Surprised to see that the standard CO2 level on the space station is 4000 ppm

Related reading "I’m living in a carbon bubble. Literally."[0] which has a lot of great basic charts and graphs on everyday data related to CO2. Information in this reading that is relevant to 4000 PPM on the Space Station

  -Typical CO2 concentrations can reach 4000 ppm in a car
  -"On the International Space Station, the odds of a crew member reporting a headache double for every 1300 ppm increase in CO2 concentration [Law 2014]."
  -Nearly every cognitive function go "down" in score when CO2 PPM levels go from 1k to 2.5k. The exceptions are Focused Activity Cognitive Score goes UP and information search stays about the same. Not shown are effects over 2.5k. 

Important to note the Space Station is at a different pressure and has no gravitational effects on airflow compared to where many of the studies were done on Earth.

I do not work nor have studied in a CO2 related field but gained a fascination with how we overlook it in daily life (like many here) and have since purchased CO2 measuring devices and read various studies. I think missing from my notes here is a link to NASA done research on the effects of CO2 on astronauts on the space station.

[0]https://medium.com/@joeljean/im-living-in-a-carbon-bubble-li...

It's my understanding that the real reason we measure co2 is because it's easier to measure. Obviously, o2 is what's important, but they correlate when co2 comes from breathing out, so it's fine to just measure one. Maybe if you maintain a good o2 level, the co2 level just isn't that important?

But on the flip side... co2 is the only thing your body measures too! Which is why taking a breath of co2 from a soda bottle or something makes you breath like you're at the end of a marathon for a few seconds until it clears out. So I have no idea how 4000ppm wouldn't totally screw with your senses.

Even 1000 ppm is pretty high in practice. In offices it approaches this value, and it's no wonder that one needs pharma-strength stimulants and intense exercise to survive the day in the office. Ever wonder why one feels sleepy in meeting rooms: it's because the CO2 is approaching 2000.

At home I like it below 600 ppm, with values between 600 and 800 leading to a gradual deterioration of mental interest. Once it goes above 800, then in the absence of pharma stimulants, the harm to cognition is quickly apparent.

I'd be willing to bet that the value for optimal cognition could be closer to 200 ppm which would be close to its per-industrial value.

I start to feel weird at 1000 ppm.

Thought it might have something to do with air pressure, but the ISS is kept at sea level pressure with the same ratio of nitrogen to oxygen (or very much close enough).

Wildly speculating - maybe C02 is easier to eliminate from your body in a very low gravity environment, so they keep it higher to avoid hypocapnia?

At that level you should be able to taste it.