* Particulate matter (PM): These are small particles of solid or liquid matter suspended in the air. They can come from a variety of sources, including burning fossil fuels, industrial processes, and natural sources such as dust and pollen. PM can cause a variety of health problems, including respiratory and cardiovascular disease.

* Nitrogen oxides (NOx): These gases are produced when fossil fuels are burned. They can contribute to the formation of smog and acid rain, and they can also cause respiratory problems.

* Sulfur oxides (SOx): These gases are produced when fossil fuels are burned. They can contribute to the formation of smog and acid rain, and they can also cause respiratory problems.

* Carbon monoxide (CO): This gas is produced when fossil fuels are burned incompletely. It can cause headaches, dizziness, and even death.

* Ozone (O3): This gas is formed when sunlight reacts with NOx and VOCs. It is a major component of smog, and it can cause respiratory problems.

* Volatile organic compounds (VOCs): These are gases that are emitted from a variety of sources, including paints, solvents, and gasoline. They can contribute to the formation of smog and ozone.