CO is produced by incomplete combustion due to lack of oxygen: petrol engines, heating systems, wildfires. It is odourless and colourless, making it especially dangerous indoors. It binds to haemoglobin 200× more strongly than O₂.
Outdoors, highest levels occur in dense traffic zones and during temperature inversions. CO's atmospheric lifetime is 1–2 months. Satellites MOPITT and Sentinel-5P monitor global CO, revealing wildfire emissions and industrial pollution.