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No, that wasn't the only issue with CO2.
Quote from Herzhog http://www.archaeology.org/1103/features/werner_herzog_chauvet_cave_forgotten_dreams.html
"We had a fairly brief period of time to film. When the researchers left in early April, I had the cave practically undisturbed for filming, but only for six days, and only four hours each day. Of course, later in the season the carbon dioxide level in the branch of the cave where you have the Panel of the Lions becomes dangerously high. In other parts of the cave there is a fairly high level of radon, and it has a cumulative effect on your lungs. So, we had to move around between toxic gases and radioactive gases."
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