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Air pollutants are airborne substances (either solids, liquids, or gases) that occur in concentrations high enough to threaten the health of people and animals, to harm vegetation and structures, or to toxify a given environment. Air pollutants come from both natural sources and human activities.
Certain pollutants are called primary air pollutants because they enter the atmosphere directly --- from smokestacks and tail pipes, for example.
Other pollutants, known as secondary pollutants, form only when a chemical reaction between a primary pollutant and some other component of the air, such as water vapor or another pollutant.
| Name | Symbol | Source |
|---|---|---|
| Carbon monoxide | CO | Incomplete combustion of fuels |
| Nitrogen Oxides | NOx (NO,NO2) | High temperature/pressure combustion |
| Hydrocarbons | HC | Incomplete combustion of fuels |
| Ozone | O3 | Photochemical reactions |
| Peroxyacetlyn nitrates | PAN | Photochemical reactions |
| Sulfur Oxides | SOx (SO2,NO3) | Combustion of sulfur-containing fuels |
| Particulates | -- | Dust, dirt, soot, salt, metals, organics |
| Carbon dioxide | CO2 | Complete combustion |
| Water vapor | H2O vapor | Combustion processes, steam |
| Methane | CH4 | Organic processes |
Particulates (also called aerosols) are solid and liquid materials in the air that are of natural or anthropogenic origin. Though always small, particulates come in a wide range of sizes ranging from about 0.1 to 100 μm.
Particulates introduced directly into the air can originate from natural fires, volcanic eruptions, the ejection of salt crystals by breaking ocean waves---and as any sufferer of hay fever can tell you, by the entrainment of pollen by wind. Human activities, especially those involving combustion, produce primary and secondary particulates.
Particulates reduce visibility by increasing scattering of visible radiation, but their effect on visibility is of less importance than their impacts on health. A large body of research analyzing the effects of particulates has shown that a more specific class of particulates---those smaller than 2.5 μm ---also present serious health problems. For this reason, the Environmental Protection Agency (EPA) in July 1997 revised its regulation regarding particulates so that in the future they will be based on these so-called fine particles.
Carbon monoxide is a colorless,
odorless gas. In the natural environment it is released as a
primary pollutant by volcanic eruptions, forest fires, bacterial
action, and other processes. Though natural processes emit far
more CO into the environment than do human activities, soil
microorganisms consume it effectively and the background values
are very low.
In cities, however, inputs can greatly exceed the rate of removal and unsafe concentration can accrue. In the US, the most important source of CO is the automobile, which releases the gas a by product of incomplete combustion. In well maintained vehicles, carbon monoxide emissions are low, but poorly operating engines can cause CO concentrations to accumulate to unsafe levels. This is particularly true in confined areas, such as garages and tunnels.
In the home, an improperly vented or malfunctioning furnace can release lethal doses of CO very quickly. Carbon monoxide is also released in home fires, where it probably is responsible for a high percentage of fire-related fatalities. Cigarette smoke also releases carbon monoxide as a by-product sufficient to greatly increase its concentration in the bloodstream.
Carbon monoxide is extremely toxic. Even low levels can cause a person to immediately experience slowed reflexes, drowsiness, and a reduction or loss of consciousness. Exposure for three hours at 400 parts per million (ppm) is life threatening, and at 1600 ppm death comes within an hour. Over the long haul, it can contribute to heart disease.
Sulfur compounds in the
atmosphere can occur in gaseous or aerosol form. The
majority---roughly two-thirds---of all sulfur compounds emitted
into the atmosphere originate from natural processes. Steam
vents, such as those in Yellowstone National Park in Wyoming or
Lassen National Park in California, provide interesting examples
of the emission of sulfur compounds. The most important of these
is the bacterial release of hydrogen sulfide (HS), a particularly noxious gas that smells like
rotten eggs. Volcanic eruptions and sea spray also play an
important role in releasing sulfur compounds. Fortunately, sulfur
gases are readily dispersed in the atmosphere.
Of the anthropogenic sulfur compounds released to the atmosphere, the most important are sulfur dioxide (SO2) and sulfur trioxide (SO3). These oxides of sulfur fall under the collective designation of SOx.
Sulfur dioxide is a primary pollutant released mainly by the burning of sulfur-containing fossil fuels, particularly coal and oil used for heating and electric power. Other industrial activities, such as petroleum refining and ore smelting, also contribute SO2. Human activities tend to be concentrated over relativelly small areas, allowing SOx to attain high values over urban and industrial areas.
Sulfur dioxide is a colorless but highly corrosive gas that irritates human respiratory systems. High concentrations are associated with a number of lung problems, and even low concentrations can cause asthmatic subjects to experience severe bronchial constriction during exercise. Though widely blamed for causing respiratory problems, scientists are not sure what role high SO2 concentrations directly play in their onset. It may be that the recurence of respiratory illness during high SO2 episodes is not due directly to the presence of the gas, but rather to the particulates that often accompany high sulfur dioxide concentrations.
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| Ancient Athens structures affected by acid rain |
Sulfur trioxide can be put directly into the air as a primary pollutant, but it more commonly builds up as a secondary pollutant following reactions involving SO2. Sulfur trioxide is not by itself a major component of air pollution. However, it rapidly combines with with water droplets to form sulfuric acid, H2SO4. If this process occurs near the surface, it forms acid fog; if it occurs in clouds, subsequent precipitation of the acid compound produces acid rain. Not surprisingly, acid fog and acid rain are both capable of causing extreme environmental harm and through time can wear down human structures. Acid fog can be particularly dangerous to people because it is so easily inhaled. Buildings and monuments made of limestone are especially vulnerable to weathering from acid rain and fog.
Nitrogen oxides (also called
oxides of nitrogen) are compounds consisting of nitrogen
and oxygen atoms. The two most important of these from an air
pollution viewpoint are nitric oxide (NO) and nitrogen
dioxide (NO2). Together the two gases are
commonly referred to as NOx.
Nitric oxide is a nontoxic, colorless, and odorless gas that forms naturally by biological processes in soil and water. While millions of tons of the material are introduced into the atmosphere each year, it is highly reactive and tends to break down very quickly. Nitric oxide also forms as a by-product of high-temperature combustion associated with automobile engines, industrial manufacturing, and electric power generation. Its primary importance from an air quality perspective is that it oxidizes to form nitrogen dioxide, a major component of smog in many places.
Nitrogen dioxide is a toxic gas that gives polluted air its familiar yellow to reddish brown color, as well as a pungent odor. It is an important component in air pollution in that it is relatively toxic, corrosive, and undergoes transformations that contribute to acid deposition and other secondary pollutants. As with nitric oxide, nitrogen dioxide breaks down very readily and, as a result, NO2 concentrations in urban areas tend to rise and fall in accordance with vehicular traffic patterns. Furthermore, the rapid decay of nitrogen dioxide prevents large concentrations from occurring in rural areas surrounding source areas.
Like sulfur compounds, nitrogen oxides can cause serious pulmonary health problems. Clinical studies have shown that NO2 easily passes through bronchial passages and irritates tissue deep within the lungs. Laboratory tests have shown animals to experience severe lung damage and reduced immunity to infection when exposed to high levels of NO2.
Volatile organic compounds
(VOC), also called hydrocarbons, are materials made
entirely of carbon and hydrogen atoms. These compounds, including
methane, butane, propane, and octane, occur in both gaseous and
particulate forms. Globally, the vast majority of hydrocarbons
arrive in the atmosphere via natural processes, including plant
and animal emissions and decomposition. In the United States,
industrial activities account for the greatest proportion of
anthropogenic hydrocarbons, with automobiles also contributing a
major share. The emissions associated with automobiles arise
primarily from incomplete fuel combustion and the evaporation of
gasoline (often while filling gas tanks).
Even in cities that have high VOC concentrations, there is little evidence that these chemicals have any direct adverse health impacts. Nonetheless, they are extremely important because in the presence of sunlight they recombine with nitrogen oxides and oxygen to produce photochemical smog.
If you have ever visited Los Angeles in the summer, you have probably heard the term photochemical smog and know what it feels like. Burning eyes, sore lungs, and a subtle but unpleasant odor accompany an atmosphere with poor visibility. Photochemical smog consists of secondary pollutants that include ozone (O3}), NO2, peroxyacyl nitrate (PAN), formaldehyde, and other gases that occur in very minute quantities. As the name implies, this type of smog forms when sunlight triggers numerous reactions and transformations of gases and aerosols. Unlike the London-type smog found in many places where smoke combines with damp air (the word smog, in fact, originally derived from the terms smoke and fog), this Los Angeles-type smog usually involves dry air.
Ozone has been designated by the Environmental Protection Agency as the most important agent of photochemical smog. It can cause serious physical and environmental harm at surprisingly low concentrations, so low that the EPA established a concentration of only 0.12 ppm averaged over a 1-hour period as the maximum allowable without exceeding federal standards.
Exposure to ozone causes inflammation of air passages that can reduce lung capacity by as much as 20%. The EPA estimates that perhaps 20% of all respiratory-related hospital visits in the northeastern United States during the summer result from exposure to ozone. Although acute symptoms usually subside fairly quickly after ozone concentrations decline, research has shown that long-term exposure to ozone can cause permanent damage to lung tissue and impairment of the body's ability to resist bronchitis, pneumonia, and other diseases. Of course, ozone causes even greater problems for people with asthma and other preexisting pulmonary problems. For those people, ozone constricts lung passages to the point where breathing becomes nearly impossible, and the gas may contribute considerably to the 5000 fatalities in the United States each year from acute asthma attacks.
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| (a) Components of the respiratory system. Air travels along a pressure gradient from the oral and nasal cavities through the trachea and bronchi to the tiniest bronchioles, which terminate as lobel, microscopic sacs known as alveoli. (b) Alveoli are richly surrounded by capillary beds, facilitating the diffusion of oxygen from the lungs into the blood strem. |
Air pollution --> part of human way of life since the 13th century.
Many examples among British literature and European art:
