Vehicle Emissions
Vehicle emissions are the gases and particles that come out of a car's exhaust pipe.
Think of it this way
Blow through a straw into a jar of clear water. Small bits and gas mix into the water. The water turns cloudy fast. That is what comes out of a pipe into the air.
Explaining vehicle emissions by grade level
Cars burn fuel to run. When fuel burns, smoke comes out the back. More cars on the road means more smoke in the air. That smoke can make the air dirty.