Fill a 1-liter beaker with water.
Add 7 drops of red food coloring.
This makes a solution approximately 350 parts per million (the same as the carbon dioxide concentration in the atmosphere).
Can you still see the red color?
What if you double the concentration?
Carbon dioxide is not a pollutant in the traditional sense.
All green plants need it to grow.
Make a list of other things that have this good-in-small-amounts bad-in-large-amounts behavior.