Check out movie “Apollo 13.” Astronauts had to literally fit square pegs into round holes to prevent death from CO2 poisoning before any of the many other threats could have killed them. There also were hundreds of folks who died of CO2 poisoning in Africa when when a volcanic lake suddenly released a lot of it. CO2 is certainly not the cause of climate change (it may be an effect of it from other causes) and isn’t poisonous at common levels, but too much, or even too little of it, is dangerous. Even too much water or O2 can kill you.
My daughter giot water poisoning when she was a baby. She almost died. She did get a brain injury.