You know CO2 can kill also right? Google CO2 poisoning aquariums to see how fast. By the time there's too much, it's too late to do anything about it because everything is dead.

• 1%, as can occur in a crowded auditorium with poor ventilation, can cause drowsiness with prolonged exposure.
• At 2% it is mildly narcotic and causes increased blood pressure and pulse rate, and causes reduced hearing.
• At about 5% it causes stimulation of the respiratory center, dizziness, confusion and difficulty in breathing accompanied by headache and shortness of breath. In addition at this concentration panic attacks may occur.
• At about 8% it causes headache, sweating, dim vision, tremor and loss of consciousness after exposure for between five and ten minutes.