Ozone (O3)
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 nitrogren
oxides (NOx) and volatile organic compound (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. Click
here for more information on the health effects of ozone.
In 1997, the EPA promulgated a new ozone national ambient air
quality standard of 0.08 ppm (8 hour averaging time). Click
here
for more information about the new ozone standard.
Nitrogen Dioxide (NO2)
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.
Particulate Matter (PM-10 and PM-2.5)
Nature and Sources of the Pollutants: 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 vary widely.
Health and Other Effects: In 1987, EPA replaced the
earlier Total Suspended Particulate (TSP) air quality standard with
a PM-10 standard. The 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 micrometers
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). In 1997,
EPA promulgated a PM-2.5 standard which includes particles with a
diameter of 2.5 microns or less. These smaller particles have the
best chance of reaching the lower respiratory tract. The health-based
national ambient air quality standard for PM-2.5 is 15 micrograms
per cubic meter (measured as an annual average) and 65 micrograms per
cubic meter (measured as a daily average).
Major concerns for human health from exposure to particulate matter
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
particulate matter can also damage manmade materials and is a major
cause of reduced visibility in many parts of the U.S. Click
here for more information on the
health effects of particulate matter.
Sulfur Dioxide (SO2)
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. EPA has a secondary SO2
national ambient air quality standard of 0.50 ppm (measured over 3
hours) designed to prevent this type of environmental deterioration.
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 reduced
visibility.
Lead (Pb)
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 lead emissions.
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 [measured as a quarterly average].
Carbon Monoxide (CO)
Nature and Sourcesof 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. Healthy 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].
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