Smog is a mixture of air pollutants that form smoke and fog in the air. It is generally formed when ground level ozone, fine particles and other chemicals react on hot days. Smog can make breathing difficult and can make human beings more susceptible to cardio-respiratory diseases. People already suffering from heart or lung disease are particularly affected. The two main ingredients in smog that affect human health are ground-level ozone and fine airborne particles.
Ozone can be good: in the upper atmosphere it protects the Earth from the sun's harmful ultraviolet rays. But ground-level ozone is a colorless, highly irritating gas that forms just above the Earth's surface, produced when two primary pollutants -- nitrogen oxides and volatile organic compounds -- react in sunlight and stagnant air. Nitrogen oxides are produced primarily by burning fossil fuels, while VOCs are gases that contain carbon, usually emitted by gasoline fumes and solvents, such as those found in some paints. Sulfur dioxide is a nitrogen oxide that can be chemically transformed into acidic pollutants like sulfuric acid and sulfates.
Airborne particles, sometimes called aerosols, are microscopic particles of pollutants that can remain suspended in the air for a considerable length of time. Primary particles include windblown dust and soil, sea spray, pollen, and plant spores. But sometimes gases can condense into solids too, and when these chemical reactions occur between nitrogen oxides, sulfur dioxide, VOCs and ammonia, secondary particles are formed, including sulfates. These airborne particles are the reason smog often appears to be yellowish-brown.
The American Meteorological Society contributed to the information in the TV portion of this report.