It's something to be careful of, but doing it for a short time is probably fine.
When sitting around, people exhale 4-5% CO₂. That's 40000 - 50000 ppm. But the Navy has done studies at that level and found it doesn't have a noticeable effect¹.
Thus, CO₂ at 40,000 ppm for 2 weeks did not affect performance on multiple tests of cognitive function in physically fit young airmen, a population probably not unlike submariners.
Comments
It's something to be careful of, but doing it for a short time is probably fine.
When sitting around, people exhale 4-5% CO₂. That's 40000 - 50000 ppm. But the Navy has done studies at that level and found it doesn't have a noticeable effect¹.
[1]: https://nap.nationalacademies.org/read/11170/chapter/5#54