CO2 is hardly poisonous. It is essential for life on our planet and levels as high as 5000 ppm are considered safe on our submarines and on the space station. Current atmospheric levels are around 400 ppm. Below 150 ppm, plant-life dies off on a massive scale.
What few realize, however, is that during the last Ice Age too little CO2 in the air almost eradicated mankind. That’s when much-colder water in oceans (that were 400 feet shallower than today) sucked most of the carbon dioxide from the air; half of North America, Europe and Asia were buried under mile-high glaciers that obliterated everything in their paths; and bitterly cold temperatures further retarded plant growth.
In fact, Earth’s atmosphere had only about 180 parts per million CO2, compared to today’s 400 ppm: 0.018% then versus 0.040% today.
Worrying that 400 ppm is too high is like worrying about your fuel tank overflowing when it reaches the 1/8 mark during filling.