If we let factories pour smoke into the air, our air will become polluted. Polluted air can make our eyes sting and fill with tear. It can make paint on buildings fade and peel off. It can eat away metal. It can kill trees and hurt other plants.
Polluted air hurts almost everything it touches.
Trees, water, and air are some of our most important natural resources. We need to take care of them so they will not be spoiled and wasted. This good use of natural resources is called conservation.