Chapter 18 -- Air Pollution
	After studying this chapter, you should be able to: describe the major categories and sources of air pollution. distinguish between conventional or "criteria" pollutants and unconventional types as well as explain why each is important. analyze the origins and dangers of some indoor air pollutants. relate why atmospheric temperature inversions occur and how they affect air quality. evaluate the dangers of stratospheric ozone depletion and radon in indoor air. understand how air pollution damages human health, vegetation, and building materials. compare different approaches to air pollution control and report on clean air legislation. judge how air quality around the world has improved or degraded in recent years and suggest what we might do about problem areas.

The Air Around Us

• Smoke, haze, dust, odors, corrosive gases, noise, and toxic compounds are present nearly everywhere, even in the most remote, pristine wilderness.
• Air pollution is generally the most widespread and obvious kind of environmental damage.
• Over the past twenty years, air quality has improved appreciably in most cities in Western Europe, North America and Japan.
• Air quality in the developing world has been getting much worse.

Natural Sources of Air Pollution

• There are many natural sources of air quality degradation.
  • Natural fires release smoke.
  • Volcanoes spew out ash, acid mists, hydrogen sulfide, and other toxic gases.
  • Sea spray and decaying vegetation are major sources of reactive sulfur compounds in the air.
  • Trees and bushes emit millions of tons of volatile organic compounds.
  • Pollen, spores, viruses, bacteria, and other small bits of organic material are present in the air.
  • Bacterial metabolism of decaying vegetation in swamps and of cellulose in the guts of termites and ruminant animals is responsible for large methane releases.

Human-Caused Air Pollution

Primary and Secondary Pollutants

• Primary pollutants: those released directly from the source into the air in a harmful form.
• Secondary pollutants: modified to a hazardous form after they enter the air or are formed by chemical reactions as components of the air mix and interact.
  • Solar radiation often provides the energy for these reactions.
• Fugitive emissions: those that do not go through a smokestack (e.g. dust from soil erosion, strip mining, rock crushing, and building construction).

Conventional or Criteria Pollutants

• The U.S. Clean Air Act of 1970 designated seven major pollutants for which maximum ambient air levels are mandated.

• These seven conventional or criteria pollutants contribute the largest volume of air-quality degradation.

Table 18.1 -- Estimated fluxes of pollutants and trace gases to the atmosphere
Species	Sources	Approximate Annual Flux (millions of metric tons/year)
		Natural	Anthropogenic
CO2 (carbon dioxide)	Respiration, fossil fuel burning, land clearing, industrial processes	370,000	23,900*
CH4 (methane)	Rice paddies and wetlands, gas drilling, landfills, animals, termites	155	350
CO (carbon monoxide)	Incomplete combustion, CH4 oxidation, biomass burning, plant metabolism	1,580	930
NMHC (nonmethane hydrocarbons)	Fossil fuels, industrial uses, plant isoprenes and other biogenics	860	92
NOx (nitrogen oxides)	Fossil fuel burning, lightning, biomass burning, soil microbes	90	140
SOx (sulfur oxides)	Fossil fuel burning, industry, biomass burning, volcanoes, oceans	35	79
SPM (suspended particulate materials)	Biomass burning, dust, sea salt, biogenic aerosols, gas to particle conversion	583	362
*About half of is this is taken up again by plant photosynthesis, while the rest accumulates in the atmosphere. Source: Data from Joyce E. Penner, "Atmospheric Chemistry and Air Quality" in W.B. Meyer and B.L. Turner (eds.), Changes in Land Use and Land Cover: A Global Perspective, 1994. Cambridge University Press. And UNEP 1999.

• Sulfur compounds
  • Natural sources: evaporation of sea spray, erosion of sulfate-containing dust from arid soils, fumes from volcanoes and fumaroles, and biogenic emissions of hydrogen sulfide and organic sulfur-containing compounds.

  • The predominant form of anthropogenic sulfur is sulfur dioxide from combustion of sulfur-containing fuel.
    • Sulfur dioxide is a colorless corrosive gas that is directly damaging to both plants and animals.
    • Can be oxidized to sulfur trioxide, which reacts with water vapor or dissolves in water droplets to form sulfuric acid (major component of acid rain).

  

• Nitrogen compounds
  • Nitrogen oxides: highly reactive gases formed when nitrogen in fuel or combustion air is heated to temperatures above 650deg.C in the presence of oxygen, or when bacteria in soil or water oxidize nitrogen-containing compounds.
  • Nitrogen oxides combine with water to make nitric acid, which is a major component of atmospheric acidification.
  • Excess nitrogen also causes fertilization and eutrophication of inland waters and coastal seas.

• Carbon Oxides
  • Carbon dioxide (CO₂) is the predominant form of carbon in the air.
  • Usually considered nontoxic and innocuous, increasing levels of carbon dioxide appears to be causing a global climate warming.
  • Burning of fossil fuels is estimated to add between 5 and 5.5 billion tons of carbon to the atmosphere each year.
  • Uncertainty exists about where the extra carbons goes.
  • Carbon monoxide: colorless, odorless, nonirritating but highly toxic gas.
  • About 90 percent of the carbon monoxide in the air is consumed in photochemical reactions that produce ozone.

• Metals and Halogens
  • Many toxic metals are mined and used in manufacturing processes or occur as trace elements in fuels, especially coal.
  • Lead
    • Worldwide lead emissions amount to about 2 million metric tons per year, or two-thirds of all metallic pollution.
    • Most lead is from leaded gasoline.
    • An estimated 20 percent of all inner-city children suffer some degree of mental retardation from high environmental lead levels.
  • Mercury
    • Two largest sources of atmospheric mercury appear to be coal-burning power plants and waste incinerators.
  • Other toxic metals of concern are nickel, beryllium, cadmium, thallium, uranium, cesium, and plutonium.
  • Halogens (fluorine, chlorine, bromine, and iodine) are highly reactive and generally toxic in their elemental form.
  • About 600 million tons of highly persistent chlorofluorocarbons (CFCs) are used annually worldwide in spray propellants, refrigeration compressors, and for foam blowing.
    • CFCs diffuse into the stratosphere where they release chlorine and fluorine atoms that destroy the ozone shield that protects the earth from U.V. radiation.

• Particulate material
  • Particulate material: all atmospheric aerosols, whether solid or liquid.
  • Includes dust, ash, soot, lint, smoke, pollen, spores, algal cells, and many other suspended materials.
  • Particulates often are the most apparent form of air pollution since they reduce visibility and leave dirty deposits on windows, painted surfaces, and textiles.
  • Respirable particles smaller than 2.5 micrometers are among the most dangerous of this group because they can be drawn into the lungs.

  

• Volatile organic compounds
  • Volatile organic compounds (VOCs): organic chemicals that exist as gases in the air.
  • Plants are the largest source of VOCs.
  • A large number of other synthetic organic chemicals, such as benzene, toluene, formaldehyde, vinyl chloride, phenols, chloroform, and trichloroethylene, are released into the air by human activities.
    • These chemicals play an important role in the formation of photochemical oxidants.
  • Of the 188 air toxics listed in the Clean Air Act, about two-thirds are VOCs and most of the rest are metal compounds.
  • EPA has identified 33 chemical compounds considered to be the greatest threat to public health in urban areas.

Table 18.2 -- Urban air toxics of greatest concern
Acetaldehyde	Coke oven emissions	Manganese compounds
Acrolein	Dioxins	Mercury compounds
Acrylonitrile	1,2-dibromoethane	Methylene chloride
Arsenic compounds	1,3-dichloropropane	Nickel compounds
Benzene	Propylene dichloride	Polychlorinated biphenyls
Beryllium compounds	Ethylene dichloride	Polycyclic organic matter
1,3-butadiene	Ethylene oxide	Quinoline
Cadmium compounds	Formaldehyde	1,1,2,2-tetrachlorethane
Carbon tetrachloride	Hexachlorobenzene	Tetrachloroethylene (perchloroethylene)
Chloroform	Hydrazine	Trichloroethylene
Chromium compounds	Lead compounds	Vinyl chloride
Source: U.S. EPA 1999

• Photochemical oxidants

  

  • Photochemical oxidants: products of secondary atmospheric reactions driven by solar energy.

  • One of the most important reactions involves formation of singlet (atomic) oxygen by splitting nitrogen dioxide (NO₂).

  • Then the atmoic oxygen reacts with another molecule of O₂ to make ozone (O₃).

  • Ozone formed in the stratosphere provides a valuable shield for the biosphere by absorbing incoming ultraviolet radiation.

  • In ambient air, however, O₃ is a strong oxidizing reagent and damages vegetation, building materials, and sensitive tissues.

Unconventional Pollutants

• EPA has authority under the Clean Air Act to set emission standards (regulating the amount released) for certain unconventional or noncriteria pollutants that are considered especially hazardous or toxic.
• Examples of these unconventional pollutants include asbestos, benzene, beryllium, mercury, polychlorinated biphenyls, and vinyl chloride.
• Aesthetic degradation: includes any undesireable changes in the physical characteristics or chemistry of the atmosphere (e.g. noise, ordors and light pollution).

Indoor Air Pollution

• The EPA has found that indoor concentrations of toxic air pollutants are often higher than outdoors.
• People generally spend more time inside than out and therefore are exposed to higher doses of these pollutants.
• Smoking is the most important air pollutant in the United States in terms of human health.
• In some cases, indoor air in homes has concentrations of chemicals that would be illegal outside or in the workplace.
• "Green design" principles can make indoor spaces both healthier and more pleasant.
• In less-developed countries of Africa, Asia, and Latin America where such organic fuels as firewood, charcoal, dried dung, and agricultural wastes make up the majority of household energy, smoky, poorly ventilated heating and cooking fires represent the greatest source of indoor air pollution.

Climate, Topography, and Atmospheric Processes

• Topography, climate, and physical processes in the atmosphere play an important role in transport, concentration, dispersal, and removal of many air pollutants.
• Wind speed, mixing between air layers, precipitation, and atmospheric chemistry all determine whether pollutants will remain in the locality where they are produced or will go elsewhere.

Inversions

• Temperature inversions: occur when a stable layer of warmer air overlays cooler air, reversing the normal temperature decline with increasing height and preventing convection currents from dispersing pollutants.

• Several mechanisms create inversions.
  • Cold front slides under an adjacent warmer air mass or when cool air subsides down a mountain slope to displace warmer air in the valley.

  • Rapid nightime cooling in a valley or basin where air movement is restricted.
    • The cool air slides in under contaminated air, squeezing it up against the cap of warmer air above and concentrated the pollutants accumulated during the day.

Dust Domes and Heat Islands

• Tall buildings in large cities create convective updrafts that sweep pollutants into the air.
• Temperatures in the center of large cities are frequently 3deg. to 5deg.C higher than surrounding countryside.
• Stable air masses created by this "heat island" over the city concentrate pollutants in a "dust dome".

Long-Range Transport

• Fine aerosols and industrial pollutants can be carried great distances by the wind.
• Some of the most toxic and corrosive materials delivered by long-range transport are secondary pollutants, produced by the mixing and interaction of atmospheric contaminants as they travel through the air.
• Somoa, Greenland, and even Antactica and the North Pole, all have heavy metals, pesticides, and radioactive elements in their air.
• The Inuit people of Broughton Island, well above the Arctic Circle, have higher levels of polychlorinated biphenyls in their blood than any other known population, except victims of industrial accidents.

Stratospheric Ozone

• In 1985, a distrubing discovery was announced: ozone levels in the stratosphere over the South Pole were dropping precipitously during September and October every year as the sun reappears at the end of the long polar winter.

• Why are we worried about stratospheric ozone?
  • In the upper atmosphere, where it screens out dangerous U.V. rays from the sun, ozone is an irreplaceable resource.

  

• Exceptionally cold temperatures in Antactica play a role in ozone losses.

• Humans release a variety of chlorine-containing molecules into the atmosphere (e.g. chlorofluorocarbons and halon gases).
  • Because these molecules are so stable, they persist for decades or even centuries once released.
  • When they diffuse out into the stratosphere, the intense U.V. irradiation releases chlorine atoms that destroy ozone.
  • At a 1989 conference, eighty-one nations agreed to phase out CFC production by the end of the century.
  • Alternatives to CFCs exist including hydrochlorofluorocarbons (HCFCs) which release much less chlorine per molecule.
  • CFC production in industrialized countries has fallen nearly 80 percent since 1989.

Effects of Air Pollution

Human Health

• Heart attacks, respiratory diseases, and lung cancer all are significantly higher in people who breathe dirty air, compared to matching groups in cleaner environments.
• Conditions are often much worse in other countries than Canada or the United States.
• The United Nations estimates that at least 1.3 billion people around the world live in areas where air is dangerously polluted.
• The most common route of exposure to air pollutants is by inhalation, but direct absorption through the skin or contamination of food and water are also important pathways.
• Because they are strong oxidizing agents, sulfates, SO₂, NO_x, and O₃ act as irritants that damage delicate tissues in the eyes and respiratory passages.
• Carbon monoxide binds to hemoglobin and decreases the ability of red blood cells to carry oxygen.
• Some important chronic health effects of air pollutants include bronchitis and emphysema.
  • Bronchitis: persistent inflammation of bronchi and bronchioles (large and small airways in the lung) that cause a painful cough and involuntary muscle spasms that constrict airways.
  • Emphysema: an irreversible obstructive lung disease in which airways become permanently constricted and alveoli are damaged or even destroyed.
• Half of all lungs examined at autopsy in the United States have some degree of alveolar deterioration.
• Smoking is undoubtedly the largest cause of obstructive lung disease and preventable death in the world.

Plant Pathology

• In the early days of industrialization, fumes from furnaces, smelters, refineries, and chemical plants often destroyed vegetation and created desolate, barren landscapes around mining and manufacturing centers.
  • Copper-nickel smelter at Sudbury, Ontario, is a notorious example of air pollution effects on vegetation and ecosystems.
• There are two probable ways that air pollutants damage plants.
  • They can be directly toxic, damaging sensitive cell membranes much as irritants do in human lungs.
  • They can act as metabolic regulators or plant hormones and disrupt normal patterns of growth and development.
• Synergistic effects: effects caused following exposure to two factors which together is more than the sum of exposure to each factor individually.
• Pollutant levels too low to produce visible symptoms of damage may still have important effects.

Acid Deposition

• Acid precipitation: the deposition of wet acidic solutions or dry acidic particles from the air.

• By the 1940's, it was known that pollutants, including atmospheric acids, could be transported long distances by wind currents.

• pH and atmospheric acidity
  • acidity is described in terms of pH (the negative logarithm of the hydrogen ion concentration in a solution).
  • pH scale ranges from 0 to 14 with 7, the midpoint, being neutral.
  • Values less than 7 indicate progressively greater acidity, while above 7 are progressively more alkaline.
  • Normal, unpolluted rain generally has a pH of about 5.6 due to carbonic acid created by CO₂ in the air.

• Aquatic effects
  • Generally, reproduction is the most sensitive stage in fish life cycles.
  • Eggs and fry of many species are killed when the pH drops to about 5.0.
  • This level of acidification (pH 5.0) can also disrupt the food chain by killing aquatic plants, insects, and invertebrates on which fish depend for food.
  • There are several ways acids kill fish.
    • Alters body chemistry
    • Destroys kills and prevents oxygen uptake
    • Causes bone decalcification
    • Disrupts muscle contraction.
  • Acid water leaches toxic metals, such as mercury and aluminum, out of soil and rocks.
  • Studies in the Adirondack Mountains of New York revealed that about half of the high altitude lakes are acidified and have no fish.
  • Much of the western United States has relatively alkaline bedrock and carbonate-rich soil, which counterbalance acids from the atmosphere.
  • Sulfates account for about two-thirds of the acid deposition in eastern North America and most of Europe, while nitrates contribute most of the remaining one-third.

• Forest damage
  • In the early 1980s, disturbing reports appeared of rapid forest declines in both Europe and North America.
  • A 1980 survey on Camel's Hump Mountain in Vermont showed that seedling production, tree density, and viability of spruce-fir forests at high elevations had declined about 50 percent in 15 years.
    • By 1990, almost all the red spruce, once the dominant species on the upper part of the mountain, were dead or dying.
  • European forests also are dying at an alarming rate.
    • In 1982, German foresters estimated only 8 percent of their forests showed pollution damage.
    • By 1983, some 34 percent of the forest was affected.
    • By 1985, more than 4 million hectares (about half the total) were reported to be in a state of decline.
  • Similar damage is reported in Czechoslovakia, Poland, Austria, and Switzerland.
  • Researchers at the Hubbard Brook Experimental Forest in New Hampshire have shown that forest soils have become depleted of natural buffering reserves of basic cations such as calcium and magnesium through years of exposure to acid rain.
  • Plant pathogens and insect pests may damage trees or attack trees debilitated by air pollution.

• Buildings and monuments
  • In cities throughout the world, some of the oldest and most glorious buildings and works of art are being destroyed by air pollution.
  • Air pollution also damages ordinary buildings and structures by corroding steel in reinforced concrete in the buildings as well as roads and bridges.

• Visibility reduction
  • Foul air obscuring the skies above industrialized cities has long been recognized as a problem.
  • Pollution affects rural areas as well (e.g. Grand Canyon National Park and Shenandoah National Park).

Air Pollution Control

Moving Pollution to Remote Areas

• Among the earliest techniques for improving local air quality was moving pollution sources to remote locations and/or dispersing emissions with smokestacks.

Particulate Removal

• Filters remove particle physically by trapping them in a porous mesh of cotton cloth, spun glass fibers, or asbestos-cellulose, which allows air to pass through but holds back solids.
• Electrostatic precipitators are the most common particulate controls in power plants.
  • Fly ash particles pick up an electrostatic surface charge as they pass between large electrodes in the effluent stream.
  • Performance depends on particle size and chemistry, strength of the electric field, and flue gas velocity.

Sulfur Removal

• Sulfur removal can be done a variety of ways either by using low-sulfur fuel or by removing sulfur from effluents.
• Fuel switching and fuel cleaning
  • Switching from soft coal with a high sulfur content to low-sulfur coal can greatly reduce sulfur emission.
  • Changing to another fuel, such as natural gas or nuclear energy, can eliminate all sulfur emissions as well as those of particulates and heavy metals.
  • Alternative energy sources, such as wind and solar power, are preferable to either fossil fuel or nuclear power, and are becoming economically competitive.
  • Coal can be crushed, washed, and gassified to remove sulfur and metals before combustion.
• Limestone injection and fluidized bed combustion
  • Sulfur emissions can be reduced as much as 90 percent by mixing crushed limestone with coal before it is fed into a boiler.
  • A relatively new technique for burning, called fluidized bed combustion, offers several advantages in pollution control.
• Flue gas desulfurization
  • Crushed limestone, lime slurry, or alkali can be injected into a stack gas stream to remove sulfur after combustion.
• Sulfur recovery processes
  • Sulfur can be removed from effluent gases by processes that yield a usable product, such as elemental sulfur, sulfuric acid, or ammonium sulfate.

Nitrogen Oxide Control

• Staged burners, in which the flow of air and fuel are carefully controlled, can reduce nitrogen oxide formation by as much as 50 percent.
• The approach adopted by U.S. automakers for NO_x reductions has been to use selective catalysts to change pollutants to harmless substances.
• Raprenox (rapid removal of nitrogen oxides) is a new technique for removing nitrogen oxides that was developed by the U.S. Department of Energy Sandia Laboratory in Livermore, California.

Hydrocarbon Controls

• Closed systems that prevent escape of fugitive gases can reduce many hydrocarbon emissions (e.g. positive crankcase ventilation (PCV) systems in automobiles).
• Controls on fugitive losses from valves, pipes, and storage tanks in industry can have a significant impact on air quality.
• Afterburners are often the best method for destroying volatile organic chemicals in industrial exhaust stacks.

Clean Air Legislation

• The Clean Air Act of 1963 was the first national legislation in the United States aimed at air pollution control.
  • Federal grants were provided to states to combat pollution, but the act was careful to preserve states' rights to set and enforce air quality regulations.
  • It became obvious that some pollution problems cannot be solved on a local basis.
• In 1970, an extensive set of amendments essentially rewrote the Clean Air Act.
  • These amendments identified the "criteria pollutants" and established national ambient air quality standards.
  • Standards are divided into two categories.
    • Primary standards: intended to protect human health.
    • Secondary standards: set to protect materials, crops, climate, visibility, and personal comfort.
• In 1990, the Clean Air Act was extensively rewritten and updated including provisions to address the following issues: acid rain, urban smog, toxic air pollutants, ozone protection, marketing pollution rights, and volatile organic compounds.
• In 1997, further changes were made to the Clean Air Act; ambient ozone standards will be lowered from 0.12 ppm to 0.08 ppm.
• The EPA estimates that costs of these measures could be as high as $8.5 billion per year, but that they should save 15,000 lives, cut hospital admissions for respiratory illnesses by 9.000, and reduce chronic bronchitis cases by 60,000 each year.
• The EPA won't fully implement these latest standards for ozone and fine soot until 2008 to give states a chance to set up monitoring systems and to find ways to eliminate pollution in the most cost-effective manner.
• California has gone further than the federal government in making specific plans for air pollution control.

Table 18.3 -- National Ambient Air Quality Standards (NAAQS)
Pollutant	Primary (Health-Based Averaging Time)	Standard Concentration
TSPa	Annual geometric meanb 24 hours	50 ug/m3 150 ug/m3
SO2	Annual arithmetic meanc 24 hours	80 ug/m3 (0.03 ppm) 120 ug/m3 (0.14 ppm)
CO	8 hours 1 hour	10 mg/m3 (9 ppm) 40 mg/m3 (35 ppm)
NO2	Annual arithmetic meanc	80 ug/m3 (0.05 ppm)
O3	Daily max 1 hour avg.	235 ug/m3 (0.12 ppm)
Lead	Maximum quarterly avg.	1.5 ug/m3
aTotal suspended particulate material. bThe geometric mean is obtained by taking the nth root of the product of n numbers. This tends to reduce the impact of a few very large numbers in a set. cAn arithmetic mean is the average determined by dividing the sum of a group of data points by the number of points.

Table 18.4 -- Air Pollutant Standards Index
Rating	Description	Health Effects	Suggested Actions
500	Disaster	Very hazardous to all; serious injury and excess deaths, especially in sensitive persons	Stay inside with doors and windows closed; avoid all physical activity
400	Emergency	Hazardous to general population; grave injury possible	Avoid outdoor exercise; young, elderly, and ill should reduce all activity
300	Warning	Very unhealthy for all; serious threat to young, elderly, or ill	Elderly or those with heart or lung disease stay indoors
200	Alert	Irritation of eyes and lungs; aggravation of existing disease	Sensitive persons stay indoors
100	Moderate	NAAQS maximum permissible levels	Avoid traffic and congestion
50	Good	No known short-term effects	No restrictions

Current Conditions and Future Prospects

• Clean Air Act goals have not been achieved; however, air quality has improved dramatically in the last decade in terms of the major large-volume pollutants.

• The EPA estimates that emissions of particulate materials decreased 78 percent, lead fell 98 percent, SO₂ declined 32 percent, and COshrank 23 percent.

• Because automobiles are the main source of NO_x, cities where pollution is largely from traffic still have serious air quality problems.

• The major metropolitan areas of many developing countries are growing at explosive rates to incredible sizes and environmental quality is still abysmal in many of them (e.g. Mexico City and many large cities in China).

• As political walls came down across Eastern Europe and the Soviet Union at the end fo the 1980s, horrifying environmental conditions in these centrally planned economies were revealed.

• Not all is pessimistic, however. There have been some spectacular successes in air pollution control (e.g. Sweden and West Germany).

Summary

• Air pollution: chemical or physical changes brought about by either natural processes or human activities resulting in air quality degradation.
• Air pollution has existed as long as there has been an atmosphere.
• Burning forests, fossil fuels, and wastes continue to be the largest source of anthropogenic air pollution.
• The seven conventional large-volume pollutants are NO_x, SO₂, CO, lead, particulates, volatile organic compounds, and photochemical oxidants.
• Major sources of air pollution are transportation, industrial processes, stationary fuel combustion, and solid waste disposal.
• Indoor air pollutants, including formaldehyde, asbestos, toxic organic chemicals, radon, and tobacco smoke comprise some unconventional pollutants.
• Long-range transport of pollutants and photochemical reactions in trapped inversion layers over urban areas are important processes affecting the distribution of pollutants.

Questions for Review

1. Define primary and secondary air pollutants.
2. What are the seven "criteria" pollutants in the original Clean Air Act? Why were they chosen?
3. What are the two main categories of materials in the EPA's list of 33 air toxics of greatest concern?
4. What is acid deposition? What causes it?
5. What is an atmospheric inversion and how does it trap air pollutants?
6. How do electrostatic precipitators, baghouse filters, flue gas scrubbers, and catalytic converters work?
7. What is the difference between ambient standards and emission limits?
8. Describe the health effects and suggested actions for each of the levels of the pollution standards index (PSI).
9. Which of the conventional pollutants has decreased most in the recent past and which has decreased least?

Questions for Critical Thinking

1. Some authors limit pollution to human-caused materials. If a natural source releases the same toxic chemicals as a factory, is one pollution and the other not?
2. What might be done to improve indoor air quality? Should the government mandate such changes?
3. Why do you suppose that air pollution is so much worse in Eastern Europe than in the West?
4. Suppose air pollution causes a billion dollars in crop losses each year but controlling the pollution would also cost a billion dollars. Should we insist on controls?
5. Utility managers once claimed that it would cost $1,000 per fish to control acid precipitation in the Adirondack lakes and that it would be cheaper to buy fish for anglers than to put scrubbers on power plants. Suppose that was true. Does it justify continuing pollution?
6. The ban on chlorofluorocarbons may mean that billions of people won't be able to have refrigerators and that millions will die each year from food poisoning. Is that justified by saving stratospheric ozone?
7. Is it possible to have zero emissions of pollutants? What does zero mean in this case?
8. If there are thresholds for pollution effects (at least as far as we know now), is it reasonable or wise to depend on environmental processes to disperse, assimilate, or inactivate waste products?
9. Should the EPA ban all 2-cycle engines (chain saws, outboard motors, lawnmowers, jet -skis)? Why or why not?
10. Do you think that we should continue to use ambient air quality standards or change to absolute emission standards for all pollutants?