Air Pollution


Air Pollution and Control

Air pollution and its effects are not a recent occurrence, as evidenced by the following quote:

". . . whosoever shall be found guilty of burning coal shall suffer the loss of his head."

King Edward II, circa 1300 a.d.

A Brief History

Air pollution has become a widespread problem during this century. There have been several documented episodes of air pollution in which loss of human life occurred:

  • In 1931, in the heavily industrialized area of Manchester England, over 500 persons lost their lives due to exposure to particulates and acids in the air. In 1948, in the steel mill town of Donora, Pennsylvania, about twenty persons died and several thousand became ill from the effects of airborne contaminants.

    Today, an estimated 3000 cancer deaths yearly are attributable to second hand tobacco smoke alone.

What is Air Pollution?

We earlier defined pollution as something that poses a potential risk to human health or the environment.

  • Then, air pollution is the presence, in the outdoor air or the air inside a structure, of substances which pose a potential threat to human health and/or the environment.

  • The presence of particles which decrease visibility is also now considered to be a form of air pollution as evidenced by the fact that the U.S. EPA has negotiated an agreement with a major western power company to improve the air quality in the Grand Canyon National Park area.

  • The U.S. EPA estimates that 50 percent of the U.S. population is exposed to air pollutants exceeding current federal standards.

Sources and Types of Air Pollution

Major Sources of Air Pollution

Activity Air Pollutants Created Detrimental Effect
Energy production from fossil fuels Carbon dioxide, sulfur oxides, particulates Increase in greenhouse gases, acidic precipitation
Automobiles, other transportation sources Carbon dioxide, nitrogen and sulfur oxides, products of incomplete combustion Increase in greenhouse gases, acidic precipitation
Refrigeration devices including home, commercial, and vehicles Chlorofluorocarbons Destruction of the stratospheric ozone layer
Industrial manufacturing Various depending on the industry and process, including toxic materials Destruction of the stratospheric ozone layer, toxic emissions

Types of Air Pollutants

Air pollutants can be classified by several different methods, including gaseous and particulate; conventional and toxic; conventional, acidic and toxic; or greenhouse, acidic, toxic and particulate.

  • Some pollutants may fall into more than one category. Others may not fit well within any of these categories.

    Greenhouse gases include carbon dioxide [CO2], nitrous oxide [N2O], chlorofluorocarbons, and some other organic compounds. Acidic pollutants include sulfur oxides [SOx], nitrous oxides [NOx], hydrochloric acid [HCl], and other acids.

    Toxic pollutants include carbon monoxide [CO], many organic compounds such as benzene, PCBs, dioxins, and furans, and inorganics such as lead, arsenic, beryllium, mercury, and asbestos.

  • Particulate pollutants include materials formed from combustion and mechanical processes.

Federal Regulations Relating to Air Quality and Control

Note: The first air pollution regulations were passed in England to reduce the emissions from the burning of coal. They were passed in 1273 a.d. That's right, 1273.

Air Pollution Control Act of 1955, PL 84-159

  • Established federal funding for air pollution research, federal technical assistance and training

Air Pollution Control Act Amendments of 1960, PL 87-761

  • A continuance of the APCA of 1955, and a study of human health effects caused by motor vehicle emissions

The Clean Air Act of 1963, PL 88-206

  • Matching grants to state and local government (federal share of 66 to 75 percent), increased research and training, efforts to control air pollution from federal facilities

Motor Vehicle Air Pollution Control Act of 1965, PL 89-272

  • Required automobile exhaust emission standards to be met in 1968

The Air Quality Act of 1967, PL 90-148

  • Time tables for establishment of air quality criteria for different pollutants, state or federal enforcement of air quality limits. Program was understaffed, under funded and unsuccessful.

The Clean Air Act Amendments of 1970, PL 91-604

  • The establishment of national ambient air quality standards for particulates, carbon monoxide, sulfur oxides, hydrocarbons, and others.

  • National emission standards for existing and new facilities, fines and criminal penalties for intentional violation, new stricter automobile emission standards, additional research funding

The Clean Air Amendments Act of 1977, PL 95-95

  • Continuance of the 1970 requirements, additional restrictions for "non-attainment areas."

The Clean Air Act of 1990, PL 

  • A complete revamping of the air pollution control regulations, including compliance time tables (3 to 20 years) for major noncompliance areas.

  • Tighter emission standards for vehicles, reformulated gasolines, air toxics requirements, acid rain controls, new permitting program with stiffer civil and criminal penalties

One Success Story

Twenty years ago, an important topic in the air pollution field was lead in the air.

  • Tetraethyl lead was used for years as an octane boosting additive in gasolines.

  • Lead is known to cause lower intelligence and behavior problems in children and high blood pressure in adults.

  • It has also been suspected of causing defects in developing fetuses.

Today, little is heard about lead in the atmosphere. Why? Because, atmospheric lead has been almost eliminated.

  • Beginning with the 1965 Motor Vehicle Air Pollution Control Act, the U.S. began a gradual phase out of lead as a gasoline additive.

  • Lead was completely phased out of gasoline in 1995.

Lead is a regulatory success story in the US.

  • Many other industrialized nations have also phased out lead. However, most developing nations are still using tetraethyl lead as a gasoline additive. In 1988 Mexico, a developing nation, began a program to reduce lead additives in gasoline.

    Almost certainly, in the future we will witness an increased effort among developing and underdeveloped nations to reduce their levels of pollution, including air pollutants.

The Atmosphere

Gas Concentration, % by volume
Nitrogen 78.1
Oxygen 21.0
Argon 0.9
Carbon dioxide* 3.3 x 10-2
Hydrogen 5 x 10-5
Ozone 1 x 10-6
Methane* 2 x 10-4

*The concentration of these gases has been significantly modified by human activity over the last 100 years.

Acidic Pollutants

Acidic pollutants reduce the pH of natural precipitation or consume alkalinity. Acid precipitation was recognized as early as 1852 by Robert Smith in Manchester, England, where coal was used extensively.

  • The acidity also increases the leaching of metals bound in the soil.

  • Adding to the problem in some cases are toxic metals present in the precipitation which originated at the pollution source.

And, not all acidity reaches the surface in precipitation.

  • Acidic particles can contact surfaces and become dry deposition. Up to 40 percent of the total acidity reaching the surface can be deposited this way.

  • Thus acid deposition would be a better term than acid rain or acid precipitation although the latter terms are more common.

Effects of Acidic Precipitation

  • It consumes alkalinity in natural wasters, eventually depressing the pH of the water. Most aquatic life are very sensitive to pH changes. In some cases, the acid precipitation causes the release of metals previously bound in the soil. Aluminum is thought to be a major factor in acid precipitation and aquatic toxicity.

    Acid precipitation is corroding many structures (bridges, buildings, etc.) at a rate several times previous levels

  • Acid precipitation is causing the destruction of sculptures and paintings at an alarming rate

The Extent of the Problem

  • A significant portion of the soils and lakes in the northeastern United States and southeastern Canada have already been affected by acidic precipitation.

  • Acidic precipitation is primarily the result of pollutant discharges from coal fired power plants such as those in the Ohio Valley of the mid-west.

  • Emissions from motor vehicles exacerbates the problem.

  • The mid-west power plant emissions are transported by wind to the northeastern U.S. and southeastern Canada.

  • Other regions of the U.S. are in imminent danger from acidic precipitation.

    These areas include the northern Minnesota area, as well as the area surrounding several major metropolitan areas.

    A large portion of central and northern Europe have already been damaged from acidic precipitation, including much of the Scandinavian states, as well as portions of Switzerland and Germany.

    • Depressed pH levels in precipitation are recorded in central Europe from the 1870's.
    • In recent years, the pH of precipitation in much of Europe has been 4.0 to 4.5.

  • Acid precipitation varies in strength across the nation, and the world.

    • Due to long atmospheric residence times, these pollutants are often deposited in locations hundreds of miles from their emission sources.

    • The Ohio River valley is an industrialized area that contributes significant amounts of acidic precipitation to the northeastern U.S. and southeastern Canada, literally hundreds of miles from their source.

Controlling Acid Emissions

  • Convert from high sulfur coal to low sulfur coal

  • Use nuclear power

    Recent changes in the regulatory process allow for faster permitting of nuclear power plants

    • There is little chance that nuclear power will supply a greater fraction of power in the foreseeable future
    • Liability risk, technical/operational problems, and environmental opposition are strong reasons not to build additional nuclear power plants
  • Wet scrubber

    A caustic spray neutralizes the acidity

    This produces a sludge which must be landfilled or otherwise disposed of

  • Fluidized bed combustion

    The coal is combusted in a bed of limestone

    The limestone neutralizes the acidity as it is produced

Particulate Pollutants

Fine particle pollution is smaller than 1 micron

  • Most are created by condensation processes following combustion. Examples include

    coal power plant smoke

    tobacco smoke

    home fire places

    forest fires

  • This type of particulate is by far the most harmful because the small particles penetrate into the alveoli of the lungs before depositing

Large particle pollution is greater than 1 micron

  • Created by mechanical processes such as

    industrial machining operations

    sanding or other mechanical surface preparations for wood, steel

Effects of particulate pollutants

  • Small particles pass through the nasal area into the lungs. There is a much greater risk of deposition in the lungs for these small particles

  • Removal of Particulate Pollutants

    The bag house filter is common for removal of particulates

    • Its operating principle is similar to a home vacuum cleaner

  • Electrostatic precipitator

    Small particles are statically charged

    The electrically charged metal plates attract the particles, removing them

  • Wet scrubber, discussed earlier

Green House Gases

Greenhouse gases are those gases which impede the exit of reflected solar energy.

Greenhouse Gases Include:



Fraction of trapped energy attributable to gas

Annual increase in gas conc., percent

Carbon dioxide Fossil fuel combustion 0.66 0.5
Chlorofluorocarbons Vehicle, residential and commercial refrigeration systems, foams, aerosol propellants 0.10 4
Methane Cattle, rice paddies 0.20 0.9
Nitrous oxide Combustion processes 0.04 0.25

Carbon dioxide

  • Carbon dioxide is responsible for two-thirds of the additional solar radiation trapped in the atmosphere. And, the atmospheric concentration of carbon dioxide is increasing at a rate of 0.5 percent per year.

  • Although the atmospheric life of CO2 is not well established, it is probably on the order of 50 to 100 years.

Chlorofluorocarbons (CFCs)

  • Where CFCs are only responsible for ten percent of the excess solar energy trapped by the atmosphere, they are increasing at a rate of four percent per year.

  • Even worse, CFCs have estimated atmospheric life times of 50 to 400 years.

  • Thus, even if we stopped burning fossil fuels and stopped producing CFCs today, we could expect those already in the atmosphere to affect the earth's temperature for at least the next one or two centuries.

Effects of CFCs

  • Estimates of how great these greenhouse gases will affect the earth vary.

  • Temperature records are not of sufficient accuracy and number prior to the late 1800's to assist in a global temperature record.

  • However, since that period, there has been a gradual warming of the earth. The global average temperature has probably increased 0.3 to 0.6C since about 1860.

    There are strong indications that the global temperature will increase more rapidly in the 21st century.

  • The average global temperature is expected to increase 2 to 4C (4 to 9F) during the next century. This corresponds roughly to a doubling of the atmospheric CO2 level.

  • Human activity has already caused a 25 percent increase in global atmospheric CO2. This increase in global temperature will have far reaching effects on the planet.

    Although the exact effects of global warming are not certain, experts believe that significant changes in the earth's weather patterns are possible, in conjunction with alterations in the ocean currents.

  • It has been predicted that increased CO2 levels will also result in an increase in global precipitation.

  • The increased temperature will result in greater evaporation rates over the oceans. However, some areas may still have reduced rainfall amounts.

Atmospheric Ozone Destruction

The stratosphere is the layer of atmosphere above the troposphere, from 12 to 70 km high.

  • In this region, and above, reactions involving atmospheric chemicals, pollutants, and light occur.

  • The oxygen in the upper atmosphere, above approximately 150 km, absorbs damaging short wave—high energy radiation protecting life on earth

  • In the upper atmosphere, sunlight provides energy that breaks apart the molecular oxygen, O2, in the atmosphere. The separate atomic oxygen, O, then combines with molecular oxygen forming ozone, O3. This ozone absorbs ultraviolet radiation emitted by the sun. It protects us.

Chlorofluorocarbons (CFCs) Destroy Ozone

  • These are the same chemicals implicated in global warming. When emitted to the atmosphere CFCs migrate to the upper layers.

    The CFCs are eventually broken down, releasing their chlorine.

    The chlorine then reacts with ozone destroying it.

  • The world first used CFCs in the 1930s.

    Global atmospheric CFC concentrations did not increase substantially until the 1960s. Scientists became concerned about their impact on the ozone layer in the 1970s. Then, in 1985 scientists detected a "hole" in the ozone layer over the Antarctic region. More recently, a similar hole was detected over the arctic region.

Control of CFC Emissions

  • CFC emissions can be controlled mainly by finding and using alternative chemicals for refrigeration and air conditioning, foam blowing agents, and solvent degreasing.

  • And, by developing better alternative refrigeration processes which do not require refrigerant gases similar to CFCs.

  • This process is being forced by a variety of national laws world wide, and by the Montreal Protocol, an international agreement signed in Montreal Canada in 1987. In essence, the Montreal Protocol requires the phaseout of the most dangerous CFCs by 1997 for developed countries, and by 2007 for developing nations.

    This agreement should lead to substantial reductions in global CFC emissions. This phaseout of CFCs will result in an improvement in stratospheric ozone.

    However, because CFCs have atmospheric lifetimes of 60 to 400 years, near term expectations are poor. Further reductions in US CFC emissions, in excess of the Montreal Protocol, are required by the 1990 Clean Air Act Amendments. But, with CFCs implicated in both greenhouse warming and ozone depletion, former President George Bush issued a presidential directive banning production of CFCs by 1996, faster than either the Montreal Protocol or the Clean Air Act Amendments.

    Elimination of CFC emissions in the U.S. is important because the United States is responsible for approximately one-quarter to one-third of world CFC emissions.

  • In an effort to accomplish the goal of CFC elimination, many industries have joined a global effort to find acceptable substitutes to CFCs.

    In the short term the substitute may be hydrofluorocarbons (HFCs). However, HFCs also damage the ozone layer, but have much shorter atmospheric lives than CFCs.

    Thus, although not ideal, they are much better than CFCs.

    In the longer term, it is expected that other substitute chemicals and possibly different refrigeration processes, will be developed which have zero ozone depleting potential, and are not greenhouse gases.

In Summary

Since the 1970s we have improved much in air pollution reduction and control. We understand air emissions, their sources, and their effects much better now than then. We are doing much better in many air pollution aspects, but much improvement is still needed

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Last revised 7/28/97