High CO2 Levels on the ISS

You have noticed that the CO₂ levels on the International Space Station are very high compared to standards for indoor air on Earth. In fact is can be up to 10x that of fresh air on Earth. It is definitely one of the challenges for astronauts living on the ISS! You can read more about the tolerable levels and standards in the Let's Talk Science background article here. To our knowledge, "NASA has set the maximum allowable 24-hour average CO₂ on board the ISS at 5 250 ppm (4.0 mmHg)." Sometimes during the mission, the astronauts are breathing piped in pure oxygen but the atmosphere in the space station is generally much like Earth except for higher levels of CO₂, which has been a challenge to decrease.

The effect of high levels of CO₂ on astronauts’ biological and psychological well-being is one of the things being studied. While there is no doubt that astronauts are very fit people who can adapt to compromised air quality, it still has nagging effects. The crew completes a weekly questionnaire for the ESA Space Headaches investigation. The Space Headaches investigation collects information that may help in the development of methods to alleviate associated symptoms and improvement in the well-being and performance of crew members in space. Headaches during spaceflight can negatively affect mental and physical capacities of crew members, which can influence performance during a space mission.

How does the life support system actually scrub the air and remove CO₂?

The Carbon Dioxide Removal Assembly system (CDRA) on the ISS works to remove CO₂ from the cabin air and feed it into the newly installed Sabatier Assembly, which will further help in reducing the total air borne CO2. This combines the CO₂ with waste hydrogen from the Oxygen Generator Assembly to produce methane, which is dumped overboard, and water. This creates an environmentally safe crew cabin as well as water for the crew. Carbon dioxide removal, or CO₂ scrubbing, involves the use of heterogeneous granules of a synthetic rock called zeolite (also known as a molecular sieve). When the cabin fans blow air through the bed of rocks, CO₂ and water stick to the zeolite, while everything else passes through. The zeolite is regenerated by heating it and exposing it to the vacuum of space.

The CO2 scrubbing systems are too taxing on the energy systems to run at a capacity needed to constantly keep the CO2 at levels comparable to earth. They will increase its capacity from time to time to counteract any complaints from astronauts. This video describes other technical issues that they have to contend with: https://www.youtube.com/watch?v=mEXewwQbGZQ&feature=youtu.be

Sources: 

http://explorecuriocity.org/Explore/ArticleId/6421/carbon-dioxide-on-earth-and-on-the-iss.aspx

https://www.nasa.gov/pdf/146558main_RecyclingEDA%28final%29%204_10_06.pdf

https://www.nasa.gov/mission_pages/station/research/news/sabatier.html

https://corporate.evonik.com/en/pages/article.aspx?articleId=97251