Carbon monoxide (
CO) is a colorless, odorless, and tasteless gas that is slightly less dense than air. It is toxic to
hemoglobic animals (including humans) when encountered in concentrations above about 35
ppm, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal biological functions. In the atmosphere, it is spatially variable and short lived, having a role in the formation of ground-level
ozone.