5000 PPM is the absolute limit of exposure in 8 hours. 6000 PPM 24/7 you start running into health problems.
I went back and checked at 6000 ppm there are some minor health effects but its ready breathable. Physical exertion would be very difficult but over time a young healthy person could probably adapt. The highest estimates I've seen referred to in literature for the Jurassic was 5000 ppm BTW.