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The study of air pollution encompasses an examination of pollution emissions from industrial, agricultural, household, and other sources; the fate, transportation, and interaction of multiple pollutants in the atmosphere; atmospheric conditions and ambient air quality levels; the health and environmental effects of air pollution; the political and economic strategies and consequences associated with air pollution legislation; and community level movements and activist responses to air pollution largely in the context of environmental justice.
Types of Pollution
Air pollution can be categorized as gaseous or particulate matter, primary or secondary, and toxic or nontoxic. Air pollution and related formal and informal prevention measures are also generally divided between outdoor or indoor environments.
One of the most pervasive outdoor pollutants with serious health and environmental risks is particulate matter. Particulate matter is classified in two ways, having a diameter less than 2.5 micrometers (PM2.5) and particulate matter with a diameter less than 10 micrometers (PM10). Although both forms of particulate matter present serious health risks, PM2.5 is generally considered more dangerous because of its small size – approximately 1/30 the width of a human hair. When breathed, PM2.5 can lodge deep in human lung tissue and even enter the bloodstream, causing severe health problems. Smaller particulate matter also tends to have a longer residence time in the atmosphere. As a result, PM2.5 can travel farther and affect a greater population.
The principle sources of PM2.5 and PM10 include combustion from motor vehicles, power plants, and other industrial sources, as well as biomass burning. One of the most severe sources of particulate matter is diesel-powered vehicles. Airborne dust, soils, and salts are other common forms of large particulate matter that occur in areas susceptible to high winds. The combination of these sources can produce a haze in the sky in both urban and rural areas, often in the late afternoon.
The composition of the earth’s atmosphere is 78 percent nitrogen (N2), 21 percent oxygen (O2) and 0.9 percent argon (Ar). The other 0.1 percent of atmosphere consists of trace gasses such as carbon dioxide (CO2), neon (Ne), helium (He), methane (CH4), and water vapor. Except in extreme situations, most pollutants are considered trace gases and do not greatly alter the overall composition of the atmosphere. Even at extremely low levels, however, gaseous pollutants can have serious health and environmental consequences.
Important gas contaminants include: Carbon monoxide (CO), chlorofluorocarbons (CFCs), nitrogen oxides (NOx), hydrocarbons (HC), sulfur dioxide (SO2), and carbon dioxide (CO2).
Carbon monoxide (CO) is formed when carbon-based substances are burned with an insufficient supply of air. The majority of CO in the United States is emitted from motor vehicles due to the incomplete combustion of gasoline. Areas within road and highway corridors are therefore subject to high levels of CO. When inhaled, the health effects of CO include headaches, respiratory illness, and-in extremely high doses-death. Carbon monoxide is a “criteria” pollutant as outlined under the Clean Air Act.
Chlorofluorocarbons (CFCs) were used historically as refrigerants, cleaning solvents, and as propellants in aerosol canisters. CFC use was largely banned under the 1987 Montreal Protocol due to its impact on the ozone layer. The State of Oregon banned CFCs as early as 1975. Their long atmospheric lifetime-ranging from 55 to 140 yearsand regenerative capabilities make CFCs particularly devastating to the upper atmosphere.
Nitrogen oxides (NOx) are a combination of nitric oxide (NO) and nitrogen dioxide (NO2). Common in urban areas, they are produced from natural sources and fuel combustion. Over half of anthropogenic NOx in the United States results from motor vehicle activity. The major health effects of NOx are pulmonary-related. Nitrogen oxides are designated as a “criteria” pollutant under the Clean Air Act and contribute to the formation of acid rain and smog.
Hydrocarbons (HC) are chemical compounds that contain both elemental hydrogen and carbon (e.g., methane). Hydrocarbons such as benzene are varieties of volatile organic compounds (VOCs), which are characterized by their ability to vaporize easily at room temperature. Hydrocarbons are most commonly released into the atmosphere when petroleum fuel is only partially burned or unburned. The majority of this pollution occurs through the exhaust pipe of motor vehicles or as a consequence of fuel evaporation. Hydrocarbons react with NOx to create smog, and in the case of benzene and other hydrocarbon varieties, may contain toxic compounds.
Sulfur dioxide (SO2) exists in gaseous form and can react with other chemicals to create tiny sulfate particles. It is one of six “criteria” pollutants under the Clean Air Act. Roughly two thirds of SO2 comes from fuel combustion at power plants, especially those burning coal. SO2 contributes to acid rain and can cause respiratory illness when inhaled.
Carbon dioxide (CO2) occurs naturally in the earth’s atmosphere from the combustion of biomass, animal respiration, and outgassing from the earth’s surface. Carbon dioxide is also emitted into the atmosphere through the anthropogenic burning of fossil fuels for energy and transportation purposes. While the presence of carbon dioxide at low concentrations is normal, CO2 is an effective green house gas that can alter the earth’s atmosphere.
Toxic air pollutants, commonly referred to as hazardous air pollutants (HAPs), present acute risks to humans and the environment. Possible risks include cancer, birth defects, damage to the immune system, as well as serious neurological and respiratory ailments. While toxic pollution is typically breathed from the air, some toxins like mercury and lead can settle into soils and enter into plants and other animals and eventually be ingested up the food chain to humans.
The U.S. Environmental Protection Agency (EPA) works with state and local governments to regulate and reduce the release of 188 toxic pollutants. Some examples of toxic pollutants include perchlorethlyene, which is a common effluent of dry cleaning facilities; methylene chloride, which is found in many industrial solvents and paint strippers; and benzene, which is found in gasoline. Other common air toxics are toluene, dioxin, and asbestos and metals such as mercury and lead compounds.
Lead (Pb) is another of the six effluents designated as “criteria” pollutants by the Clean Air Act. Whereas in 1970 the vast majority of lead pollution came from motor vehicles, now most lead pollution is the result of metals processing activities such as lead smelters. Today, the biggest concerns regarding lead occur in “hot spots” immediately surrounding these processing facilities. Between 1980 and 1999, ambient lead levels in the atmosphere dropped 94 percent. The overall reduction in lead pollution since 1978 is attributed to the gradual phase out of leaded gasoline. The health effects of lead are severe and include organ, neurological, brain, and cardiovascular damage.
High concentrations of toxic air pollutants in impoverished and minority communities in the face of pro-business city planning agendas have galvanized grassroots environmental justice activists. Environmental justice organizations speak out against the inequitable distribution of air pollution within these neighborhoods. The goal of these movements varies from one to the next but usually promotes private polluting facilities moving to other locations or adopting cleaner practices; holding governments accountable to communities; and public involvement and control over decisions influencing air quality.
In order to gain political traction, these groups provide evidence of environmental injustices. Some communities practice “popular epidemiology” by locating and recording health incidents throughout neighborhoods, while others mobilize around air quality reports using data from Toxic Release Inventories (TRI). The TRI database caters specifically to the public and provides information on facility-specific toxic emissions into the environment.
In all areas of the world, people spend much of their time inside while at work and at home. The prevalence of indoor air pollution is therefore a serious problem that affects both developed and developing nations. Indoor air pollution consists of gas compounds and particulate matter, as well as toxics and nontoxics. Throughout the world, forms of indoor air pollution include: Radon filtering into the home from below ground; volatile organic compound (VOC) emissions from office supplies; furnishings, paints, and other household solvents; lead particles from dried paint; and particulate matter from wood burning fireplaces and secondhand smoke from cigarettes.
Asbestos is another serious health risk in indoor environments although a number of product bans and reformed building construction codes have eliminated the use of asbestos-based material in homes. Because it is carcinogenic (cancer causing), asbestos was one of the first hazardous air pollutants regulated under the 1970 Clean Air Act.
In developing nations the burning of biomass for heating and cooking purposes is a major health hazard for nearly 2 billion people. The World Health Organization (WHO) estimates that approximately 1.6 million people die each year as a result of exposure to indoor air pollution from biomass burning. The main groups impacted by indoor smoke are women and children because they are responsible for most cooking and heating activities. The 2002 World Summit on Sustainable Development in Johannesburg launched the Partnership for Clean Indoor Air. The principle aim of this coalition of organizations is to reduce indoor air pollution from household energy use around the world.
Secondary pollutants are not directly emitted into the atmosphere. Rather, they are formed when primary pollutants react with other gases, sunlight, or water vapor and undergo a chemical change in the atmosphere. Two examples of secondary pollutants are tropospheric ozone and components of acid rain such as nitric and sulfuric acids.
Tropospheric ozone (O3) is a photochemical oxidant and is formed through a reaction involving sunlight, NOx , and hydrocarbons. Ozone is an important component of smog that impacts urban residents around the world. Respiratory and eye irritation are the most common symptoms of photochemical smog exposure. Mega-cities such as Beijing, Los Angeles, Mexico City, and Athens all suffer from pronounced smog levels. Wind and precipitation are two principal means for removing smog.
The formation of smog is aided by the presence of an inversion layer that creates a so-called “lid” trapping pollutants over the city. An inversion layer, which forms when warm air resides over cold air, prevents old, stale, and polluted air from escaping vertically out of an urban system. Under these stable atmospheric conditions, surface level pollution accumulates creating elevated smog levels.
In 2005, the American Lung Association used recent county-by-county air quality data to conduct a “State of the Air” report. County level data on two pervasive pollutants-ozone and particulate matter-were included in the report. According to the study, in 2005 over 52 percent of U.S. residents lived in counties with unhealthful levels of either ozone or particle pollution. (The results were based on the EPA’s Air Quality Index.) Despite California’s firm commitment to reduce dangerous air pollution, fivemetropolitan areas around the state make the top 10 list for short-term particulate matter pollution while six make the ozone pollution top-10 list.
Nitric acid (HNO3) is an easily soluble, acidic gas. It is formed when nitrogen dioxide (another secondary pollutant) reacts with water. Sulfuric acid (H2SO4) is also highly acidic and contributes to the formation of acid rain when reacting with water. Acid rain is typically most severe downwind from major power and industrial plants, although pollution from cities can also lead to acid rain. For example, areas of the Northeastern United States experience acid rain originating from Midwest industrial belts.
Sources of Pollution
Pollution sources are both environmental and anthropogenic. Environmental sources include fires, volcanoes and other geothermal activities, livestock, trees (most notably species of pine), the earth’s substrate (which can experience radioactive decay), and agricultural fields and deserts, both of which produce dust storms when exposed to high winds.
Anthropogenic sources are categorized as stationary, mobile, or area sources. Stationary sources (commonly referred to as “point sources”) are generally fixed to a single location or point. These sources include industrial activity such as combustion-fired power plants, oil refineries, and other wood, natural gas, oil, and coal burning facilities. Other stationary sources include landfills and pollution directly resulting from agricultural activity such as loose soil, chemical applications, and controlled burns. Mobile sources include automobiles such as passenger cars, public transportation vehicles, heavy duty trucks, and mobile construction equipment. Aircraft, locomotives, and marine vessels are other varieties of mobile pollution sources. Area sources include stationary and nonroad sources that are too numerous or small to accurately account for in strict emissions inventories and are therefore assigned to a general area.
Legislation and Mitigation
Air quality legislation can focus on the pollution source by regulating single-source or total regional emissions. Legislation is also often based on air quality standards, aiming to limit the ambient level of pollution in the atmosphere over a given period of time. Still other legislation is technology-driven and regulates facilities based on equipment standards. While these regulations ultimately target the polluting facility, they usually operate through a central air quality officer at the national, state, county, or municipal scale.
Air pollution in the United States is regulated at both the national and state scale. National Ambient Air Quality Standards (NAAQS) are established by the U.S. Environmental Protection Agency’s Office of Air Quality Planning and Standards (U.S. EPA OAQPS). The standards establish maximum allowable pollution levels for a given area. The NAAQS consists of primary and secondary standards designed to meet human health and nonhuman health (crops, structures, and animals) requirements respectively.
The standards are directed toward six criteria pollutants: ozone (O3), particulate matter (PM2.5, PM10), carbon monoxide (CO), sulfur dioxide (SO2), nitrogen oxides (NOx) and lead (Pb). The EPA was instructed to maintain records of ambient air quality levels in accordance with 1990 amendments to United States Clean Air Act of 1970.
The 1970 Clean Air Act (CAA) was a landmark legislative accomplishment outlining federal rules and regulations for stationary and mobile air pollution sources. Aside from mandating the NAAQS, the CAA also created standards for hazardous pollutants through the National Emission Standards for Hazardous Air Pollutants (NESHAPS). Through New Source Performance Standards, the CAA created strict stationary source emissions standards. Perhaps the most significant achievements of the act is that it allows states to devise their own air quality implementation and enforcement plans, assuming they are no less stringent than federal standards. States must submit a State Implementation Plan (SIP) to the EPA in order to demonstrate a capacity to achieve minimum pollution criteria.
California is a notable example of a state aggressively enacting numerous stringent air pollution regulations. For example, in 2002, California passed legislation known as the Pavley Bill, creating vehicle emissions standards requiring that greenhouse gas emissions from new vehicles be reduced by 22 percent by the 2012 model year and 30 percent by the 2016 model year. By 2006, ten other states had adopted these strict California standards, which in turn put pressure on car manufacturers to fill this growing “green” automobile market.
Mobile source emissions are also controlled by regulating vehicle fuel composition. Examples of fuel-based regulation are initiatives to reformulate gasoline by eliminating toxics such as benzene and the 1995 ban on leaded gasoline for passenger vehicles in the United States. Another set of measures intended to reduce mobile source emissions focus on zoning and land use ordinances that reduce total vehicle miles driven. Examples include “smart growth” initiatives such as carpool incentives and urban growth boundary designations.
Because air is fluid and political boundaries are porous (in terms of both the movement and eventual impacts of air pollution) national and subnational regulatory frameworks are insufficient alone. Multilateral Environmental Agreements (MEAs) have been created to govern cross boundary pollution between nations and international air-related environmental problems. One example is the 1979 UN Economic Commission for Europe (UNECE) Convention on Long-Range Trans-boundary Air Pollution. The 1987 Montreal Protocol banning the production of CFCs and the 1997 Kyoto Protocol designed to regulate global greenhouse gas emissions are examples of international accords that emphasize “local” pollution prevention measures to prevent potentially calamitous global impacts.
Monitoring and Enforcement
Air quality modeling supports air pollution regulations and increases public awareness by predicting pollution concentrations and levels of human exposure. Pollution modeling requires synthesizing atmospheric inputs with emissions data from the National Emission Inventory (NEI). The EPA compiles NEI data on criteria and hazardous air pollutants from a variety of state and local agencies as well as industry.
A number of programs are used to mitigate pollution by enforcing standards, reducing the financial cost of compliance and improving levels of public awareness. Permitting programs help sources achieve mandated emissions standards. Permitting requirements are outlined under the Clean Air Act in two forms, construction permit programs and operating permit programs. Construction permits include New Source Review (NSR) permits. NSR permits are issued by air pollution agencies and regulate pollution through technological standards. New sources of pollution and significant emission increases at existing sources must satisfy certain state-of-the-art pollution control technology requirements in order to avoid penalties.
Under Title V of the Clean Air Act, operating permits are issued to certain pollution sources. These permits outline industry-based performance standards and require sources to monitor and share with the EPA their compliance with these requirements. A number of source monitoring measures are available for most facilities, although fugitive emissions (unaccounted for effluent releases) present an ongoing challenge to most monitoring efforts.
Emissions trading programs and “command and control” enforcement procedures are both used to enforce air pollution compliance. Emissions trading programs cap total emissions within a given region but allow sources with more expensive control costs to purchase pollution “credits” from sources experiencing cheaper control costs. The goal of these programs is to achieve emissions reduction in the most cost efficient manner. Command and control enforcement relies on government control to achieve emissions reductions. Under these programs, facilities that violate industry standards are fined by government regulatory enforcement bodies.
- Devra Davis, When Smoke Ran Like Water (Basic Books, 2002);
- Thad Godish, Air Quality (CRC Press, 2004);
- Gerald Markowitz and David Rosner, Deceit and Denial: The Deadly Politics of Industrial Pollution (University of California Press, 2004);
- United States Environmental Protection Agency, “Air Topics,” www.epa.gov.