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Indoor Air Quality and Cognition: The CO2 You Breathe

Published: September 7, 2026 · Updated: September 7, 2026

By Sabina Malikova, Sabina Malikova Design Office.

You can walk into a bedroom, a meeting room or a home office feeling clear-headed, and within an hour feel your focus dissolve into a vague fog you cannot quite name. Often the cause is invisible and odourless: the carbon dioxide you exhale, quietly accumulating in a room that does not breathe. In my work I treat air not as a background condition but as one of the most direct levers we have over how well the brain performs during

an ordinary day.We spend the overwhelming majority of our lives indoors, moving between sealed rooms that trap far more than warmth. Modern buildings are deliberately airtight for energy efficiency, which is excellent for heating bills and unfortunate for the air we think in. The result is quietly familiar: the freshest room in the house at eight in the morning can become the dullest by mid-afternoon, without a single visible thing having changed.What the science saysResearch shows that indoor CO2 is

far more than a question of stuffiness. In controlled studies, once concentrations climb above roughly 1000 ppm, measurable declines appear in decision-making, information processing and sustained attention, the cluster of sensations people casually call ‘brain fog’. A landmark controlled-exposure experiment (PMC3548274) found that higher CO2 levels were associated with significantly lower scores across most dimensions of complex decision-making, even when temperature and other factors were held steady. Later work on high-performing green buildings (PMC4892924) linked better-ventilated, low-CO2 interiors with meaningfully

higher cognitive scores, and a broader review (Europe PMC 30172928) points to 1000 ppm as a practical threshold where these effects tend to emerge. None of this suggests fresh air makes anyone a genius. It suggests that stale, CO2-rich air may quietly hold the brain below its own natural baseline.What makes this so easy to miss is that CO2 gives…

Related reading

• Longevity by Design methodology — the four measurable pillars
• CLCI — circadian lighting explained
• BCS — the biophilic connection score
• WHO Europe 2018 — environmental noise guidelines

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