Peopfe
^•4 , 5s .f^WJ:^"''
*
-------
-------
Little
iackground
The last two decades have witnessed a growing national
commitment to protecting public health and welfare
from the effects of air pollution. Recent federal
legislation, particularly the 1977 Amendments to the
1970 Clean Air Act, have guided federal and state
control agencies, industry, and a concerned public in a
cooperative effort aimed at solving the technological and
social problems involved in cleaning the Nation's air.
As part of this effort, many states have developed
wide-ranging regulatory, enforcement, and
administrative programs to reduce emissions of air
pollution from a great variety of sources. These
abatement efforts have been augmented by federal air
pollution regulations and control measures directed
toward emissions from new motor vehicles and certain
industrial sources. Indirect federal assistance has
included dissemination of information on new air
pollution control technology, studies of the health
effects of air pollution, and the coordination of state
efforts.
What has been accomplished by this considerable
expenditure of time, material, and funds? How does air
quality vary across the United States, and how has it
improved during recent years? To help answer these
questions, the U.S. Environmental Protection Agency
(EPA) published the National Air Quality and Emissions
Trends Report, 1977* in December 1978. It contained
estimates of the quality of the Nation's air, emission
levels by source category, the exposure of selected
urban populations to air pollution, and comparisons with
figures from previous years.
fltfl
I
-------
Summary
More than 8,000 air monitors across the country
measure and record air pollution levels. This report
covers data accumulated from these stations relative to
five principal pollutants. The data is largely quite
encouraging. Not all reports are good, but there is more
good news than bad about the quality of the Nation's air.
For example:
• The level of particulates in the air (particles such as
soot and dust) decreased 32 percent between 1960 and
1979.
• Sulfur dioxide in large urban areas decreased 67
percent from 1964 to 1979.
• Carbon monoxide in center-city locations fell 36
percent from 1972 to 1979 declining seven percent per
year.
• Ozone (smog) has shown no long-term national trend,
from 1974-1979, which is consistent with the trend in
ozone precursor emissions. The precursor, volatile
organic compounds, has remained relatively constant,
although the contribution from various types of sources
has changed significantly. Emissions from transportation
have decreased, while emissions from industrial
processes increased. While a 3 percent improvement in
mi 5
ot
-------
A Little
Background
The last two decades have witnessed a growing national
commitment to protecting public health and welfare
from the effects of air pollution. Recent federal
legislation, particularly the 1977 Amendments to the
1970 Clean Air Act, have guided federal and state
control agencies, industry, and a concerned public in a
cooperative effort aimed at solving the technological and
social problems involved in cleaning the Nation's air.
As part of this effort, many states have developed
wide-ranging regulatory, enforcement, and
administrative programs to reduce emissions of air
pollution from a great variety of sources. These
abatement efforts have been augmented by federal air
pollution regulations and control measures directed
toward emissions from new motor vehicles and certain
industrial sources. Indirect federal assistance has
included dissemination of information on new air
pollution control technology, studies of the health
effects of air pollution, and the coordination of state
efforts.
What has been accomplished by this considerable
expenditure of time, material, and funds? How does air
quality vary across the United States, and how has it
improved during recent years? To help answer these
questions, the U.S. Environmental Protection Agency
(EPA) published the National Air Quality and Emissions
Trends Report. 1977* in December 1978. It contained
estimates of the quality of the Nation's air, emission
levels by source category, the exposure of selected
urban populations to air pollution, and comparisons with
figures from previous years.
Highlights of that detailed report with additional
recent data are contained in this booklet.
w Street
'Copies can be obtained from the EPA Office of Air Quality
Planning and Standards, Research Triangle Park, N.C. 27711.
-------
Summary
More than 8,000 air monitors across the country
measure and record air pollution levels. This report
covers data accumulated from these stations relative to
five principal pollutants. The data is largely quite
encouraging. Not all reports are good, but there is more
good news than bad about the quality of the Nation's air.
For example:
• The level of particulates in the air (particles such as
soot and dust) decreased 32 percent between 1960 and
1979.
• Sulfur dioxide in large urban areas decreased 67
percent from 1964 to 1979.
• Carbon monoxide in center-city locations fell 36
percent from 1972 to 1979 declining seven percent per
year.
• Ozone (smog) has shown no long-term national trend,
from 1974-1979, which is consistent with the trend in
ozone precursor emissions. The precursor, volatile
organic compounds, has remained relatively constant,
although the contribution from various types of sources
has changed significantly. Emissions from transportation
have decreased, while emissions from industrial
processes increased. While a 3 percent improvement in
ozone levels was observed between 1978 and 1979,
which followed a similar decrease in emissions, it will
take a longer period of time to establish whether this is
an actual trend.
• Nitrogen dioxide levels have increased 15 percent
between 1975 and 1979 at 180 sites with five years of
data, corresponding to a 12 percent increase in
emissions. The increase in emissions is due to an
increase in emissions from transportation, as well as
increasing emissions from fuel combustion in stationary
sources. While this trend is cause for concern, it is
important to realize that only 3 percent of the nitrogen
dioxide measurements at 933 sites with data exceeded
the health-related standard in 1979.
While no one who lives near a large city or industrial
complex would claim, on the basis of these figures, that
the millenium has been reached, the records of ambient
air monitors across the United States do show that we
are making significant progress against air pollution.
The source of the encouraging figures, their
significance in terms of health, and what they tell us
about programs for pollution abatement are discussed m
the pages which follow.
US. Environment-' •" election
-------
Clarifying the
Dicture
Most of .our information about pollution levels is derived
from two related sources. (1) estimates of pollution
emissions,* and (2) measurements of ambient air quality.
Emission estimates and pollution measurements are re-
ferred to throughout this booklet, so it is important at the
outset to have a clear understanding of what they involve.
Emission estimates are what the name implies—
estimates of the amount and kinds of pollution being
generated by automobiles, factories, and other sources,
based on the best available engineering calculations.
Emission estimates tell us how much total pollution is
being released into the air in a given area.
Ambient air quality measurements, on the other hand,
tell us the amount and kinds of pollutants that are in the
air we breathe at a given time and place. Measurements
of ambient air quality are taken routinely by monitoring
stations operated by state and local agencies.
So, emission estimates tell us how much and what
kinds of pollution are being generated; ambient air
quality readings tell us how much of that is in the air
we breathe. Differences between estimates and measure-
ments (readings) occur because of such things as local
changes in weather and wind direction, the use of tall
smokestacks, and pollution blown in from distant points.
Air monitoring stations have been operating for a
number of years. There are thousands of air monitors
across the country routinely measuring local levels of air
pollution. Sufficient data has now been compiled from
these monitors to reveal local and national trends in
levels of five of the most widespread pollutants.
The five are: particulates (particles such as soot and
dust), sulfur dioxide, carbon monoxide, ozone, and
nitrogen dioxide. All five are known or suspected causes
of illness or disease, and they often occur in concentra-
tions that are above the health standards set by EPA.
By monitoring the levels of these pollutants locally,
officials can identify potential health problems and take
steps to correct them. Studying cumulative records from
many monitoring stations reveals reglorTakarjd national
trends in air pollution which may require broader
corrections.
For example, the cumulative record of monitoring
stations has confirmed that air pollution does not
recognize state lines. Wind and weather patterns
regularly transport pollutants from their source to places
far away. An example is the current pressing problem of
acid rain which falls in the Northeast and which may
have its origins from sulfur dioxide and nitrogen oxide
emissions transported from the industrial Midwest.
'Emission estimates Jor 1979 are preliminary and may be
revised slightly to reflect the latest information being gathered
by EPA.
-------
Particulates
Total suspended participate matter is the general term
for particles found in the atmosphere. In addition to soot
and dust, particulates are composed of organic matter
and compounds containing sulfur, nitrogen, and metals.
Some particles may be formed in the air as a result of
various chemical and physical processes, so the
chemical composition of particulates varies widely,
depending upon location and time of year. Certain
components of particulates are considered to be inactive
in the human body. Others, such as sulfates, nitrates,
and metals, are being studied to determine their
contribution, if any, to the adverse health effects
observed from elevated particulate levels. When
airborne particles are inhaled, they may irritate the
respiratory system, or damage the clearance mechanism
of the lungs, thereby contributing to acute respiratory
illnesses in much the same way as gaseous pollutants
do. Prolonged inhalation of certain components of
airborne particles may increase the number of cases
and the severity of chronic respiratory diseases.
Ambient levels of total suspended particles (largely
soot and dust) decreased 32 percent during the 20-year
period from 1960 to 1979, an improvement of about 2
percent per year. From 1970 to 1979, particulate
emissions decreased 50 percent due to the control of
industrial emissions. Actual measurements of air
quality, however, did not show the same rate of
improvement. The difference is attributed to low-level
fugitive emissions from industry and to windblown dust.
Dust levels remain fairly stable over time.
Some sections of the country show more
improvement than others. The Northeast, Great Lakes
and Southern states show high rates of improvement,
while Western states show little change. In the West,
agricultural and natural sources continue to be major
contributors of windblown particulates.
Despite the improvements, total suspended
particulates remain a problem. Approximately 21
percent of the Nation's population live in areas where
the annual standard is exceeded. Although there has
been a nationwide decrease in levels of total suspended
particulate matter, there is evidence that atmospheric
levels of particulates in some size ranges are Increasing.
This is indicated by the increasing levels of small
particulates such as sulfates, and the deterioration of
visibility in the Southwest and non-urban areas of the
East. These patterns are consistent with the growth of
emission sources outside large metropolitan areas.
-------
National Trend in
Werage Paniculate
.evels. 1960-1979
Graph A)
This shows year-by-year changes in average ambient
particulate levels based on measurements taken
throughout the country. Nationally, ambient particulate
levels measured in 95 urban areas with a long history of
data dropped dramatically between 1960 and 1971. This
was followed by a more gradual decline from 1972 to
1979, as measured at more than 3000 monitoring sites.
The rapid improvement in the late 1960's was largely due
to the emission control programs of state and local air
pollution control agencies. The improvement in air quality
during the latter half of this period is due mainly to control
of traditional particulate sources such as fuel combustion,
solid waste disposal operations, and industrial process
emissions. The slight reversal in the downward trend in
1976 is due to an increase in windblown dust caused by
abnormally dry weather.
Mational Trend in
'articulate Emissions
Graph B)
The dramatic 50 percent reduction in particulate
emissions from 1970 to 1979 results largely from
installation of control equipment on industrial processes,
reduced coal burning by non-utility users, installation of
control equipment by electric utilities that burn coal, and
a decrease in the burning of solid waste.
Total Suspended
Particulate
Concentration
ug/m3
70 71 72
Emissions,
Million Tons/Year
99 Transportation • Fuel Combustion in Stationary
Sources Industrial Processes •Solid Waste and
Miscellaneous
-------
Sulfur
Dioxide
Sulfur dioxide is one of a number of sulfur-containing
compounds found in the atmosphere. It enters the air
primarily from the burning of coal and oil, but also from
various other industrial processes. Studies of serious air
pollution episodes have found an increase in death rates
among people with existing heart and lung disease
when high concentrations of sulfur dioxide are present
in combination with high concentrations of total
suspended paniculate matter. Even when
concentrations are below the level of a serious episode,
there is a noticeable increase in acute and chronic
respiratory disease.
Sulfur dioxide reacts in the atmosphere to form other
compounds such as sulfuric acid, sulfates, and sulfites.
These may be more irritating to the respiratory system
than sulfur dioxide. However, not enough is known
about these pollutants at present to permit EPA to take
any specific regulatory steps other than controlling
sulfur dioxide, which generally lowers the
concentrations of the other sulfur compounds.
Levels of sulfur dioxide (SO2) in the air over the
Nation's urban areas decreased by 67 percent from
1964 to 1979. Improvement was most rapid during the
1966 to 1971 period because of an increased use of
cleaner burning fuels in the residential, commercial and
industrial sectors of most urban areas. Local and state
air pollution regulations led to a switch from coal and
high sulfur oil to natural gas and low sulfur oil. Between
1970 and 1979, the improvement continued with the
national average SO2 levels (ambient readings) dropping
44 percent. A corresponding 7 percent decrease in
sulfur oxides emissions was observed between 1970
and 1979. The greater improvement in ambient levels
reflects sulfur dioxide trends in urban areas, where
most of the emission reductions have taken place.
Offsetting the emission reductions in the urban areas
was the location of new sources, such as fossil fuel
power stations, in rural areas.
Nationally, the sulfur dioxide problem has diminished
to the point that only a small number of urban areas
now exceed the air quality standard. Some regions
outside major urban areas continue to have high sulfur
dioxide because of single sources, such as non-ferrous
smelters. Today, these individual sources pose the
greatest obstacle to the regional attainment of air
quality standards for sulfur dioxide.
Current plans for converting power plants from oil to
coal could increase levels of SO2.
-------
National Trend in
Average Sulfur Dioxide
Levels, 1965-1979
(Graph A)
Nationally, annual average sulfur dioxide levels measured
at 32 urban locations declined dramatically from 1966 to
1971. This was followed by a more gradual decline from
1972 to 1979 as measured at more than 1,000 monitoring
sites. The improvement in air quality during the first half
of this period is due mainly to (1) the change from coal
with a high sulfur content to natural gas, electricity, and
low sulfur oils from residential and commercial space
heating; (2) strict local emission regulations, which
caused the reduction in sulfur content of coal and fuel oil
or required the installation of control equipment to remove
sulfur; and (3) the location of major new sources, such as
fossil-fuel burning power plants, away from urban areas.
In recent years, most urban areas have attained the sulfur
dioxide standards and are now working to maintain these
lower levels rather than reduce sulfur oxides emissions.
further. There are still problems, however, with large rural
power plants and smelters.
National Trend in
Emission of Sulfur
Oxides, 1970-1979
(Graph B)
Sulfur oxides emissions declined 7 percent from 1970 to
1979. The moderate decline in emissions parallels a
considerable reduction in sulfur dioxide levels in urban
areas. This difference between emission and air quality
trends arises because the use of high sulfur fuels has
shifted from urban areas to a growing number of
sources outside of densely populated areas where there
are fewer other sources.
55
Sulfur
Dioxide
Concentration
ug/m3
77 78 79
Transportation • Fuel Combustion in Stationary
Sources Industrial Processes
-------
Carbon
Monoxide
•30C
73 74 75 76 77 78 79 80
MS Ambient Carbon Monoxide
(parts per million)
• Gasoline Consumption
(millions of gallons)
Source: New Jersey
Department of Environmental
Protection
Carbon monoxide is a byproduct of the incomplete
burning of fuels—notably by cars and trucks. It is also
released by some industrial processes. In some urban
areas, automobiles and other modes of transportation
are responsible for over 99 percent of these emissions,
although any city with heavy traffic may have a potential
problem from carbon monoxide. In some cases, the
problem is highly localized with only a few street
corners experiencing high carbon monoxide levels. In
other cases, the problem is spread throughout the
center-city area and along major commuter corridors.
Inhaled, carbon monoxide enters the blood stream and
binds chemically to hemoglobin, the substance that
carries oxygen to the cells. This reduces the amount of
oxygen delivered to all tissues of the body. The percentage
of hemoglobin inactivated by carbon monoxide depends
on the amount of air breathed, the concentration of
carbon monoxide in air, and the length of exposure.
Cigarette smoke also contains carbon monoxide.
Therefore, cigarette smokers have a portion of their
hemoglobin inactivated by this source as well as by
external air pollution.
Carbon monoxide weakens the contractions of the
heart, reducing the amount of blood pumped to various
parts of the body and, as a result, reduces oxygen
available to the muscles and various organs. In a
healthy person, this effect significantly reduces the
ability to perform physical exercise. In a patient with
heart disease who is unable to compensate for the
decrease in oxygen, the effect can threaten life. People
with coronary artery disease show changes in their
electrocardiograms after having been exposed to carbon
monoxide from heavy freeway traffic.
Individuals with anemia, emphysema, and other lung
disease, as well as those living at high altitudes, are likely
to be more susceptible to the effects of carbon monoxide.
Even at relatively low concentrations, carbon monoxide
can affect mental function, visual acuity, and alertness.
Nationally, ambient carbon monoxide levels in center-
city locations have shown a steady decline. From 1972
to 1979 carbon monoxide levels dropped at a rate of 7
percent per year with an overall reduction of about 36
percent. The greatest improvements occurred at sites
traditionally having the worst carbon monoxide problem.
Estimates of nationwide carbon monoxide emissions
from highway vehicles, in contrast, show only a 7
percent decrease since 1972. The smaller reduction in
CO emissions is largely due to a 35 percent increase in
total vehicle miles travelled since 1970. The
improvement in average concentrations is greater than
the reduction in CO emissions, because the trend
reflects levels at traffic-saturated monitoring sites in the
center city. These sites have recorded little or no change
in vehicle miles travelled. Therefore, the ambient carbon
monoxide trend reflects the reduction in emissions from
new cars brought about by federal standards on vehicle
emissions.
-------
New Jersey
Carbon Monoxide
Levels
(Graph A)
The significant improvement in ambient carbon
monoxide levels at all sites, even after accounting for
changing weather conditions, is attributable to state and
federal emission reduction programs.
Short-term changes in emission rates can also signifi-
cantly influence carbon monoxide levels. Witness the
effect of the 1974 gasoline shortage when a substantial
drop in gasoline consumption caused a rapid reduction
in carbon monoxide levels during the winter of 1973-74.
National Trend in
Average Carbon
Monoxide Levels,
1972-1979
(Graph B)
This graph shows year-by-year changes in annual average
carbon monoxide levels at 223 urban sites over the 8 year
period, 1972-1979. Carbon monoxide levels improved at a
rate of 7 percent per year with an overall reduction of
about 36 percent. The improvement reflects levels at
traffic-saturated monitoring sites in the center-city. Since
these sites have experienced little or no change in the
number of vehicles in their vicinity, the improvement in
carbon monoxide levels reflects the reductions in
emissions from new cars brought about by federal
standards on vehicle emissions.
National Trend in
Emissions of
Carbon Monoxide,
1970-1979
(Graph C)
Carbon monoxide emissions in 1979 were 5 percent
lower than in 1970. Highway vehicles are the main
source of this pollutant, and there was an increase of
35 percent in total vehicle miles travelled during this
period. The increase in traffic offsets the decrease in
emissions per car achieved by recent pollution control
measures. Outside transportation, relatively small
reductions in carbon monoxide emissions were obtained
in solid waste disposal and agricultural burning.
Carbon Monoxide
Concentration
mg/m3
Emissions
Million Tons/Year
70 71 72 73 74 75 76 77 78 79
,* Transportation HFuel Combustion in Stationary Sources
Industrial Processes •Solid Waste and Miscellaneous
-------
10
Ozone
Unlike the other pollutants discussed here, ozone is not
emitted directly by specific sources. Instead, it is formed
in the air by chemical reactions between nitrogen oxides
and volatile organic compounds, such as the vapors of
gasoline, chemical solvents, and combustion products of
various fuels. Since these reactions are stimulated by
sunlight, ozone reaches peak levels in most parts of the
country during the summer. This type of pollution first
gained attention in the 1940's as Los Angeles "smog."
Since then, photochemical smog has been observed
frequently in many other cities as well.
Ozone, the main constituent of photochemical oxidants,
and peroxyacyl nitrates are associated with a number of
health effects in humans. Peroxyacyl nitrates and other
chemicals, such as aldehydes, cause the eye irritation
that is characteristic of photochemical pollution.
Ozone severely irritates the mucous membranes of
the nose and throat. It impairs normal functioning of the
lungs and reduces the ability to perform physical
exercise. Its effects are more severe in individuals with
chronic lung disease. The length of exposure, frequency
of exposure, and ozone concentration are significant
factors in determining the effects. Individuals with
asthma or diseases of the heart and circulatory system
experience symptoms at lower ozone concentrations. It
also appears that ozone in combination with sulfur
dioxide has a greater effect on respiratory function than
either pollutant alone.
Ozone has long been a major air pollution problem in
Los Angeles and other parts of southern California.
These areas have abundant sunlight and high emissions
from heavy motor vehicle traffic—a major source of
volatile organic compounds and nitrogen oxides. In
recent years, ozone has become increasingly important
in most major urban areas as motor vehicle traffic
has increased and the levels of other pollutants have
dropped. Particularly affected is the urban area from
Washington to Boston as well as areas around Chicago,
Milwaukee, Denver, and Houston.
Measurements of ozone taken over a 6-year period,
1974-1979, do not reveal a long-term national trend.
This is consistent with the trends in the ozone precursor
emissions, volatile organic compounds. The trend in
volatile organic compounds has remained relatively
stable, although the contribution from various types of
sources has changed significantly. Emissions from
transportation have decreased despite an 18 percent
increase in vehicle miles driven between 1974 and
1979, because of the increasing effectiveness of
emissions controls. Emissions from industrial sources
have increased because of increased industrial
production. Between 1978 and 1979 an encouraging 3
percent drop in ozone levels was observed,
corresponding to a 3 percent decrease in emissions. The
decrease in emissions occurred, primarily, because
volatile organic compound emissions from industrial
-------
11
The National Trend in
Ozone Levels, 1974-
1979
(Graph A)
processes remained relatively unchanged, while
emissions from motor vehicles dropped. This drop is
largely due to the increased level of emissions control
and a reduction in motor vehicle miles driven between
1978 and 1979.
This shows year-by-year changes in the composite May
through October average of daily maximum hour ozone
values at 230 urban sites over the 6-year period, 1974-
1979. Ozone reveals no long-term trend. The lack of trend
is consistent with a similar lack of trend in volatile organic
compound emissions.
National Trend in
Emissions of Volatile
Organic Compounds
(Graph B)
Volatile organic compound emissions in 1979 were 4
percent lower than in 1970. The main sources of this
pollutant are transportation and industrial processes.
Emissions from transportation decreased, despite a 35
percent increase in vehicle miles driven during this
period, while industrial process emission increased over
the decade. Additional decreases occurred in the solid
waste and miscellaneous categories.
Ozone
Concentration,
pphm
70 71
Emissions,
Million Tons/Year
Transportation •Fuel Combustion in Stationary
Sources Industrial Processes •Solid Waste and
Miscellaneous • Non-Industrial Organic Solvent
-------
12
Ozone Northeast
Corridor Population
Exposure
(Summer 1979)
(Map)
High ozone levels have been measured for many years in
the densely populated northeast corridor which extends
from Washington to Boston. This map of the counties
in the region shows the estimated number of days in
one summer during which the ozone levels exceeded
0.12 part per million of air (the current standard).
About 61 percent of the population in this region lives
in areas which experienced 5 or more days above the
ozone standard. Approximately 30 percent lives in areas
which experienced 10 or more days above the standard.
These areas include most of Connecticut, eastern Long
Island, and a few counties in Pennsylvania and New
York. Large areas with less severe ozone levels—less
than 5 days above the standard—include central Massa-
chusetts and sections of Maryland and Pennsylvania.
Estimated Number of Days
Exceeding Ozone Standard
In Summer 1979
• Oto4
• 5to9
• 10 or more
76
77
78
79
Nitrogen Dioxide
Concentration
ug/m3
B
30
70 71 72 73 74
Emissions
Million Tons/Year
75 76 77 78 79
H Transportation •Fuel Combustion in Stationary
Sources 'l Industrial Processes
-------
13
Nitrogen
Dioxide
National Trend in
Average Nitrogen
Dioxide Levels,
1975-1979
(Graph A)
National Trend in
Emissions of Nitrogen
Oxides, 1970-1979
(Graph B)
Nitrogen dioxide is one of a family of nitrogen oxides.
The oxides important to air pollution control usually
come from high-temperature combustion. Nitrogen
dioxide plays a major role in the atmospheric reactions
which produce photochemical oxidants (smog) and is
primarily responsible for smog's yellow-brown color.
Continued or frequent exposure to high levels of
nitrogen dioxide can cause pulmonary edema. At the
present time, however, such high concentrations seldom
occur.
The health effects of exposure to high concentrations
of nitrogen dioxide for short periods are not clear and
are still being studied. Individuals with chronic
bronchitis and emphysema may have their symptoms
aggravated. Animal studies suggest that such exposure
impairs resistance to infection.
In the years 1975 through 1979 nitrogen dioxide
levels increased about 15 percent at 180 trend sites
with 5 years of data, while emissions for the same
period were up 12 percent. In 1979, only 3 percent of
the nitrogen dioxide measurements at 933 sites with
data exceeded the health-related ambient air quality
standard.
Most of the increase in nitrogen dioxide emissions
came from motor vehicles and electric utility generating
plants. Although the emission rates for motor vehicles
and generating plants has been steadily reduced,
increased demands have more than offset the
reductions. During the 1974-1979 period the number of
miles travelled by all types of motor vehicles increased
substantially and higher electricity demands caused
utilities to burn more fuel.
These are year-by-year changes in average nitrogen
dioxide levels based on measurements obtained at 180
sites with 5 years of data. There is an increasing trend
over the period with nitrogen dioxide levels rising 15
percent. The increase in nitrogen dioxide levels
corresponds to increases in nitrogen oxides emissions
from transportation and fuel combustion in stationary
sources.
Nitrogen oxides emissions increased 18 percent from
1970 to 1979. The increase in nitrogen oxides
emissions resulted primarily from increased fuel use by
stationary sources and increased highway motor vehicle
travel. Vehicle miles driven increased 35 percent over
the decade. During this same period industrial process
emissions remained relatively constant, while solid
waste and miscellaneous emissions decreased.
-*.' c*» •
-------
------- |