Most people in the eastern side of the U.S. use a house furnace to heat their house up. But what they don't realize is that in a house furnace, there is fire. Fire lets off a smoke which causes pollution to get into the air. Once pollution is in the air, it forms the killer smog.
Another thing is a gas-car. The gas-car uses an engine to power the car. But to power the engine, you need gas. "What does gas do after it gets absorbed and used by the engine?" you ask. Well after the gas gets used by the engine, the engine turns the gas into electricity leaving a liquid water behind. Then the water gets evaporated into a smoke that comes out as pollution which makes the killer smog.