Question

which material is a common indoor air pollutant?
smog
natural gas
vehicle exhaust
oxygen

Explanation:

Smog is mainly an outdoor pollutant formed from vehicle exhaust and industrial emissions in the atmosphere. Vehicle exhaust is also predominantly an outdoor pollutant from vehicles on roads. Oxygen is a natural, non - pollutant gas that is essential for life. Natural gas can be used indoors (e.g., in gas stoves) and if there is incomplete combustion or a leak, it can be an indoor air pollutant.

Answer:

natural gas