Cognitive testing in controlled chambers shows decision-making performance drops measurably as CO₂ rises above 1,000 ppm and falls sharply by 2,500 ppm. In a UK office, that maps directly onto the post-lunch slump: occupancy is highest, ventilation rate is fixed, and CO₂ climbs through the afternoon. The brain reads it as oxygen deprivation — even though oxygen is fine — and triggers drowsiness.
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Why under-ventilated rooms make you tired
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What 'enough ventilation' means in a UK office
Approved Document F requires at least 10 litres per second of fresh air per person in an office. CIBSE Guide A and the WELL Building Standard set higher targets where cognitive performance matters (15 L/s/person and above). Most UK offices that report fatigue are running at 4–7 L/s/person — often because occupancy has grown since the AHU was sized.
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How to confirm it — the ventilation audit
The diagnostic is a ventilation assessment: airflow measurements at AHU intake, supply grilles and extracts, plus continuous CO₂ logging at desk height across one or two working weeks. The combined dataset tells you the actual delivered fresh-air rate per person, where the system is losing air, and whether the problem is plant capacity or balancing.
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What the fix usually looks like
If the AHU has capacity, balancing and grille adjustments solve most cases at low cost. If occupancy has outgrown plant capacity, options are: reduce desk density, add demand-controlled ventilation (CO₂-driven dampers), or upgrade the AHU. The test data converts the conversation from "the building is stuffy" into a costed engineering decision.
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Frequently asked questions
Why am I always tired at the office?
The most common cause is rising CO₂ from under-ventilation, which is well documented to reduce cognitive performance and produce drowsiness. Heat above 24 °C, low humidity and VOC exposure compound the effect. A two-week IAQ monitoring study identifies which factor dominates in your building.
Is fatigue a sign of sick building syndrome?
Yes. Persistent fatigue, difficulty concentrating and headaches in the same group of people, easing in the evening and at weekends, are part of the WHO definition of sick building syndrome. The most common underlying cause in UK offices is inadequate fresh-air supply.
What's the right test for an under-ventilated office?
A ventilation assessment combining continuous CO₂ logging at desk height with airflow measurements at the AHU and grilles. This tells you whether the system is delivering the litres-per-second-per-person specified in Approved Document F and CIBSE Guide A.
Can opening windows fix it?
In a naturally ventilated office, partially — but it's weather-dependent, brings outdoor pollutants in, and competes with heating costs. In a mechanically ventilated building, opening windows often unbalances the AHU and makes the problem worse. The test result determines the right fix.