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As clean air moves across Earth’s surface, it collects the products of natural events (volcanic eruptions and dust storms) and human activities (emissions from cars and smokestacks).  These potential pollutants, called primary pollutants, mix with the air in the troposphere, and may react with one another or with the basic components of air to form new pollutants, called secondary pollutants.  Eventaully they return to the earth as particles, droplets, or chemicals dissolved in precipitation. Air pollution is not new, but it has mushroomed since the Industrial Revolution.  Table 17-1 lists the major classes of pollutants found in outdoor air.  In MDCs most pollutants enter the atmosphere from the burning of fossil fuels from stationary sources (power plants) and mobile sources (motor vehicles which are responsible for 80-88% of the air pollution).  Outdoor pollution in industrialized countries comes mostly from five groups of primary pollutants:

· carbon oxides
· nitrogen oxides (mostly NO and NO2, or NOx)
· sulfur oxides 
· volatile organic compounds (mostly hydrocarbons)
· suspended particulates all produced primarily by combustion of fossil fuels.  


You may be inhaling more air pollutants indoors than outside.  As many as 150 dangerous chemicals, in concentrations 10-40 times higher than those outdoors, occur in the typical American home.  The health risks from exposure to such chemicals are magnified because people spend 70-90% of their time indoors.  In 1990 the EPA placed indoor air pollution at the top of the list of 18 sources of cancer risk.  Some important indoor air pollutants:

Source:  Air fresheners, mothball crystals
Threat:  Cancer

Source:  Dry-cleaning-fluid fumes on clothes
Threat:  Nerve disorders, damage to liver and kidneys, possible cancer

Source:  Furniture stuffing, paneling, particle board, foam insulation
Threat:  Irritation of eyes, throat, skin, and lungs; nausea; dizziness

Source:  Tobacco smoke, wood stoves
Threat:  Lung cancer

Source:  Carpets, plastic products
Threat:  Kidney and liver damage

Source:  Radioactive soil and rock surrounding foundation, water supply
Threat:  Lung cancer

Tobacco Smoke
Source:  Cigarettes
Threat:  Lung cancer, respiratory aliments, heart diseases

Methylene Chloride
Source:  Paint strippers and thinners
Threat:  Nerve disorders, diabetes

Carbon Monoxide
Source:  Faulty furnaces, unvented gas stoves and kerosene heaters, wood stoves
Threat:  Headaches, drowsiness, irregular heartbeat

Source:  Pipe insulation, vinyl ceiling and floor ties 
Threat:  Lung disease, lung cancer

Nitrogen Oxides
Source:  Unvented gas stoves and kerosene heaters, wood stoves
Threat:  Irritated lungs, children’s colds, headaches

Source:  Chlorine-treated water in hot showers
Possible threat:  Cancer


Photochemical smog is a mixture of primary and secondary pollutants that forms when some of the primary pollutants interact under the influence of sunlight.  The resulting mix of more than 100 chemicals is dominated by ozone, a highly reactive gas that harms most living organisms.  Virtually all modern cities have photochemical smog, but it is much more common in cities with sunny, warm, dry climates and lots of motor vehicles.
The hotter the day, the higher the levels of ozone and other components of  smog.  

Nitrogen oxides are key ingredients needed to form photochemical smog.  As car traffic increases in the morning, NOx levels rise and begin reacting with volatile organic compounds--mostly hydrocarbons, which are released by vehicles, gas stations, oil refineries, dry cleaners, print shops, and vegetation--in the presence of sunlight to yield photochemical smog.  On a sunny day the smog builds up to peak levels by early afternoon, irrigating people’s eyes and respiratory tracts.  People are most vulnerable between 11 AM and 4 PM.  

Thirty years ago cities like London, Chicago, and Pittsburgh burned large amounts of coal and heavy oil, which contain sulfur impurities, in power plants and factories and for space heating. During winter such cities suffered from industrial smog, consisting mostly of a mixture of sulfur dioxide, suspended droplets of sulfuric acid formed from some of the sulfur dioxide, and a variety of suspended solid particles. Today in most parts of the world coal and heavy oil are burned only in large boilers with good pollution control or with tall smokestacks, so industrial smog, sometimes called gray-air smog, is rarely a problem. 


The frequency and severity of  smog in an area depends on: climate, topography, population density, amount of industry, and fuels used. In areas with high average annual precipitation, rain and snow help cleanse the air. Winds also help sweep pollutants away and bring in fresh air but may transfer some pollutants to distant areas.  

Hills and mountains tend to reduce the flow of air in valleys below and allow pollutant level to build up at ground level. Building in cities also slow wind speed and reduce dilution and removal of pollutants.

Sometimes, however, weather conditions trap a layer of dense, cool air beneath a layer of less dense, warm air in an urban basin or valley. This is called a temperature inversion, 
a thermal inversion, or an inversion layer.  In effect, a lid of warm air covers the great region and prevents the upward-flowing air currents that would disperse pollutants from developing. These inversions usually last for only a few hours; but sometimes, when a high-pressure air mass stalls over an area, they last for several days. Then air pollutants at ground level build up to harmful and even lethal levels.  This describes the Los Angeles basin, which has 11.9 million people, 8.5 million cars, and thousands of factories, and thermal inversion at least half of the year.

To reduce local air pollution and meet government standards without having to add expensive air pollution controlled devices, coal-burning power plants, ore smelters, and industries began using smokestacks up to 1,000 feet high to spew pollutants above the inversion layer. As this practice spread in the 1960s and 1970s, pollution in downwind areas began to rise. In urban areas large quantities of NOx are released by motor vehicles.

As sulfur dioxide and nitrogen oxides are transported by prevailing winds, they form secondary pollutants such as nitric acid vapor, sulfuric acid droplets, and particles of sulfate and nitrate salts. These chemicals descend to the earth in wet form as acidic rain, snow, fog, and cloud vapor and in dry form as acidic particles, resulting in acid deposition, commonly called acid rain.  These acidic components remain in the atmosphere for only a few days, so acid deposition occurs on a regional rather than on a global basis.

Acidity of substances in water is commonly expressed in terms of pH, with pH values greater than 7 being alkaline and pH values less than 7 acidic. Natural precipitation is slightly acidic, with a pH of 5.0-5.6. However, typical rain in the eastern United States is about 10 times more acidic, with a pH of 4.3, and in some areas is 100 times more acidic, with a pH of 3-as acidic as vinegar.  And some cities and mountains downwind from cities are bathed in acid fog as acidic as lemon juice, with a pH of 2.3-about 1,000 times the acidity of normal precipitation. 


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