Summary:
- Pollutants include materials that are naturally occurring but are added to the atmosphere so that they are there in larger quantities than normal.
- Pollutants dirty the air, change natural processes in the atmosphere, and harm living things.
- Air pollution started to be a problem when early people burned wood for heat and cooking fires in enclosed spaces such as caves and small tents or houses.
- But the problems became more widespread as fossil fuels such as coal began to be burned during the Industrial Revolution.
- Seven of the 10 of the smoggiest cities are in California.