Carbon monoxide (CO) is a colorless, tasteless, and odorless gas or liquid. CO is formed when incomplete combustion happens, and can cause poisoning if breathed. It also participates in chemical reactions that produce ozone, indirectly contributing to climate change.
When a combustion (burning) reaction happens, the oxygen reacts with the fuel and transforms it into carbon dioxide (CO2) and water. Incomplete combustion happens where there is not enough oxygen to burn the fuel, creating CO as a byproduct.
source: usfa.fema.gov
Car engines - Fire places
Power plants - Tobacco smoke
Volcanos - Incense
Forest fires - Boilers
Biomass burning - ...and more!