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Nature and Sources of the Pollutant: Ground-level ozone (the primary
constituent of smog) is the most complex, difficult to control, and pervasive of the six
principal pollutants. Unlike other pollutants, ozone is not emitted directly into the air
by specific sources. Ozone is created by sunlight acting on NOx and VOC emissions in the
air. There are literally thousands of sources of these gases. Some of the more common
sources include gasoline vapors, chemical solvents, combustion products of various fuels,
and consumer products. They can originate from large industrial facilities, gas stations,
and small businesses such as bakeries and dry cleaners. Often these "precursor"
gases are emitted in one area, but the actual chemical reactions, stimulated by sunlight
and temperature, take place in another. Combined emissions from motor vehicles and
stationary sources can be carried hundreds of miles from their origins, forming high ozone
concentrations over very large regions. Approximately 50 million people lived in counties
with air quality levels above EPA's health-based national air quality standard in 1994.
The highest levels of ozone were recorded in Los Angeles. High levels also persist in
other heavily populated areas like the Texas Gulf Coast and much of the Northeast.
Health and Other Effects: Scientific evidence indicates that
ground-level ozone not only affects people with impaired respiratory systems (such as
asthmatics), but healthy adults and children as well. Exposure to ozone for 6 to 7 hours,
even at relatively low concentrations, significantly reduces lung function and induces
respiratory inflammation in normal, healthy people during periods of moderate exercise. It
can be accompanied by symptoms such as chest pain, coughing, nausea, and pulmonary
congestion. Recent studies provide evidence of an association between elevated ozone
levels and increases in hospital admissions for respiratory problems in several U.S.
cities. Results from animal studies indicate that repeated exposure to high levels of
ozone for several months or more can produce permanent structural damage in the lungs.
EPA's health-based national air quality standard for ozone is 0.12 ppm (measured at the
highest hour during the day). Ozone is also responsible for several billion dollars of
agricultural crop yield loss in the U.S. each year. Ozone also damages forest ecosystems
in California and the eastern U.S.
Nature and Sources of the Pollutant: Particulate matter is the term
for solid or liquid particles found in the air. Some particles are large or dark enough to
be seen as soot or smoke. Others are so small they can be detected only with an electron
microscope. Because particles originate from a variety of mobile and stationary sources
(diesel trucks, wood stoves, power plants, etc.), their chemical and physical compositions
Health and Other Effects: In 1987, EPA replaced the earlier Total
Suspended Particulate (TSP) air quality standard with a PM-10 standard. The new standard
focuses on smaller particles that are likely responsible for adverse health effects because
of their ability to reach the lower regions of the respiratory tract. The PM-10 standard
includes particles with a diameter of 10 micrograms or less (0.0004 inches or one-seventh
the width of a human hair). EPA's health-based national air quality standard for PM-10 is
50 micrograms per cubic meter (measured as an annual average) and 150
micrograms per cubic meter (measured as a daily average).
Major concerns for human health from exposure to PM-10 are: effects on breathing and
respiratory systems, damage to lung tissue, cancer, and premature death. The elderly,
children, and people with chronic lung disease, influenza, or asthma, tend to be
especially sensitive to the effects of particulate matter. Acidic PM-10 can also damage
manmade materials and is a major cause of reduced visibility in many parts of the U.S.
Nature and Sources of the Pollutant: Carbon monoxide is a colorless
odorless poisonous gas formed when carbon in fuels is not burned completely. It is a
byproduct of motor vehicle exhaust, which contributes more than two-thirds of all CO
emissions nationwide. In cities, automobile exhaust can cause as much as 95 percent of all
CO emissions. These emissions can result in high concentrations of CO, particularly in
local areas with heavy traffic congestion. Other sources of CO emissions include
industrial processes and fuel combustion in sources such as boilers and incinerators.
Despite an overall downward trend in concentrations and emissions of CO, some metropolitan
areas still experience high levels of CO.
Health and Other Effects: Carbon monoxide enters the bloodstream and
reduces oxygen delivery to the body's organs and tissues. The health threat from CO is
most serious for those who suffer from cardiovascular disease. Health individuals are also
affected, but only at higher levels of exposure. Exposure to elevated CO levels is
associated with visual impairment, reduced work capacity, reduced manual dexterity, poor
learning ability, and difficulty in performing complex tasks. EPA's health based national
air quality standard for CO is 9 parts per million (ppm) [measured over 8 hours].
Nature and Sources of the Pollutant: Nitrogen dioxide belongs to a
family of highly reactive gases called nitrogen oxides (NOx). These gases form when fuel
is burned at high temperatures, and come principally from motor vehicle exhaust and
stationary sources such as electric utilities and industrial boilers. A suffocating,
brownish gas, nitrogen dioxide is a strong oxidizing agent that reacts in the air to form
corrosive nitric acid, as well as toxic organic nitrates. It also plays a major role in
the atmospheric reactions that produce ground-level ozone (or smog).
Health and Other Effects: Nitrogen dioxide can irritate the lungs and
lower resistance to respiratory infections such as influenza. The effects of short-term
exposure are still unclear, but continued or frequent exposure to concentrations that are
typically much higher than those normally found in the ambient air may cause increased
incidence of acute respiratory illness in children. EPA's health-based national air
quality standard for NO2 is 0.053 ppm (measured as an annual average). Nitrogen oxides are
important in forming ozone and may affect both terrestrial and aquatic ecosystems.
Nitrogen oxides in the air are a potentially significant contributor to a number of
environmental effects such as acid rain and eutrophication in coastal waters like the
Chesapeake Bay. Eutrophication occurs when a body of water suffers an increase in
nutrients that reduce the amount of oxygen in the water, producing an environment that is
destructive to fish and other animal life.
Nature and Sources of the Pollutant: Sulfur dioxide belongs to the
family of sulfur oxide gases (SOx). These gases are formed when fuel containing sulfur
(mainly coal and oil) is burned, and during metal smelting and other industrial processes.
Health and Other Effects: The major health concerns associated with
exposure to high concentrations of SO2 include effects on breathing, respiratory illness,
alterations in pulmonary defenses, and aggravation of existing cardiovascular disease.
Major subgroups of the population that are most sensitive to SO2 include asthmatics and
individuals with cardiovascular disease or chronic lung disease (such as bronchitis or
emphysema) as well as children and the elderly. EPA's health-based national air quality
standard for SO2 is 0.03 ppm (measured on an annual average) and 0.14 ppm (measured over
24 hours). Emissions of SO2 also can damage the foliage of trees and agricultural crops.
Together, SO2 and NOX are the major precursors to acid rain, which is associated with the
acidification of lakes and streams, accelerated corrosion of buildings and monuments, and
Nature and Sources of the Pollutant: Smelters and battery plants are
the major sources of the pollutant "lead" in the air. The highest concentrations
of lead are found in the vicinity of nonferrous smelters and other stationary sources of
Health Effects: Exposure to lead mainly occurs through inhalation of
air and ingestion of lead in food, paint, water, soil, or dust. Lead accumulates in the
body in blood, bone, and soft tissue. Because it is not readily excreted, lead can also
affect the kidneys, liver, nervous system, and other organs. Excessive exposure to lead
may cause anemia, kidney disease, reproductive disorders, and neurological impairments
such as seizures, mental retardation, and/or behavioral disorders. Even at low doses, lead
exposure is associated with changes in fundamental enzymatic, energy transfer, and other
processes in the body. Fetuses and children are especially susceptible to low doses of
lead, often suffering central nervous system damage or slowed growth. Recent studies show
that lead may be a factor in high blood pressure and subsequent heart disease in
middle-aged white males. Lead may also contribute to osteoporosis in postmenopausal women.
EPA's health-based national air quality standard for lead is 1.5 micrograms per cubic
meter (æg/m3) [measured as a quarterly average].