Does CO₂ affect how you think? What the research shows so far

Lab studies have found that people score lower on decision-making tests when the CO₂ in a room rises to around 1,000 ppm, a level many closed bedrooms pass at night. Not every study has found it, and new research keeps coming out. What's already clear is what a high reading tells you: the room isn't getting enough outside air for the people in it. That you can measure, and fix, starting tonight.

What the lab studies found

• Lawrence Berkeley National Laboratory, 2012. 22 adults spent 2½ hours at a time in a test room at about 600, 1,000 and 2,500 ppm, with pure CO₂ added to the incoming air and the ventilation unchanged. At 1,000 ppm, scores were lower on six of nine decision-making scales, and at 2,500 ppm seven were much lower.
• Harvard and Syracuse, 2016. 24 office workers spent six workdays over two weeks in a lab set up like an office, without knowing what the air was like each day. Scores on a decision-making test were lower on days the researchers raised CO₂ to about 950 ppm, and lower again at about 1,400.
• Technical University of Denmark, 2017. 25 people spent just over four hours in a test room. When the CO₂ came from the people themselves in a poorly ventilated room, at 3,000 ppm, they reported more headache, fatigue, sleepiness and difficulty thinking clearly, and did worse on some tests. When pure CO₂ was added to fresh air instead, the same tests didn't change. The researchers put the effects down to bioeffluents: everything else people give off, which builds up along with the CO₂ when a room is short of fresh air.

What the standards say

ASHRAE, the society behind US ventilation standards, reviewed this research in its position document on indoor CO₂ (approved in February 2025). It counts six studies that linked about 1,000 ppm to lower performance; in three of them, the effect grew with the concentration, which ASHRAE takes as a sign of a real effect. Other studies found no effect, and ASHRAE calls for more research. It considers CO₂ nontoxic up to 5,000 ppm, and treats indoor CO₂ as an indicator of how much outside air each person in a room is getting.

Sleep research has gone further for bedrooms. A 2025 review of 17 sleep studies for ASHRAE found people slept measurably worse once bedroom CO₂ reached about 1,000 ppm, and recommends keeping bedrooms below 1,000, preferably below 800. Why 800. The sleep studies.

What it means for your bedroom

• Use CO₂ as your fresh-air gauge. A high number means the room is short of outside air for the people in it, and everything else people and the room give off is staying in with it. The Danish study points at that mix as what people notice.
• Aim for 800 or less while you sleep. That's the bedroom goal from the sleep research, on average across the night.
• Measure three ordinary nights. They show whether your bedroom stays near 800 or climbs well past it. If it climbs, a free change for two or three nights (the door open a few inches, a fan) is the first thing to try. Free fixes to try first
• Waking up with headaches? Check for carbon monoxide first. The CPSC lists headache, dizziness, sleepiness and confusion among the symptoms of CO poisoning. Make sure your CO alarms work, and see a doctor about symptoms that keep coming back. CO alarm vs CO₂ monitor

HouseExhale turns a few nights of readings into one plain answer about your bedroom's fresh air, what to try first, and whether it worked. Check my bedroom