Question

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

Explanation:

Smog is an outdoor pollutant formed from vehicle exhaust and industrial emissions. Vehicle exhaust is mainly from cars outside. Oxygen is not a pollutant. Natural gas can be a common indoor air pollutant, for example, if there is a leak in a gas - powered appliance like a stove or heater.

Answer:

natural gas