Care to elaborate? How exactly would widespread atmospheric concentrations of +/- 800ppm carbon dioxide NOT be harmful for overall human health in the long-term?
Because that's normal indoor concentration and we would have known if people died from it. A lot of people are exposed to such concentrations long-term and they survive fine. While people spending nearly all their time indoors would benefit from more fresh air, it is for more important reasons than actually the air.
Below 1000 ppm people will be just fine. Above that cognitive abilities begin to drop, like if you're in a crowded room when concentrations might reach a few thousand, but even if you're stuck in a room like that for life, it's not that which is going to kill you.
Cognitive and physical abilities also drop if you live 2000 masl, but you don't die from it, and even though you would perform better at sea level, it would be a stretch to say that living at 2000 masl is harmful.