Rich wrote:

Now I realize this still isn't the 24/7/365 exposure you're referring
to, so I'm quite interested what the maximum exposure a human could
endure would be?

Ah, found even another reference with a guideline:

"Carbon Dioxide (CO2). The earth's atmosphere normally con-
tains 300 ppm of carbon dioxide. At considerably higher concen-
trations, it can asphyxiate people by reducing the amount of oxy-
gen present.

Manure decomposition and the normal breathing process of
animals can increase the level of carbon dioxide in confined
spaces. Typical concentrations inside ventilated buildings range
from 1,000 ppm during well-ventilated periods to 10,000 ppm dur-
ing winter. The effects of excessive concentrations of carbon
dioxide are presented in Table 3. The NIOSH maximum recommended
safe carbon dioxide concentration for workers is 5,000 ppm."
(http://www.animalgenome.org/edu/PIH/104.html)