Everyone knows that too much carbon dioxide is bad. It warms our planet, raises sea levels, and generally wreaks havoc on our environment (why leaders all over the world have gathered in Paris this week to talk about carbon emissions). Now, scientists have discovered a different way that CO2 harms us: if we’re exposed to too much of it indoors—like in an office—it hurts our ability to think, which may ultimately affect both our well-being and our job performance.

Of course, people have known for a while that exceedingly high levels of CO2 are unsafe. For example, if you’re exposed to CO2 at 90,000 parts-per-million (ppm) for five minutes, you’ll die. The Occupational Safety and Health Administration sets an exposure limit way below that level, at 5,000 ppm on CO2 over an eight-hour work shift. Thankfully, most office buildings have an even lower concentration than that. At those lower levels, CO2 is considered harmless—it’s measured in buildings so people know how well-ventilated a space is, but no one’s ever considered CO2 a direct pollutant.

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But a recent study by scientists at Harvard and Syracuse suggests that those lower CO2 levels we consider harmless—concentrations found in many office buildings—are actually high enough to impair human health.

For their study, published in Environmental Health Perspectives, researchers recruited 24 knowledge workers who normally spend their time in an office (like architects, designers, and engineers) and had them work eight-hour days in a simulated office. For several of those workdays, the researchers manipulated the level of CO2 in the office, so that it fell at either a low, moderate, or high level (~550 ppm, 945 ppm, or 1,400 ppm) for the day.

The lowest level is pretty close to what you’d breathe outdoors, and represents a very well-ventilated building, whereas 945 ppm is the typical CO2 level found in most offices. Even 1,400 ppm isn’t unrealistic—study author Joseph Allen says that these are all concentrations you would get in standard office buildings. “It was our goal to make sure these simulations were tied to real world environments,” says Allen, an assistant professor at the Harvard T.H. Chan School of Public Health. “We didn’t want to test the exotic or extreme, we wanted to test conditions that most of us find ourselves in.” Indoor CO2 concentrations are determined by the number of people in a space (our breath is the source of CO2), the ventilation rate of the building, and the concentration of CO2 outdoors. (So poor ventilation and lots of people breathing in a space would cause high levels of CO2 indoors.)

We tested conditions that most of us find ourselves in.

During the CO2 trials, participants went about their normal workday, and both they and the researchers were blind to the air conditions in the simulated office. In the afternoon, Allen and his team gave participants a 1.5 hour cognitive assessment to test how that day’s CO2 level affected their high-order decision-making skills. The results showed a clear trend: less CO2 improves cognitive function. Compared to the lowest CO2 level (550 ppm), people scored 15% worse on the test for the moderate CO2 day (945 ppm) and 50% worse on the high CO2 day (1,400 ppm). The researchers also broke down participants’ cognitive function into different categories, and found that higher CO2 levels most hurt people’s ability to use information, respond to crises, and strategize—types of thinking closely related to productivity, says Allen.