Carbon dioxide(CO2) is not poisonous. We know this since when we breathe in, a whole cocktail of gases enters our lungs, such as oxygen, nitrogen, methane and carbon dioxide. The cells in our body diffuse carbon dioxide into our blood which is then transported back to the aveoli in our lungs, which then diffuses into the aveoli themselves and we breathe it out.
 
 However, if we lived in an atmosphere where only carbon dioxide was present, we would die, since our cells have adapted to using oxygen in order to respire and function.
 
 Hope this helps!!