Air Pollution

• particles. In industrial areas and where there is heavy vehicular traffic, these contain carbon and hydrocarbons from the incomplete combustion of fuels. The hydrocarbons include a variety of polycyclic aromatic hydrocarbons (PAHs) that have been shown to cause mutations [Link].

  Inhaled particles smaller than 2.5 µm ("PM_2.5") are taken deep into the lungs and may be deposited there. Chronic exposure to these small particles has been linked to

  • reduced lung development and function in adolescents;
  • an increased risk of asthma;
  • increases in the incidence of heart disease and lung cancer later in life;
  • a significant increase in the frequency of germline mutations in the sperm of exposed mice.

  The U.S. Environmental Protection Agency (EPA) has established an air quality standard of an annual average of no more than 15 µg of these particles in a cubic meter of air. (Los Angeles averaged 20 µg/m³ in 1999 and 2000.)

  A survey of 217 counties in the U.S. showed an association between a reduction of 10 µg PM_2.5 with an increase in life expectancy of almost 1 year. Conversely, studies in Europe showed that an increase of 10 µg PM_2.5 was associated with a decrease in life expectancy of about 1 year.

• sulfur dioxide (SO₂). These are produced from the oxidation of fuels (e.g., coal and oil) containing sulfur compounds.
• carbon monoxide (CO). Also produced from the incomplete combustion of fuels.
• volatile organic compounds (VOCs) including PAHs like benzopyrene, a notorious carcinogen). These are produced from the incomplete combustion of gasoline.
• nitrogen oxides ("NO_X". These are produced by the chemical union of O₂ and N₂ in the cylinders of internal combustion engines.
• ammonia (NH₃). The widespread use of nitrogen-containing fertilizers in agriculture has led to the release of large amounts of ammonia to the atmosphere. The ammonia combines readily with nitrogen oxides forming PM_2.5 particles of ammonium nitrate and resulting haze.

Photochemical smog

• nitrogen oxides (NO_X) oxidize
• volatile organic compounds (VOCs)

These secondary pollutants are damaging to plant life and lead to the formation of photochemical smog. PAN is primarily responsible for the eye irritation so characteristic of this type of smog.

The figure outlines representative reactions leading to the formation of photochemical smog. Radicals are atoms or molecules with unpaired electrons. They are very reactive chemically.