Question

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

Explanation:

Smog is an outdoor pollutant formed by the reaction of pollutants in the presence of sunlight. Vehicle exhaust is also an outdoor pollutant. Oxygen is not a pollutant. Natural gas can be a common indoor air pollutant as it can leak from appliances and release methane and other compounds.

Answer:

natural gas