I read your link and the concentrations / toxicity levels seemed awfully high to me, but I was going from memory.
Anyway, I went back and actually found the reference I was "quoting" ( if my faulty memory can be considered a quote ) and here is some of what I found.
The NIOSH Recommended Exposure Limit for CO2 in workplaces is 0.5% (5,000 ppm) for a 40-hour workweek (TWA = Total Weekly Average) and 3.0% for a 15-minute short-term exposure limit; (STEL) a level of 4.0% is designated as "immediately dangerous to life or health"
This from Wikipedia:
Due to the health risks associated with carbon dioxide exposure, the U.S. Occupational Safety and Health Administration says that average exposure for healthy adults during an eight-hour work day should not exceed 5,000 ppm (0.5%). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary health issues is significantly less. For short-term (under ten minutes) exposure, the U.S. National Institute for Occupational Safety and Health (NIOSH) and American Conference of Government Industrial Hygienists (ACGIH) limit is 30,000 ppm (3%). NIOSH also states that carbon dioxide concentrations exceeding 4% are immediately dangerous to life and health. [32]
Adaptation to increased levels of CO2 occurs in humans. Continuous inhalation of CO2 can be tolerated at three percent inspired concentrations for at least one month and four percent inspired concentrations for over a week. It was suggested that 2.0 percent inspired concentrations could be used for closed air spaces (e.g. a submarine) since the adaptation is physiological and reversible. Decrement in performance or in normal physical activity does not happen at this level.[33][34]
These figures are valid for pure carbon dioxide. In indoor spaces occupied by people the carbon dioxide concentration will reach higher levels than in pure outdoor air. Concentrations higher than 1,000 ppm will cause discomfort in more than 20% of occupants, and the discomfort will increase with increasing CO2 concentration. The discomfort will be caused by various gases coming from human respiration and perspiration, and not by CO2 itself. At 2,000 ppm the majority of occupants will feel a significant degree of discomfort, and many will develop nausea and headaches. The CO2 concentration between 300 and 2,500 ppm is used as an indicator of indoor air quality.
Now, that last paragraph bothers me as it seems to contradict earlier levels / effects, although it commits itself to "pure" CO2, and seems to address indoor exposures.
But one thing seems to be clear, and that is the higher concentrations refer to a limited exposure time, be it a day, week, or monthly period.
Continuous exposure appears to require much lower levels.
As a matter of safety, I could probably feel fairly secure as long as "atmospheric" levels of CO2 remained below 1000 ppm. ( .1 percent )
This is probably way lower than what we can tolerate without consequence, but in this day and age, I'm not sure who to trust any more with the data.
I guess more research is in order to determine what is and isn't "safe" or "optimal" for the planet and most especially, humans.
In the meantime, I will buy a canary, and keep it close to me at all times.
If I find it dead at the bottom of it's cage, I'll start to worry.
I've never given any thought to what the human body might be able to adapt to, in partucular concerning higher/lower levels of CO2 concentrations. Would humans develop larger lungs or other mechanisms to deal with higher levels of CO2 over long periods of time? Particularly if those changes slowly occurred over very long periods of time? Maybe we're not all gonna die...