NATIONAL AIR QUALITY AND EMISSIONS TRENDS REPORT, 1992

Chapter 1:  Executive Summary

1.1  Introduction

This is the twentieth annual report 1-19 documenting air pollution
trends in the United States.  As in previous years, the primary
emphasis is on those pollutants for which the U.S. Environmental
Protection Agency (EPA) has established National Ambient Air
Quality Standards (NAAQS).  EPA set these standards to protect
public health and welfare.  Primary standards are designed to
protect public health, while secondary standards protect public
welfare, such as effects of air pollution on vegetation,
materials and visibility.  For the first time, this report
discusses air toxics, another set of pollutants regulated under the
Clean Air Act.  Air toxics are those pollutants known to or
suspected of causing cancer or other serious health effects, such
as reproductive effects or birth defects.  Because ambient data on
air toxics is limited, this report simply provides an
introduction to the subject and an overview of the types of air
toxics information that future reports may provide as additional
data becomes available.  

The analyses in this report focus on comparisons with the primary
standards in effect in 1992 to examine changes in air pollution
levels over time, and to summarize current air pollution status. 
The six pollutants with National Ambient Air Quality Standards are:
carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone
(O3), particulate matter whose aerodynamic size is equal to or less
than 10 microns (PM-10), and sulfur dioxide (SO2).  It is important
to note that the discussions of ozone in this report refer to
ground level, or tropospheric, ozone and not to
stratospheric ozone.  Ozone in the stratosphere, miles above the
earth, is a beneficial screen from the sun's ultraviolet rays. 
Ozone at ground level, in the air we breathe, is a health and
environmental concern and is the primary ingredient of what is
commonly called smog.

The report tracks two kinds of trends: air concentrations, based on
actual direct measurements of pollutant concentrations in the air
at selected sites throughout the country; and emissions, which are
estimates of the total tonnage of these pollutants released into
the air annually based upon the best available engineering
calculations.  The estimates of emissions in this report differ
from those reported last year.  Emissions are now reported in units
of short tons per year (2,000 pounds), rather than in metric tons
(2,205 pounds) as used in earlier reports.  Also, the report
reflects a mixture of new estimation
methodologies for fuel combustion, industrial, and transportation
sources, and includes data obtained from a new model which was used
to update mobile source emissions in response to concerns about
possible underestimates of the mobile source contribution to total
emissions.  This is discussed in Chapter 3. 

The first three chapters of this report cover trends in the six
pollutants with National Ambient Air Quality Standards.  Chapter 4
presents the information on air toxics. Chapter 5 includes a
detailed listing of selected 1992 air quality summary statistics
for every metropolitan statistical area (MSA) in the nation. 
Chapter 6 presents 1983-92 trends for 23 cities throughout the U.S. 
Chapter 7 presents summary air pollution statistics from other
countries to provide a broader range of air pollution information.

1.2  Major Findings

CARBON MONOXIDE

Air Concentrations

        1983-92:    34 percent decrease (8-hour second high at 308 sites)
                    94 percent decrease (8-hour exceedances at 308 sites)

        1991-92:     7 percent decrease (8-hour second high at 390 sites)

Emissions

        1983-92:   25 percent decrease 

        1991-92:    4 percent decrease 

Overview

Trends.   Improvements continued with the 1983-92 ten year period
showing 34 percent improvement in air quality levels and a 25
percent reduction in total emissions.  The air quality improvement
agrees more closely with the estimated 30 percent reduction in
highway vehicle emissions.  This progress occurred despite
continued growth in miles of travel in the U.S.  Transportation
sources account for approximately 80 percent of the nation's CO
emissions.  The 30 percent decrease in highway vehicle emissions
during the 1983-92 period occurred despite a 37 percent increase in
vehicle miles of travel.  Estimated nationwide CO emissions
decreased 4 percent between 1991 and 1992.

Status.  In November 1991, EPA designated 42 areas as nonattainment
for CO.  Based upon the magnitude of the CO concentrations, 41 of
these areas were classified as moderate and 1 (Los Angeles) was
classified as serious.  In September 1993, Syracuse, NY became the
first of these 42 nonattainment areas to be redesignated as an
attainment area.  

Some Details.  The first major clean fuel program under the 1990
Clean Air Act Amendments is the oxygenated fuel program implemented
by state and local agencies following EPA guidelines.  Increasing
the oxygen content of gasoline reduces CO emissions by improving
fuel combustion, which is typically less efficient at cold
temperatures.  On November 1, 1992, new oxygenated fuel programs
began in 28 metropolitan areas.  These programs generally run from
November through February and preliminary results suggest greater
CO air quality improvements, with peak CO levels declining 13
percent in areas with the new oxy-fuel program as compared to a 3
percent decline in non-program areas.

CO EFFECTS

      Carbon monoxide enters the bloodstream and reduces the
delivery of oxygen to the body's organs and tissues.  The health
threat from carbon monoxide is most serious for those who suffer
from cardiovascular disease, particularly those with angina or
peripheral vascular disease.  Healthy individuals also are affected
but only at higher levels.  Exposure to elevated carbon monoxide
levels is associated with impairment of visual perception, work
capacity, manual dexterity, learning ability and performance of
complex tasks.

LEAD

Air Concentrations

        1983-92:   89 percent decrease 
                    (maximum quarterly average at 203 sites) 

        1991-92:    9 percent decrease 
                    (maximum quarterly average at 235 sites)

Emissions

        1983-92:   89 percent decrease in total lead emissions    
                    (96 percent decrease in lead emissions 
                    from transportation sources)

        1991-92:    3 percent increase in total lead emissions    
                    (6 percent decrease in lead emissions 
                    from transportation sources)

Overview

Trends.   Ambient lead (Pb) concentrations in urban areas
throughout the country have decreased 89 percent since 1983.  Total
Pb emissions have also dropped 89 percent since 1983 due
principally to reductions from automotive sources.  The drop in Pb
consumption and subsequent Pb emissions was brought about by the
increased use of unleaded gasoline in catalyst-equipped cars (99
percent of the total gasoline market in 1992) and the reduced Pb
content in leaded gasoline.

Status.   In 1991, EPA announced that 12 areas would be designated
as nonattainment because of recorded violations of the National
Ambient Air Quality Standard for lead.  EPA also designated as
"unclassifiable" 9 other areas for which existing air quality data
are insufficient at his time to designate as either attainment or
nonattainment.  On April 22, 1993, EPA designated one of the
unclassifiable areas as nonattainment.

Some Details.   The large reduction in lead emissions from
transportation sources has changed the nature of the ambient lead
problem in the U.S.  In 1983, estimated lead emissions were 49,232
tons and 91 percent was due to transportation sources. In 1992,
estimated lead emissions had dropped to 5,176 tons and
transportation sources accounted for 31 percent, due to the
remaining fraction of leaded gasoline sales.  Remaining lead
nonattainment problems are associated with point sources, such as
smelters, battery plants, and solid waste disposal.  Consequently,
EPA's current monitoring and control strategies target these kinds
of specific sources.

PB EFFECTS

      Exposure to lead can occur through multiple pathways,
including inhalation of air and ingestion of lead in food, water,
soil or dust.  Lead accumulates in the body in blood, bone and    
soft tissue.  Because it is not readily excreted, lead also affects
the kidneys, liver, nervous system and blood-forming organs. 
Excessive exposure to lead may cause 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 homeostatic      
mechanisms in the body.  Fetuses, infants and children are
especially susceptible to low doses of lead, often suffering
central nervous system damage.  Recent studies have also shown that 
lead may be a factor in high blood pressure and subsequent heart
disease in middle-aged white males.

NITROGEN DIOXIDE

Air Concentrations

            1983-92:    8 percent decrease (annual mean at 183 sites)

            1991-92:    3 percent decrease (annual mean at 235 sites)

Emissions:  Nitrogen Oxides (NOx)                            

            1983-92:    5 percent increase 

            1991-92:    1 percent decrease

Overview

Trends.   Nitrogen oxide emissions are estimated to have increased
5 percent since 1983, with a 9 percent increase in fuel combustion
emissions.  Air quality improved 8 percent since 1983.  The two
primary source categories of nitrogen oxide emissions, and their
contribution in 1992, are fuel combustion (51 percent) and
transportation (45 percent).  Since 1983, emissions from highway
vehicles have remained relatively constant.

Status.   In November 1991, EPA designated Los Angeles as the only
nonattainment area for NO2.

Some Details.   In recent years, Los Angeles was identified as the
only location not meeting the National Ambient Air Quality Standard
for nitrogen dioxide.  In 1992, all monitoring locations in Los
Angeles reported data meeting the federal standard.  This is the
first step towards Los Angeles being redesignated as an attainment
area for nitrogen dioxide.

The scientific community has expressed concerns that previous EPA
emission estimates have underestimated the contribution of
transportation sources.  An extensive off-highway survey did indeed
show marked increases in off-highway emissions from 1983 to 1992. 
However, highway emissions using a new model (MOBILE5) stayed
relatively flat.  The major increases, using new methodologies, are
from electric utilities, industrial sources, and off-highway mobile
sources.  Last year's trend from 1982-1991 showed an 8 percent
decrease in NOx, whereas this year's trend shows a 5 percent
increase.  As methodologies continue to improve, we expect to see
variance in the estimates in future years as well.

NO2 EFFECTS

      Nitrogen dioxide can irritate the lungs and lower resistance
to respiratory infection (such as influenza).  The effects of
short-term exposure are still unclear but continued or frequent   
exposure to concentrations higher than those normally found in the
ambient air may cause increased incidence of acute respiratory
disease in children.  Nitrogen oxides are an important      
precursor both to ozone and to acidic precipitation and may affect
both terrestrial and aquatic ecosystems.  Atmospheric deposition of
NOx is a potentially significant contributor to ecosystem effects
including algal blooms in certain estuaries such as the Chesapeake
Bay.  In some western areas, NOx is an important precursor to
particulate matter concentrations.  

OZONE

Air Concentrations

            1983-92:    21 percent decrease 
                        (second highest daily max 1-hour at 509 sites)                            
                        
                        65 percent decrease 
                        (exceedance days at 509 sites) 

            1991-92:    7 percent decrease 
                        (second highest daily max 1-hour at 672 sites)

Emissions:  Volatile Organic Compounds (VOC)

            1983-92:    11 percent decrease (+5 percent for NOx)             
            1991-92:    3 percent decrease (-1 percent for NOx)

Overview

Trends.   Ground level ozone, the primary constituent of smog, has
been a pervasive pollution problem for the U.S.  Ambient trends
during the 1980s were influenced by varying meteorological
conditions.  Relatively high 1983 and 1988 ozone levels are likely
attributable in part to hot, dry, stagnant conditions in some areas
of the country.  The 1992 levels were the lowest of the 1983-92
period.  While the complexity of the ozone problem and the effects
of meteorological conditions warrant caution in interpreting the
data, there have been recent control measures, such as lower Reid
Vapor Pressure for gasoline resulting in lower fuel volatility and
lower NOx and VOC emissions from tailpipes.  Emission estimates for
volatile organic compounds (VOCs), which contribute to ozone
formation, are estimated to have improved by 11 percent since 1983. 
However, these VOC emission estimates represent annual totals.  NOx
emissions, the other major precursor factor in ozone formation,
increased 5 percent between 1983 and 1992.  While these annual
emission totals are the best national numbers now available,
seasonal emission trends would be preferable.  

Status.   In November 1991, EPA designated 98 nonattainment areas
for O3.  Based upon the O3 concentrations in these areas, EPA
classified 43 areas as marginal, 31 as moderate, 14 as serious, 9
as severe, and 1 (Los Angeles) as extreme.  In June 1992, Kansas
City became the first of these 98 nonattainment areas to be
redesignated as an attainment area.  In December 1992, Cherokee
County, SC became the second.  In September 1993, Greensboro, NC
and Knoxville, TN were also redesignated as attainment areas for
ozone.  

Some Details.   Year to year ozone trends are affected by changing
meteorological conditions.  The 21 percent improvement between 1983
and 1992 is in part due to 1983 being a relatively high year for
ozone.  New statistical techniques to account for meteorological
influences suggest an improvement of 10 percent for the 10-year
period.

O3 EFFECTS

      The reactivity of ozone causes health problems because it
damages lung tissue, reduces lung function and sensitizes the lungs
to other irritants.  Scientific evidence indicates that ambient   
levels of ozone not only affect people with impaired respiratory
systems, such as asthmatics, but healthy adults and children, as
well.  Exposure to ozone for 6 - 7 hours at relatively low      
concentrations has been found to significantly reduce lung function
in normal, healthy people during periods of moderate exercise. 
This decrease in lung function often is accompanied by such
symptoms as chest pain, coughing, nausea and pulmonary congestion.
Though less well established in humans, animal studies have
demonstrated that repeated exposure to ozone for months to years
can produce permanent structural damage in the lungs and accelerate
the rate of lung function loss and aging of the lungs.  Ozone is
responsible each year for agricultural crop yield loss in the U.S.
of several billion dollars and causes noticeable foliar damage in
many crops and species of trees.  Forest and ecosystem studies
indicate that damage is resulting from current ambient ozone
levels. 

PARTICULATE MATTER

Air Concentrations:  Particulate Matter (PM-10)

            1988-92:    17 percent decrease 
                        (based on arithmetic mean at 652 sites)

            1991-92:    9 percent decrease 
                        (based on arithmetic mean at 652 sites)   

Emissions:  PM-10

            1983-92:    3 percent decrease

            1988-92:    8 percent decrease 

            1991-92:    2 percent increase 

Overview

Trends.  In 1987, EPA replaced the earlier total suspended
particulate (TSP) standard with a PM-10 standard.  PM-10 focuses on
the smaller particles likely to be responsible for adverse health
effects because of their ability to reach the lower regions of the
respiratory tract.  Ambient monitoring networks have been revised
to measure PM-10 rather than TSP.  Although PM-10 trends data are
limited, ambient levels decreased 17 percent between 1988 and 1992. 
PM-10 emissions from sources historically included in inventories
are estimated to have decreased 8 percent since 1988 and 3 percent
since 1983.  Nationally, fugitive sources (such as emissions from
agricultural tilling, construction, and unpaved roads) provide 6-8
times more tonnage of PM-10 emissions than sources historically
included in emission inventories.

Status.  In November 1991, EPA designated 70 areas as nonattainment
for PM-10.  

Some Details.  Because many PM-10 monitoring networks
evolved from previously established Total Suspended Particulate
networks, emphasis is being placed on evaluating current PM-10
monitoring networks to be certain that they adequately characterize
problems from these finer particles.  New monitoring techniques,
such as low cost portable monitors, are being used as a tool in
these evaluations.

PM EFFECTS

      Based on studies of human populations exposed to high
concentrations of particles (often in the  presence of sulfur
dioxide), and laboratory studies of animals and humans, the major
effects of concern for human health include effects on breathing
and respiratory symptoms, aggravation of existing respiratory and
cardiovascular disease, alterations in the body's defense systems 
against foreign materials, damage to lung tissue, carcinogenesis
and premature mortality.  The major subgroups of the population
that appear likely to be most sensitive to the effects of      
particulate matter include individuals with chronic obstructive
pulmonary or cardiovascular disease, individuals with influenza,
asthmatics, the elderly and children.  Particulate matter      
causes damage to materials, soiling and is a major cause of
substantial visibility impairment in many parts of the U.S. 

SULFUR DIOXIDE

Air Concentrations

            1983-92:   23 percent decrease (arithmetic mean at 476 sites)                                    
                       31 percent decrease (24-hour second high at 476 sites)           
                          
            1991-92:    7 percent decrease (arithmetic mean at 557 sites)

Emissions:  Sulfur Oxides (SOx)

            1983-92:    no change
 
            1991-92:    < 1 percent decrease

Overview

Trends.   Since 1983, SOx emissions were unchanged while average
air quality improved by 23 percent.  This difference occurs because
the historical ambient monitoring networks are population-oriented
while the major emission sources tend to be in less populated
areas. 

Status.   Almost all monitors in U.S. urban areas meet EPA's
ambient air quality standards for SO2.  Dispersion models are
commonly used to assess ambient SO2 problems around point sources
because it is frequently impractical to operate enough monitors to
provide a complete air quality assessment.  Currently, there are 46
areas designated nonattainment for SO2.  Current concerns focus on
major emitterstotal atmospheric loadings and the possible need for
a shorter-term standard.  Seventy percent of all national SOx
emissions are generated by electric utilities.

Some Details.   The Acid Rain provisions of the 1990 Clean Air Act
Amendments include a goal of reducing SOx emissions by 10 million
tons relative to 1980 levels.  The focus of this control program is
an innovative market-based emission allowances trading program
which will provide affected sources flexibility in meeting the
mandated emission reductions.  This is EPA's first large-scale
regulatory use of market-based incentives and the first allowance
trade was announced in May 1992.  This program is coordinated with
the air quality standard program to ensure that public health is
protected while allowing for cost effective reductions of SO2.

SO2 EFFECTS

      The major health effects of concern associated with high
exposures to sulfur dioxide include effects on breathing,
respiratory illness and symptoms, alterations in the lung's
defenses, and aggravation of existing respiratory and
cardiovascular disease.  The major subgroups of the      
population most sensitive to sulfur dioxide include asthmatics and
individuals with chronic lung disease (such as bronchitis or
emphysema) or cardiovascular disease.  Children and the elderly   
may also be sensitive.  Sulfur dioxide produces foliar damage on
trees and agricultural crops.  It and nitrogen oxides are major
precursors to acidic deposition (acid rain), which is associated  
with a number of effects including acidification of lakes and
streams, accelerated corrosion of buildings and monuments and
visibility impairment.

1.3  Some Perspective

It is important to realize that many of these air quality
improvements during the past ten years occurred even in the face of
growth of emissions sources.  More detailed information on these
emission trends and the updated estimation methodologies are
contained in a companion report.20

While progress has been made, it is important not to lose sight of
the magnitude of the air pollution problem that still remains. 
About 54 million people in the U.S. reside in counties which did
not meet at least one air quality standard based upon data
submitted to EPA's data base for the single year 1992.  Ground
level ozone is the most common contributor with 45 million people
living in counties that exceeded the ozone standard in 1992.  This
is the first year that no areas had measured values exceeding the
nitrogen dioxide standard; previous reports had identified Los
Angeles with annual means not meeting the nitrogen dioxide
standard.  With respect to sulfur dioxide, it is important to note
that while no measured data were submitted to EPA's data base
showing exceedances in 1992, the current sulfur dioxide problems in
the U.S. are associated with point sources and typically identified
by modelling rather than by routine ambient monitoring.  These
statistics, and associated qualifiers and limitations, are
discussed in Chapter 5.  These population estimates are based only
upon a single year of data, 1992, and only consider counties with
monitoring data for that pollutant.  As noted in Chapter 5, there
are other approaches that would yield different numbers.  In 1991,
EPA issued a rule formally designating areas that did not meet air
quality standards.21 Based upon these designations, EPA estimated
that 140 million people live in ozone nonattainment areas.  This
difference between the 140 million and 54 million population
figures is because the formal designations are based upon three
years of data, rather than just one, to reflect a broader range of
meteorological conditions.  Also, the boundaries used for
nonattainment areas may consider other air quality related
information, such as emission inventories and modeling, and may
extend beyond those counties with monitoring data to more fully
characterize the ozone problem and to facilitate the development of
an adequate control strategy.  For the pollutant lead, EPA's
aggressive effort to better characterize lead point sources has
resulted in new monitors that have documented additional problem
areas.   

Finally, it should be recognized that this report emphasizes those
six pollutants that have National Ambient Air Quality Standards. 
As discussed in Chapter 4, there are other pollutants of concern. 
According to industry estimates, more than 2.0 billion pounds of
toxic pollutants were emitted into the atmosphere in 1991, compared
to 2.2 billion pounds for the previous year.22,23  They are
chemicals known or suspected of causing cancer or other serious
health effects (e.g., reproductive effects).  Control programs for
the pollutants discussed in this report can be expected to reduce
these air toxic emissions by controlling particulates, volatile
organic compounds and nitrogen oxides.  However, Title III of the
Clean Air Act Amendments of 1990 provided specific new tools to
address routine and accidental releases of these toxic air
pollutants.  The statute established an initial list of 189 toxic
air pollutants.  Using this list, EPA published a list of the
industry groups (or "source categories") for which EPA will develop
emission standards.  EPA will issue standards for each listed
source category, requiring the maximum degree of emissions
reduction that has been demonstrated to be achievable.  These are
commonly referred to as maximum achievable control technology
(MACT) standards.  EPA is also implementing other programs to
reduce emissions of chlorofluorocarbons, halons, and other
pollutants that are depleting the stratospheric ozone layer, and
pollutants contributing to acid deposition.

1.4  References

1.  The National Air Monitoring Program: Air Quality and Emissions Trends - 
    Annual Report, EPA-450/1-73-001a and b, U. S. Environmental Protection 
    Agency, Office of Air Quality Planning and Standards, Research Triangle 
    Park, NC 27711, July 1973.

2.  Monitoring and Air Quality Trends Report, 1972, EPA-450/1-73-004, U. S. 
    Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, December 1973.

3.  Monitoring and Air Quality Trends Report, 1973, EPA-450/1-74-007, U. S. 
    Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, October 1974.     

4.  Monitoring and Air Quality Trends Report, 1974, EPA-450/1-76-001, U. S. 
    Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, February 1976.

5.  National Air Quality and Emissions Trends Report, 1975, EPA-450/1-76-002, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, November 1976.

6.  National Air Quality and Emissions Trends Report, 1976, EPA-450/1-77-002, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, December 1977.

7.  National Air Quality and Emissions Trends Report, 1977, EPA-450/2-78-052, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, December 1978.

8.  1980 Ambient Assessment - Air Portion, EPA-450/4-81-014, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, February 1981.

9.  National Air Quality and Emissions Trends Report, 1981, EPA-450/4-83-011, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, April 1983.
    
10. National Air Quality and Emissions Trends Report, 1982, EPA-450/4-84-002, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, March 1984.

11. National Air Quality and Emissions Trends Report, 1983, EPA-450/4-84-029, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, April 1985.

12. National Air Quality and Emissions Trends Report, 1984, EPA-450/4-86-001, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, April 1986.

13. National Air Quality and Emissions Trends Report, 1985, EPA-450/4-87-001, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, February 1987.
    
14. National Air Quality and Emissions Trends Report, 1986, EPA-450/4-88-001, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, February 1988.

15. National Air Quality and Emissions Trends Report, 1987, EPA-450/4-89-001, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, March 1989.     

16. National Air Quality and Emissions Trends Report, 1988, EPA-450/4-90-002, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, March 1990.     

17. National Air Quality and Emissions Trends Report, 1989, EPA-450/4-91-003, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, February 1991.

18. National Air Quality and Emissions Trends Report, 1990, EPA-450/4-91-023, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, November 1991.

19. National Air Quality and Emissions Trends Report, 1991, EPA-450/R-92-001, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, October 1992.

20. National Air Pollutant Emission Estimates, 1900-1992, EPA-454/R-93-032, 
    U. S. Environmental Protection Agency, Office of Air Quality Planning and 
    Standards, Research Triangle Park, NC 27711, October 1992.

21. Federal Register, November 6, 1991.

22. 1991 Toxics Release Inventory, EPA-745-R-93-003, U. S. Environmental 
    Protection Agency, Office of Pollution Prevention and Toxics, 
    Washington, D.C. 20460, May 1993.

23. 1990 Toxics Release Inventory, EPA 700-S-92-002, U. S. Environmental 
    Protection Agency, Office of Pollution Prevention and Toxics, 
    Washington, D.C. 20460, May 1992.