Carbon dioxide (CO2) is toxic above 6%, at 10% it can cause suffication.
The gas is present in the air you exhale, to find out how toxic it is try this...breathe normally into a plastic bag and make sure there are no leaks. It wont take long before you start feeling bad and dizzy. Then stop, or you'll begin to hyper ventilate, and get into trouble.
100% CO2...oh dear
