess CO2 and restore normal pH. But the vasodilation itself changes the hemodynamic environment inside the skull, and when CO2 stays elevated, the compensatory mechanism can only do so much.
3PubMed Central. The cerebrovascular response to carbon dioxide in humansThere is also evidence from animal studies suggesting CO2 may interfere with neurotransmitter signaling more directly. Research on coral reef fish exposed to elevated CO2 found that their abnormal behaviors could be reversed by blocking a specific receptor for GABA, one of the brain’s main inhibitory neurotransmitters. The researchers proposed that elevated CO2 disrupts the normal balance of chloride and bicarbonate ions that GABA receptors depend on, effectively flipping GABA signaling in ways that scramble sensory processing. Because GABA receptors are highly conserved across vertebrates, this mechanism has raised questions about whether similar disruption could occur in mammals, though the CO2 levels involved in the fish studies were far higher than what humans encounter indoors.
4Nature Climate Change. Near-future carbon dioxide levels alter fish behaviour by interfering with neurotransmitter functionThe Indoor Air Studies That Sounded the Alarm
For decades, indoor air quality experts treated CO2 mainly as a proxy for ventilation. If CO2 was high in a room, it meant fresh air was not coming in fast enough, and the real culprits were assumed to be volatile organic compounds, dust, and bioaerosols that accumulated alongside it. Then a pair of studies challenged that assumption by testing whether CO2 itself was the problem.
In a 2012 experiment at Lawrence Berkeley National Laboratory, researchers placed 22 participants in a chamber where they could control CO2 independently of other pollutants. At around 1,000 ppm, moderate but statistically significant drops appeared in six out of nine decision-making performance scales compared to the 600 ppm baseline. At 2,500 ppm, large and significant reductions showed up in seven of the nine scales, with some raw scores falling to as little as 6% of baseline levels.
5PubMed Central. Is CO2 an indoor pollutant? Direct effects of low-to-moderate CO2 concentrations on human decision-making performanceA follow-up study published in 2016 expanded the approach. Researchers exposed office workers to simulated green building conditions (around 550 ppm CO2), a moderate CO2 day (about 945 ppm), and a high CO2 day (about 1,400 ppm). Across seven of the nine cognitive domains tested, scores declined at each step up in CO2. On average, a 400 ppm increase was associated with a 21% decrease in cognitive scores after adjusting for individual differences. On the highest CO2 day, scores were roughly 50% lower than on the cleanest-air days.
6PubMed Central. Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and Volatile Organic Compound Exposures in Office Workers: A Controlled Exposure Study of Green and Conventional Office EnvironmentsA 2023 systematic review and meta-analysis pulled together a broader swath of the literature and confirmed that CO2 below 5,000 ppm does affect cognitive performance, with complex tasks being hit harder than simple ones. The biggest declines in complex task performance appeared at additional CO2 concentrations of 1,000 to 1,500 ppm above baseline.
7Building and Environment. Short-term exposure to indoor carbon dioxide and cognitive task performance: A systematic review and meta-analysisWhy Some Researchers Are Skeptical
The findings above generated real excitement, but not everyone in the field is fully convinced. Several replication attempts have produced weaker or conflicting results. One study exposed 31 volunteers to either about 830 ppm or 2,700 ppm CO2 for less than an hour and found that the high-CO2 condition did not produce clear-cut declines across the board. Instead, the main signal was subtler: a learning effect that researchers normally expect to see when someone takes the same cognitive test twice simply failed to appear in the high-CO2 condition, as though the gas prevented the brain from getting better at the task with practice.
8Building and Environment. Exploring the physiological, neurophysiological and cognitive performance effects of elevated carbon dioxide concentrations indoorsPart of the inconsistency may come down to methodology. The type of cognitive test matters a lot: simple reaction-time tasks tend to be more resistant to CO2 than complex decision-making or strategic planning exercises, as the meta-analysis noted. Exposure duration, whether participants know their air is being manipulated, individual differences in baseline alertness, and even the time of day can all shift results. A study of astronaut-like subjects exposed to CO2 up to 5,000 ppm for under three hours found only minor effects on cognition and decision-making, suggesting that healthy, alert individuals performing structured tasks in a controlled environment may tolerate higher levels than the office studies implied.
9npj Microgravity. Effects of acute exposures to carbon dioxide on decision making and cognition in astronaut-like subjectsThe honest read of the evidence right now is that CO2 probably does impair complex cognitive performance at concentrations common in crowded indoor spaces, but the size of the effect and the exact threshold are still being pinned down. The most dramatic numbers from the earliest studies may overstate it. The field is still young enough that reviewers have noted roughly half of the available studies show clear cognitive declines while others do not, a split that suggests real effects mixed with real methodological noise.
10Building Services Engineering Research and Technology. Possible future impacts of elevated levels of atmospheric CO2 on human cognitive performance and on the design and operation of ventilation systems in buildingsHow CO2 Disrupts Sleep and Next-Day Performance
Even if you are not awake to notice it, the CO2 level in your bedroom overnight matters. Several studies have linked higher bedroom CO2 to worse sleep quality. In one experiment, researchers found that when bedroom CO2 was lower, both objectively measured sleep quality and next-day performance on logical thinking tests improved significantly, and participants reported feeling less sleepy the following day.
11PubMed. The effects of bedroom air quality on sleep and next-day performanceA larger observational study using wrist-worn activity monitors found that people sleeping in the highest CO2 quintile had sleep efficiency about 4 percentage points lower than those in the lowest quintile, a meaningful gap when sustained night after night. That effect was comparable to, and in some cases larger than, the sleep disruption from elevated bedroom temperature or fine particulate matter.
12PubMed Central. Associations of Bedroom PM2.5, CO2, Temperature, Humidity and Noise with Sleep: an Observational Actigraphy StudyAnother study focused on naturally ventilated bedrooms during mild-weather months confirmed the pattern: higher nightly CO2 correlated with worse self-reported sleep quality, stronger perceived odor, and lower air-quality acceptability ratings.
13PubMed. Associations of bedroom air temperature and CO(2) concentration with subjective perceptions and sleep quality during transition seasonsThis matters for cognition because poor sleep is itself one of the strongest predictors of impaired thinking. There is even evidence of a feedback loop: the study that exposed 31 volunteers to 2,700 ppm CO2 found that participants who had slept less the previous night were more susceptible to becoming sleepier during the CO2 exposure. If high bedroom CO2 degrades your sleep, you arrive the next day with less resilience to any CO2-related cognitive drag at work or school.
8Building and Environment. Exploring the physiological, neurophysiological and cognitive performance effects of elevated carbon dioxide concentrations indoorsChronic Exposure and Why Duration Matters
Most of the studies discussed so far involve short exposures, typically minutes to a few hours. But many people spend years in environments where CO2 regularly exceeds 1,000 ppm: office workers in sealed buildings, submarine crews, space station astronauts, or anyone who sleeps in a small, tightly sealed bedroom. The consequences of chronic exposure are less well studied but potentially more serious.
A review focused on long-duration spaceflight noted that while humans seem able to work safely in mildly elevated CO2 for extended periods, chronic exposure to respiratory acidosis can cause bone loss, systemic inflammation, impairments in cognitive function and visuomotor skills, and sustained changes in cerebral chemistry.
14PubMed Central. Confined spaces in space: Cerebral implications of chronic elevations of inspired carbon dioxide and implications for long-duration space travelIn clinical medicine, the clearest window into chronic CO2 effects comes from patients with severe lung disease. People with advanced COPD can develop chronic hypercapnia because their lungs can no longer expel CO2 efficiently. Reviews of cognitive impairment in COPD have identified chronic hypercapnia as a contributor through inflammatory, neurochemical, and cerebrovascular changes, effects that get worse when low oxygen and high CO2 occur together.
15CHEST Pulmonary. Mechanisms and Risk Factors of Cognitive Impairment in COPDAnimal data adds another dimension. Rats exposed to high CO2 concentrations during the neonatal period developed lasting learning deficits and impaired synaptic plasticity in the hippocampus, the brain region most critical for forming new memories. The exposed animals showed reduced long-term potentiation, the cellular process believed to underlie learning, along with decreased inhibitory function in hippocampal circuits.
16PubMed. Neonatal exposure to high concentration of carbon dioxide produces persistent learning deficits with impaired hippocampal synaptic plasticityWho Is More Vulnerable
Not everyone responds to elevated CO2 the same way, and identifying who is most at risk is one of the field’s open questions. The study finding that sleep-deprived individuals became sleepier during CO2 exposure hints at a broader pattern: pre-existing stressors on the brain may amplify CO2’s effects. If your cognitive reserves are already depleted by poor sleep, fatigue, or illness, an additional hit from stuffy air may tip you over a threshold you would otherwise tolerate.
Children may be especially worth watching. A short review of indoor CO2 research noted that respiratory symptoms have been reported in children exposed to indoor CO2 above 1,000 ppm.
17Environment International. Effects of low-level inhalation exposure to carbon dioxide in indoor environments: A short review on human health and psychomotor performanceChildren also breathe faster per unit of body weight than adults, and their developing brains may be less equipped to buffer the pH shifts CO2 produces. The animal evidence showing lasting hippocampal damage from neonatal CO2 exposure adds a reason for concern, though the concentrations used in those studies were much higher than what children encounter in classrooms.
Older adults and people with preexisting respiratory or cardiovascular conditions are another group with reason to pay attention. Anyone whose body already struggles to maintain normal blood gas levels is starting from a compromised baseline. The COPD literature makes this clear: even mild elevations in blood CO2 can compound cognitive difficulties in someone whose lungs are not clearing CO2 efficiently.
Practical Ways to Keep Indoor CO2 Low
The simplest intervention is ventilation. Opening a window remains the cheapest and most effective way to bring CO2 down in most residential settings. In commercial buildings, mechanical ventilation systems can be equipped with CO2 sensors that automatically increase outdoor air supply when concentrations climb. These demand-controlled ventilation systems adjust the mix of fresh and recirculated air to maintain a target, often around 700 to 800 ppm, and they also reduce energy waste compared to running ventilation at a fixed high rate all day.
18Energy and Buildings. A robust CO2-based demand-controlled ventilation control strategy for multi-zone HVAC systemsConsumer-grade CO2 monitors have become affordable in recent years, typically using nondispersive infrared sensors that are accurate enough for home or office use. Placing one on your desk or nightstand gives you a real-time read on your air quality. If you see readings persistently above 1,000 ppm during work hours or while sleeping, that is a signal to increase ventilation. In bedrooms, cracking a window or leaving the door open to the rest of the house can make a noticeable difference, since a single sleeping person in a small, sealed room can push CO2 well past 2,000 ppm by morning.
In meeting rooms and classrooms, the problem is acute: a dozen people in a modestly sized room with no fresh air supply can push CO2 past 1,500 ppm within half an hour. Scheduling breaks, limiting room occupancy, or simply opening a door to the hallway between sessions can help. Schools that have invested in improved ventilation have generally reported better air quality scores, and the pandemic-era push for better airflow in classrooms created infrastructure that serves indoor air quality more broadly.
Rising Outdoor CO2 and the Long-Term Outlook
Everything discussed so far treats outdoor air as “clean” baseline air with low CO2, typically around 420 ppm today. But atmospheric CO2 is rising. Some projections for urban outdoor air concentrations before 2100 approach levels that the indoor-air literature has flagged as concerning.
19Nature Sustainability. Direct human health risks of increased atmospheric carbon dioxideThis creates a compounding problem. Indoor CO2 is always higher than outdoor CO2, because people are in the room exhaling it. If outdoor baseline concentrations climb from 420 to 500 or 550 ppm, the indoor peak during a crowded meeting or a night in a sealed bedroom climbs by the same amount. A room that currently peaks at 1,200 ppm might peak at 1,300 or 1,400 ppm in a few decades, pushing more people past whatever the true cognitive-impairment threshold turns out to be. Ventilation systems designed for today’s outdoor air would need to move more air to achieve the same indoor targets.
10Building Services Engineering Research and Technology. Possible future impacts of elevated levels of atmospheric CO2 on human cognitive performance and on the design and operation of ventilation systems in buildingsThe evolutionary framing is interesting here. During the entire span of human evolution, atmospheric CO2 ranged between roughly 180 and 280 ppm, and the body’s pH buffering system evolved to operate in that band. Current outdoor levels are already above anything the species experienced before industrialization, and indoor levels in tight buildings can be several times higher still.
20PubMed Central. Hypothesis: Potentially Systemic Impacts of Elevated CO2 on the Human Proteome and HealthHow the Brain Detects and Adapts to CO2
Mammals have evolved specialized sensing systems that monitor both oxygen and CO2 in the blood and trigger corrective responses. Chemoreceptors in the brainstem and carotid bodies detect changes in blood CO2 and pH, prompting you to breathe faster and deeper to blow off excess CO2. This is why a stuffy room can make you yawn or feel short of breath even before you consciously notice anything wrong.
21PubMed. Mechanisms and Consequences of Oxygen and Carbon Dioxide Sensing in MammalsAt the cellular level, some of the brain’s CO2 sensitivity runs through connexin26, a protein that forms channels between cells and whose sensitivity to CO2 has been tuned by evolution to match each species’ typical respiratory environment. Researchers comparing connexin26 across species found that mole rats, which live in underground burrows with very high CO2, have a version of the protein that requires far higher CO2 to activate than the human or rat version. Chickens, which have lower resting blood CO2, have a more sensitive version. This kind of evolutionary calibration underscores that there is no universal “safe” CO2 level across animals; each species’ physiology is matched to its historical niche.
22PubMed Central. Evolutionary adaptation of the sensitivity of connexin26 hemichannels to CO2For humans, this means our CO2 tolerance is calibrated to an atmosphere that no longer exists, and indoor environments routinely push concentrations well beyond anything our ancestors’ brains were optimized for. The body’s compensatory systems, faster breathing, vasodilation, pH buffering by the kidneys, can handle modest spikes. But the growing body of evidence suggests that when those systems are outpaced, particularly during sustained exposure or when the brain is already taxed, cognitive performance is one of the first things to slip.