Of course the EPA must regulate CO2. Just look at the dangers of Hypercapnia.
At 1% concentration of carbon dioxide CO2 (10,000 parts per million or ppm) and under continuous exposure at that level, such as in an auditorium filled with occupants and poor fresh air ventilation, some occupants are likely to feel drowsy. Carbon dioxide concentration must be over t 2% (20,000 ppm) before most people are aware of its presence unless the odor of an associated material (auto exhaust or fermenting yeast, for instance) is present at lower concentrations. Above 2%, carbon dioxide may cause a feeling of heaviness in the chest and/or more frequent and deeper respirations. If exposure continues at that level for several hours, minimal “acidosis” (an acid condition of the blood) may occur but more often is absent. Breathing rate doubles at 3% CO2 and is four times the normal rate at 5% CO2. Toxic levels of carbon dioxide: at levels above 5%, concentration CO2 is directly toxic. [At lower levels we may see the effects of a reduction in the relative amount of oxygen and not direct toxicity of CO2.]
So I guess the EPA should monitor the CO2 levels, and when they reach 10000 ppm they will have to regulate it. (I am only half-way kidding, indoor concentrations can reach these levels).