The main gaseous air pollutants that injure trees are sulfur dioxide, fluorides, and oxidants.
Sulfur dioxide comes mostly from burning coal and oil to generate electricity.
It also is produced in smelting ores that contain sulfur and in manufacturing sulfur products. Fluorides are produced by processing aluminum ore, manufacturing phosphate fertilizer and processing stone. The oxidants are formed in the atmosphere from chemical reactions powered by sunlight. They are produced mostly from industrial and auto emissions, and they are common components of smog. Low concentrations of ozone also occur naturally in the atmosphere.