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A recent (Sept. 2017) news item on the company website of Gas Sensing Solutions Innovative sensor technology assists NASA with vital wearable CO2 monitoring equipment caught my eye. A section of the article is quoted below, with the accompanying photo of Expedition 49 crewmember Kate Rubins of NASA working "...with the airlock inside of Kibo, the Japanese Experiment Module. Rubins was installing the Robotics External Leak Locator (RELL), a technology demonstration designed to locate external ISS ammonia (NH3) leaks." (quoted caption from the copy of this image found in Wikimedia Commons and in NASA Johnson's Flikr where it is dated 27-Sep-2016).

See also SST Sensing's Innovative Sensor Technology Assists NASA with Vital Wearable CO2 Monitoring Equipment and CO2Meter.com's NASA Wearable CO2 Monitor uses CozIR Sensor.

I know that CO2 buildup is an issue in confined spaces in microgravity, but the article suggests that even the baseline levels experienced by ISS astronauts might be a problem.

Question: Do ISS astronauts notice, or experience symptoms of elevated CO2 levels? Do they wear monitors regularly? I wonder if some confided sections, like end modules or the Bigelow Expandable Activity Module might be more at risk, and astronauts are more likely to use personal monitors there for safety, or if they are used strictly for data collection and archiving.

These are commercial websites selling or advocating products. I'm wondering if there are NASA or other non-commercial, authoritative sources that address this question.


Though extensive terrestrial studies have shown that this increased level will have no effect on the continued good health of those exposed to it, there must be some acknowledgement of the fact that crew members onboard the ISS still regularly experience symptoms of acute CO2 toxicity – such as headaches or lethargy.

Per this answer, the image is that of astronaut and molecular biologist Dr. Kathleen "Kate" Rubins (Ph.D.), the first person to sequence DNA in space.

enter image description here

It is generally accepted that human adaptations to microgravity conditions will cause astronauts in space to become more sensitive to elevated CO2 levels. This can result not only in physical discomfort, but also impinge on their cognitive skills and reaction times – thereby potentially leading to safety risks. On average crew members stay on the ISS for a period of around 6 months, so having a good grasp of the ongoing implications is clearly of great value.

With operational practicalities meaning that it is not possible (either technically or financially speaking) to remove enough CO2 to replicate normal conditions here on Earth, the ISS has to function with relatively high ambient CO2 concentrations present in the air. Yet its occupants have to live and work in an acceptable environment.

NASA scientists have, for many years, wanted to embark on a thorough investigation of how elevated CO2 levels impact on ISS crew members’ ability to carry out their allotted tasks, but it has taken a long time for them to be in a position to employ a method for acquiring data to a high enough degree of precision. Given that, as already mentioned, air does not mix well without the influence of gravity, wall-mounted sensors had proved themselves to be an unsuitable means by which to get an accurate reflection of a crew member’s exposure to CO2. What was needed instead were wearable CO2 sensors.

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I don't have anything public I can point to, but yes, they do notice. The CO2 target is a frequent topic of conversation between the crew and ground personnel, and the crew haven't been shy about pointing out when CO2 concentrations are too high for their liking.

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  • $\begingroup$ Thanks! In this particular case I can understand why there might not be public supporting links. $\endgroup$
    – uhoh
    Jan 25, 2018 at 1:27
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    $\begingroup$ Tristan, do not feel I have a full answer but possible reference for yours is Scott Kelly in 'Endurance: A year in space' who has a great deal to say about CO2 Levels and the discomfort they caused, to the point it is almost a theme of the book. $\endgroup$ Nov 6, 2018 at 9:52
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    $\begingroup$ @GremlinWranger if you have a copy handy, a quote or two in a short supplementary answer would be really interesting to read. $\endgroup$
    – uhoh
    Aug 12, 2019 at 17:46
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The CO2 content of fresh outdoor air is about 0.03 %. The NASA long-duration spacecraft maximum allowable CO2 concentration is 0.7 %.

Indoor air of good quality has up to 0.08 % CO2, medium quality between 0.08 and 0.1 % CO2, low quality between 0.1 and 0.14 % and poor quality above 0.14 %.

The maximum allowable workplace concentration in Germany for up to 8 hours daily is set to 0.5 %.

Though extensive terrestrial studies have shown that this increased level will have no effect on the continued good health of those exposed to it, there must be some acknowledgement of the fact that crew members onboard the ISS still regularly experience symptoms of acute CO2 toxicity – such as headaches or lethargy.

It is generally accepted that human adaptations to microgravity conditions will cause astronauts in space to become more sensitive to elevated CO2 levels. This can result not only in physical discomfort, but also impinge on their cognitive skills and reaction times – thereby potentially leading to safety risks. On average crew members stay on the ISS for a period of around 6 months, so having a good grasp of the ongoing implications is clearly of great value.

Source: SST Sensing

So yes, ISS astronauts notice or experience symptoms of elevated CO2 levels.

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Supplementary to Tristan's answer, Scott Kelly in 'Endurance' has a lot to say on carbon dioxide levels and their effects, with 27 entries in the index (separate to the 13 for the CDRA system that managed them). All pages numbers from paperback edition.

page 86:

The carbon dioxide levels are high today, nearly four millimeters of mercury. I can check it on the laptops and see exactly what the concentration of CO2 is in our air, but I don't need to - I can feel it. I can sense the levels with a high degree of accuracy based only on the symptoms I've come to know so well: headaches, congestion, burning eyes, irritability.

Page 87

On my second flight to space, again on the space shuttle, I become more aware of how CO2 was affecting me and talked to my crewmates about their symptoms. I could pinpoint those moments when the CO2 was highest just from the way I felt

page 174

The CO2 is much better now that I'm the only one exhaling on this side of the ISS. My headaches and congestion have largely cleared up, and I noticed a difference in my mood and cognition

page 285 (after a space walk, Seedra refers CDRA, the Carbon Dioxide Removal System)

Kimiya helps us remove our helmets first, which is a relief in one way. But we will miss the cleaner air: the CO2 scrubbers in the suits do a much better job than Seedra

He also describes his dis-satisfaction with the CO2 levels and wrangling with NASA over them, including arranging time on a submarine for a new program manager where the Navy limit was set at 2mm of mercury as a maximum, rather than the 4mm 'normal' above.

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