It's just a buzz word to scare non-thinking sheeple.
C02 is a good thing for plant growth. Major greenhouse pump it into their greenhouse to help the plants grow.
Too much could be bad, but looking at numbers, we are a long way off. Current world levels are 417`ish parts per million (ppm). It would need to be 40,000 ppm or so to be immediately toxic to humans.
At 200 ppm, plants stop growing, at 150 ppm, plants start to die.
Greenhouses and grow-ops shoot for 1200 to 1500ppm. At that rate, C02 has been called magic gas, because it helps plants grow so well.
So at 417 ppm currently world-wide, we are closer to all plant life dying (150ppm) than we are to plants growing like crazy (1500ppm) and even if we could reach 1500 ppm, we'd still be 1,000s of ppms away from toxic levels for humans.