Twenty-two people in a chamber at Johnson Space Center breathed air with four different amounts of carbon dioxide and the most striking dip in performance came at 1,200 ppm. The finding drew attention because the study did not show a simple pattern in which each higher level produced worse results.
The work came from a 2019 npj Microgravity study on how short carbon dioxide exposures affect thinking that matters in spaceflight. The paper focused on decision-making and a broad Cognition battery because crews on spacecraft may need to think clearly during emergencies while living in sealed habitats.
Researchers found that most of the strongest changes clustered at 1,200 ppm, even though the chamber later reached 2,500 ppm and 5,000 ppm. The authors treated that uneven pattern with caution and said longer studies are still needed before anyone can assume that brief results on the ground predict what happens over time in orbit.
Why 1,200 ppm stood out
Robert R. Scully and colleagues designed the study around a practical space question. Carbon dioxide can build up in closed vehicles and the paper notes that some spacecraft levels have been higher than the outdoor air people usually breathe on Earth.
Instead of finding a steady slide from low carbon dioxide to high carbon dioxide, the team saw the sharpest drop at the second test level. Performance on most measures in the management simulation fell below the 600 ppm baseline at 1,200 ppm, while many scores at higher concentrations moved back toward baseline or even above it.
The paper’s PubMed record summarizes the same central point. Aggregate speed, accuracy and efficiency scores across the cognitive tasks were lower at 1,200 ppm than at baseline, yet the authors did not see a clear dose-response pattern across the full set of results.
That makes the study interesting, but it also limits how far the result can travel. A non-linear pattern can reflect real biology, yet it can also reflect noise, stress, adaptation during a session, or the way several separate tasks were combined into summary scores.
How the chamber days worked
The experiment used a double-blind cross-over study with 22 healthy adults described as astronaut-like subjects. Four groups rotated through four chamber conditions: 600, 1,200, 2,500 and 5,000 parts per million of carbon dioxide.
Each exposure lasted about three hours on a morning test day and each group returned once a week for four weeks. Before entering the chamber, then again 15 minutes and 2.5 hours after entry, the volunteers completed the iPad-based thinking tests. They also repeated the battery after leaving the chamber.
The researchers placed the Strategic Management Simulation about 30 minutes after chamber entry. The task puts participants inside a crisis scenario and scores broad behaviors. Those scores include initiative, information use, focused activity and task orientation.
The full open-access paper shows how wide the second test battery was. The Cognition battery included ten tasks tied to attention, memory, abstraction, emotional recognition, risk decisions, processing speed and vigilance, which gave the team several ways to look for subtle changes.
What the management test found
The management simulation produced the clearest warning sign in the whole study. According to the paper, all measures of complex decision-making changed from the 600 ppm baseline when carbon dioxide rose to 1,200 ppm and eight of the nine measures moved downward.
Some of those declines involved behaviors that crews would want during a demanding event. Lower scores appeared in basic activity and applied activity. The paper also reported declines in task orientation, initiative and breadth of approach. Information utilization moved upward at 1,200 ppm, which shows that the pattern was mixed even at the level where the broadest drop appeared.
Higher concentrations did not extend that same picture in a straight line. At 2,500 ppm, only task orientation and applied activity still differed significantly from baseline and both of those scores were above baseline. At 5,000 ppm, focused activity level rose above baseline while basic activity fell below it.
For readers outside the lab, the main lesson is narrow and specific. The study suggests that short carbon dioxide exposure can affect complex performance in uneven ways, while it does not show that every additional rise in chamber carbon dioxide makes every important decision skill worse.
What changed in the cognition battery
The second test system looked less dramatic than the management simulation, yet it still pointed to some slowing at 1,200 ppm. When the researchers averaged the ten tasks into summary measures, aggregate speed, accuracy and efficiency were lower there than at baseline.
Individual tasks told a less uniform story. The battery sampled sensory-motor speed, working memory, spatial judgment, pattern reasoning, emotion recognition, risk taking and vigilant attention. A rise or fall in one task therefore did not automatically mean the same thing happened everywhere else.
The paper matters here because it was built for high-performing people rather than for a general classroom sample. The authors noted that Mathias Basner and colleagues had developed the tablet tests with repeated use in demanding settings in mind, which made them a useful match for a spaceflight question.
Even so, the study did not deliver a neat message that carbon dioxide damaged every corner of cognition. The signal was strongest in combined scores and in the management simulation, while the task-by-task picture stayed scattered enough that the authors avoided a broad claim of uniform mental impairment.
Why the dose pattern stayed uneven
The authors addressed the awkward part directly: their results conflicted with some earlier reports that found a more direct decline at higher carbon dioxide levels. In the abstract and discussion, they suggested that different subject populations and different bundles of other stressors may help explain why studies do not always agree.
A related NASA technical report had already described crew complaints such as headaches and lethargy under spaceflight conditions, even when older ground evidence suggested moderate levels should be safe. That gap is one reason short chamber studies remain relevant, but it also shows why one experiment cannot settle the whole question.
Duration is another major limit. This study tested acute exposures that lasted less than three hours, while actual missions can keep crews inside a carbon-dioxide-managed habitat for weeks or months. The authors therefore called for longer exposure studies rather than presenting their work as a final answer.
What the result means for space crews
Space agencies care about carbon dioxide because crews do not merely sit and breathe in orbit. They troubleshoot equipment, handle timelines, communicate with Earth and respond to off-nominal events. A small shift in high-level judgment can matter even when a person still looks alert and can finish simple tasks.
NASA has continued to examine the topic in later work. A more recent NASA citation record describes further testing of physical and cognitive performance under simulated emergency conditions, which signals that the agency still sees unresolved operational questions.
The cautious reading of the 2019 study is therefore twofold. Brief carbon dioxide exposures can coincide with measurable changes in complex performance and the size and direction of those changes did not rise in a clean stair-step pattern across the four chamber levels. That leaves mission planners with a reason to keep measuring, especially over longer durations and in crew-like settings that come closer to real spaceflight.
