The source of carbon(II)oxide that acts as air pollutant is

Carbon(II) oxide is the IUPAC-style name for carbon monoxide (CO). It is a colourless, odourless, and highly toxic gas that is a major air pollutant, especially in urban areas.

The primary source of carbon monoxide as an air pollutant is the incomplete combustion of carbon-containing fuels. When fuels such as petrol, diesel, kerosene, coal, or wood burn with an insufficient supply of oxygen, carbon is only partially oxidised to CO instead of fully oxidised to CO₂:

\[ 2\text{C} + \text{O}_2 \rightarrow 2\text{CO} \]

Vehicle exhaust emissions are the single largest contributor of CO to the atmosphere, along with industrial furnaces and domestic cooking fires that operate under oxygen-poor conditions.

Respiration produces carbon dioxide (CO₂), not carbon monoxide. Photochemical smog is a secondary pollution phenomenon caused by sunlight acting on nitrogen oxides and volatile organic compounds; it is not a source of CO itself. Decomposition of sewage releases gases such as methane (CH₄) and hydrogen sulphide (H₂S), not carbon monoxide.