Carbon dioxide (CO2) can be poisonous at very high concentrations.
If you’ve seen the movie Apollo 13, you know that one of the crises the NASA engineers had to overcome was devising a carbon dioxide filter to keep the astronauts from suffering CO2 poisoning from their own breathing in the tight quarters of the lunar module ‘lifeboat’. Back here on earth, the principal concern is the effect of CO2 as a greenhouse gas and the contribution of rising CO2 in the atmosphere to climate change.
Before the Industrial Revolution, atmospheric CO2 levels were less than 300 parts per million (ppm). Today, CO2 levels are around 390 ppm and rising. While this may be cause for concern in terms of potential climate effects, the level is far below that which can cause direct toxicity. As a point of comparison, the Occupational Health and Safety Administration (OSHA) Permissible Exposure Limit of CO2 is 5,000 ppm. In other words, current CO2 levels are less than 1/10th the OSHA limit.