Why is carbon monoxide an air pollutant?
It's dangerous to us and
Carbon monoxide (CO) is basically the result of incomplete combustion, or depriving a combustion (fire) of oxygen before it is complete. If you put a cup over a candle, it's going to release carbon monoxide for this reason.
The reason it's an air pollutant is because it's incredibly dangerous to us, as well as other animals. It's odorless, so we can't detect it without smoke detectors, but it binds to our hemoglobin, preventing it from delivering oxygen we breathe to our body. If you breathe too much CO, you'll suffocate. Air pollution can be defined as any airborne substance at a level that is dangerous to life, so this would definitely fall into that category.
It's also a secondary air pollutant because it can react with other air pollutants to create ozone (O3). While ozone is awesome in the stratosphere where it's needed to block UV-B rays, it kills chlorophyll in plants and irritates our eyes and lungs, and can cause bronchitis.