Evolution

• Air pollution is mainly a modern problem. Up until a century ago, the major cause of air pollution was sulfur dioxide, caused by volcanic eruptions. Since the industrial age, however, man-made pollutants have become the main problem. This is especially due to the combustion of fuels, the widespread use of toxic metals (especially lead) and the production of chemical and industrial products.
  
  

Effects

• Cardiopulmonary disease kills over a million people each year in the U.S. alone. Of those cases, almost a 100 percent are somewhat related to air pollution and the effects of the environment. This is especially true in children, which have reported increased levels of asthma attacks, and the elderly, who frequently show signs of toxins accumulation in their lungs and throat. Air pollution also increases the number and severity of bronchitis and respiratory allergy cases, as well as worsening cardiac conditions and emphysema.
  
  

Identification

• Air pollution can be invisible, as many of the gases are odorless and colorless, caused by issues such as radioactive decay or toxic metals. However, most places with serious pollution problems reached that position due to the heavy emission of nitrogen oxides, which produces the hazy, grayish look that has become associated with large cities.
  
  

Geography

• When it comes to air pollution in cities, China definitively holds the worst spots. In fact, 16 of the top 20 most polluted cities in the world are in China, with Linfen, in the Shanxi province, taking the first place. Several cities in Russia (and former USSR countries) also make the list, with Chernobyl and Sumgayit taking prominent place. In the U.S., Pittsburgh, Pennsylvania takes the lead, followed by several Californian cities.
  
  

Type

• There are several types of air pollutants, some of which people are barely aware of. The most common is smog, usually a concentration of carbon dioxide, mehane and chlorofluorocarbons (CFCs). All three pollutants, known as greenhouse gases, are caused directly by human activity, including the burning of fossil fuels, the use of refrigerants and the overuse of aerosol propellants. Sulfur dioxide is another common air pollutant. It is mainly brought into the air by various industrial processes, including substance discoloration, refrigeration and as a solvent.