When the air has some dust, soot or chemicals floating in it, people who are inside probably won't notice it. People who are outside might notice it.
People with asthma, a disease that can make it hard to breathe, and children who play outside a lot might feel a little strange. When you are active outdoors, for example, when you run and jump a lot, you breathe faster and take in more air. Any pollutants in the air go into your lungs.