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Greenhouse owners generally spike their air to 1200-1500ppm CO2. That's the compromise between best cost and best for the plants and is lower than these species evolved in.

As to humans, 400ppm causes zero problems, matter of fact, OSHA says 1000ppm for continuous exposure. NIOSH says 10,000 ppm for a 10 hour work shift.

For instance: https://inspectapedia.com/hazmat/Carbon%20_Dioxide_Exposure_...

>As to humans, 400ppm causes zero problems

You mean it causes zero known problems.

Medicine and biology are not yet advanced enough to say in most case whether putting the human body in a condition humans haven't experienced in 800,000 years is harmful. It took decades to learn that asbestos is harmful. If the harm caused by asbestos were cognitive decline rather than shortened lifespan, it probably would've taken longer, and cognitive decline is the effect that appears at the lowest concentration of all of the known adverse effects of co2. (One study found cognitive decline after 2.5 hours of exposure to 1000 ppm.)

Plants use co2 to make their bodies just as we use proteins, fats and carbohydrates to make our bodies, so I am not particularly reassured by knowing the plants thrive at very high co2 levels.

Graph of co2 levels over the last 800,000 years: http://kaltesonne.de/wp-content/uploads/2017/03/co2-1024x576...

The GP obviously doesn't mean that 1000+ ppm CO2 is toxic, they mean that the changes to the environment it would cause are less than ideal for humans to thrive.

1000ppm decreases human cognitive ability by ~20%. It's hard enough to deal with these problems with the brainpower we have.

[1] https://ehp.niehs.nih.gov/doi/10.1289/ehp.1510037

Humans in isolation can tolerate 400ppm. Offices and houses accumulate far more than outside it pushes to far higher inside, enough to breach health and safety recommendations, cause decreased performance and so on. We don't know where planetary equilibrium would settle at 400ppm as we're still emitting. Looking outside and no longer in isolation >400 seems to be causing some pretty wild changes.

I'd say that's far from zero problems.

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