United States
Environmental Protection
Agency
Environmental Monitoring Systems
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S4-83-025 Aug. 1983
&EPA Project Summary
Carbon Monoxide Concentrations
in Four U.S. Cities During the
Winter of 1981
David Holland and David Mage
Portable monitors were used to meas-
ure time averaged personal exposures
(10 to 30 min) to carbon monoxide.
Data were collected from January
through March 1981 in four cities
where carbon monoxide ambient levels
in excess of National Ambient Air Quali-
ty Standards have been reported: Stam-
ford, CT; Los Angeles, CA; Phoenix, AZ;
and Denver, Co. In each city, personal
exposures were measured in three ty-
pical microenvironment types: indoor,
commuting, and residential driving.
These measurements were made in
the vicinities of fixed monitoring sta-
tions that recorded ambient levels of
carbon monoxide.
The highest indoor personal expo-
sures were recorded in Denver (arith-
metic mean value of 6.1 ppm). The
highest commuting and residential
driving exposures were recorded in
Los Angeles (11.4 ppm and 7.6 ppm,
respectively). Except in Stamford, per-
sonal exposures during commuting
and residential driving activities were
higher than fixed-site ambient concen-
trations. Indoor exposures were lower
than fixed-site concentrations in all
cities except Denver.
For the four cities, the linear relation-
ships between simultaneous fixed-site
and personal exposure measurements
were inconclusive. Any relationship
that may exist between these two types
of measurements is probably very com-
plex. The exposure vs. fixed-site linear
relationships were further complicated
in this study by the large distances
separating many measurements of the
personal exposures and fixed-site con-
centrations.
Ambient concentrations were high-
est during the morning (7 to 8 a.m.)
and evening (4 to 6 p.m.) hours, with
average hourly peak levels of approxi-
mately 6 ppm. An exception to this
pattern was observed in Stamford
where average levels of 10 to 12 ppm
were recorded at one fixed site.
This Project Summary was developed
by the Environmental Monitoring Sys-
tems Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully doc-
umented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Ambient levels of carbon monoxide (CO)
are regulated according to the National
Ambient Air Quality Standards (NAAQS).
The Office of Mobile Sources is responsible
for regulating motor vehicle emissions --
the major source of CO in the urban
environment. When the NAAQS for CO are
violated, the states involved must effect
State Implementation Plans (SIPs) to re-
duce the ambient levels to meet the NAAQS
The fundamental premise is that when a
CO standard is violated at an urban moni-
toring station, the people in the area sur-
rounding the station also are exposed to
CO levels above the NAAQS, therefore, a
SIP is necessary. However, actual human
CO exposures in various indoor and out-
door activity patterns are affected by highly
localized phenomena and may be signifi-
cantly different from ambient CO levels
recorded at fixed stations that are sparsely
distributed in urban areas. These phe-
nomena include vehicle emissions, smok-
ing, and indoor ventilation conditions. Ac-
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cordingly, the "CO Winter Studies" were
designed to collect personal CO exposures
in microenvironments located in the vicinity
of fixed monitoring stations. The data from
the two types of measurements were
compared to determine if individuals were
exposed to the CO levels indicated by the
fixed monitoring stations. For the purposes
of this study, microenvironments are de-
f i ned to be specific locations with air space
of homogeneous CO concentration. Micro-
environment types are groups of similar
microenvironments.
Procedure
The study was conducted during an 8-
wk period in the winter (January to March)
of 1 981 in four U.S. cities where ambient
CO levels above the NAAQS had been
observed. Miniature portable monitors
were used to measure time averaged ex-
posures to CO for 10 to 30 min time
periods. In each city, personal exposures
were measured in three common micro
environment types: Indoor, commuting,
and residential driving. The cities and the
contractors that performed the monitoring
were: Stamford, CT (GCA Corp., Tech-
nology Division); Los Angeles, CA (Science
Applications, Inc.); Phoenix, AZ(Systems,
Science and Software, Inc.), and Denver,
CO(PEDCo Environmental, Inc.). Stamford
was selected because of the high occur-
rence of CO fixed-site measurements in
excess of the 8-h IMAAQS standard of 9
ppm. These measurements are caused by
the monitors' location near a busy inter-
section. Los Angeles was chosen beause
the average commute time is greater than
the national average of about 20 mm. Also,
a 9-person pilot study using personal ex-
posure monitors has been conducted in
Los Angeles. Phoenix was chosen because,
unlike most cities, the highest CO ambient
levels are measured during the evening
hours. These CO levels may be a result of
evening atmospheric inversions that occur
before the evening rush hour traffic period.
Denver was selected because of its high
elevation above sea level. At Denver's
altitude, more automobile exhaust may be
released into the air due to inadequate
adjustment of vehicle engines to the lower
levels of oxygen available for fuel combus-
tion.
Results
Descriptive statistics for each micro-
environment type within each city are
presented in Table 1. The personal ex-
posures within each microenvironment
type were matched by time to the concen-
tration recorded at the nearest fixed site in
each city. For Stamford, summary statistics
are provided for each of the two fixed sites.
For Los Angeles, the personal exposures
are matched to the nearest fixed site. For
Phoenix and Denver, the indoor and com-
muting personal exposures are compared
Table 1. Summary Statistics: Personal Exposures and Fixed-Site Concentrations by Microenvironment Type for Four U.S. Cities
Data source3
Microenvi-
ronment
type
Indoor
Commut-
ing
Residen-
tial
driving
Statistic
Sample size
Geometric mean
Arithmetic
mean
Standard
deviation
Median
Range
Correlation
coefficient
Sample size
Geometric mean
Arithmetic
mean
Standard
deviation
Median
Range
Correlation
coefficient
Sample size
Geometric mean
Arithmetic
mean
Standard
deviation
Median
Range
Correlation
coefficient
Stamford11
Personal
Exposure
659
3.0
5.6
8.0
2.7
0-61
0.14
1341
4.9
6.3
4.7
5.2
0-38
0.12
577
1.8
2.6
2.9
2.0
0-39
0.08
Fixed
Site 1
659
5.0
6.4
3.8
6.8
0-25
0.03
1341
6.6
9.3
5.7
9.6
0-25
0.27
577
4.9
6.2
3.8
6.3
0-29
0.26
Fixed
Site 2
659
1.3
2.0
2.2
1.0
0-12
1341
1.8
2.8
2.9
1.9
0-16
577
1.3
2.0
2.0
1.0
0-8
Los Angelesc
Personal Fixed
Exposure Site
1239
2.3
3.2
2.5
2.6
0-18
0.47
96
15.2
16.4
6.2
15.5
3-42
-0.06
807
6.2
7.6
5.0
7.0
1-38
0.41
1239
3.3
4.2
3.2
3.0
1-21
96
4.0
5.2
4.0
4.0
1-21
807
3.1
4.0
2.9
3.0
1-16
Phoenixd
Personal Fixed
Exposure Site
380
0.3
2.0
2.2
1.5
0-17
0.48
839
4.7
6.4
4.7
5.3
0-50
0.47
58
5.3
6.3
3.1
6.4
0-14
0.34
380
1.6
2.7
2.5
2.0
0-13
839
2.6
3.7
3.0
2.5
0-16
58
1.1
3.1
2.4
3.6
0-9
Denver1*
Personal
Exposure
1953
4.2
6.1
4.8
5.0
0-58
0.33
3584
7.9
10.7
7.6
9.0
0-55
0.24
526
4.2
6.0
5.1
4.5
0-45
0.02
Fixed
Site
1953
4.2
5.0
3.3
4.0
1-24
3584
5.5
6.4
3.9
5.7
1-27
526
2.0
2.9
2.2
2.4
0-10
aAveraging times of personal exposure and fixed data are not equal.
bSummary statistics are provided for each of the two fixed sites.
cPersonal exposures are matched to the nearest fixed site.
dlndoor and commuting personal exposures are compared to data from the urban fixed site.
Residential driving exposures are compared to the urban-residential fixed site.
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to data from the urban fixed site; the
residential driving exposures are com-
pared to the urban-residential fixed site.
The majority of the personal exposures
were not continuous mtime. However, the
fixed site monitors usually recorded con-
tinuous 1-h averages. Thus, the summary
statistics presented in Table 1 were ob-
tained by comparing personal exposures
that were averaged over a portion of a
given hour to hourly averages. This pro-
cedure introduces a fictitious replication
into the analysis because a given fixed
hourly average is repeated for every per-
sonal exposure. Any possible effects of
this replication should be negligible when
comparing the arithmetic means of each
type of measurement because of the large
number of personal exposures in each
microenvironment type (see sample sizes
in Table 1). However, it is important when
comparing ranges and standard deviations
that the averaging time be held constant
due to the potential for CO levels to rapidly
increase or decrease under conditions
found in urban environments. Thus, some
of the statistical results from comparing
personal exposures that usually included
only a portion of a given hour to hourly
averages should be viewed with caution.
Except for Stamford, each personal ex-
posure arithmetic mean was higher than
the corresponding fixed-site mean during
commuting and residential driving. In ad-
dition, the indoor mean for Denver is
higher than the fixed-site mean. The arith-
metic means and median values indicate
that the highest indoor exposures were
measured in Denver. As expected, the
highest commuting and residential driving
exposures were measured in Los Angeles.
Stamford fixed monitoring station 1, which
is located near heavy construction activity,
recorded the highest fixed-site concentra-
tions.
The sample correlation coefficients, which
provide an empirical measure of the linear
association between the personal exposure
and fixed-site measurements, are very low
(<0.5). The square of the correlation co-
efficient multiplied by 100 provides a
percentage measure of the total sample
variation that is accounted for in the linear
relationship. As measured by the square of
the correlation, none of the linear relation-
ships are strong within any of the micro-
environment types. Any relationship that
may exist between these two types of
measurements is probably very complex
and cannot be adequately described by a
simple linear relationship. The exposure
vs. fixed-site relationships were further
complicated in this study by the large
distances separating many of the personal
exposures and fixed-site measurements.
Plots of the hourly fixed-site arithmetic
means vs. hour of day (Figures 1 through
5) indicate that the highest CO concentra-
tions occurred in the morning and evening
hours. The morning peak occurred at ap-
12 -\
11 -
10 -
proximately 8:00 am. and the evening
peak varied from city to city. As expected,
the evening peak occurred much later in
Phoenix (around 10 p.m.). Also, the Phoenix
evening peak concentration exceeded that
of the morning peak, unlike the pattern
observed in the other cities. Again, Stam-
I
O
O
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to
68 10 12 14 16 18 20 22 24
Time of day
024
Figure 3. Hourly arithmetic means recorded at 10 fixed-site monitoring stations in Los Angeles
6 -
24
Figure 4. Hourly arithmetic means recorded at the urban fixed-site monitoring station in
Phoenix
ford monitoring station 1 recorded the An indirect approach was used to esti-
highest hourly mean concentrations. The mate the exposure of a typical commuting
morning and evening peak levels were office worker. A time-weighted average
approximately 6 ppm in the other three exposure (equation 1) was calculated for
cities. the following activities: morning commute
to office, work at office, midday eating and
shopping, and evening commute to home.
The time periods used to determine average
estimates for each activity were 8-12 a. m.
and 1-5 p.m. for work at the office, 7-9
am. for the morning commute, and 4-6
p. m. for the evening commute. The midday
hours used for the eating and shopping
activities varied from city to city due to the
different monitoring procedures used in
each city. Although the morning and eve-
ning commute time periods overlap the
assumed office hours, these times were
used to provide a representative estimate
of the average CO levels of these activities.
n
Z CO, • t,
time weighted _ i = 1
average exposure n
i=1 ' (1)
where n = number of measurements
recorded during an ac-
tivity time period for the
entire study
t, = averaging time of expo-
sure (min)
CO, = average exposure for
averaging time t,
Table 2 shows the time-weighted average
exposures in each of the five activities and
the estimated maximum 8-h average ex-
posure for a commuting office worker in
each city. The 8-h average value is a time-
weighted combination of the commuting
(maximum of the morning and evening
commute averages), office, store and
restaurant average exposures. These values
were computed by the following equation:
maximum 8-h
average =
1/8 (maximum
commuting
exposure)
+ 6/8 (office
exposure)
+ 1/16 (restaurant
exposure +
store expo-
sure) (2)
This weighted combination assumes a
1 -h commuting time, 6 hours in the office,
1/2-h in a restaurant, and 1/2-h shopping
time during the total time period of 8-h.
Due to the high CO commuting levels, the
commuting office worker in Los Angeles
received the highest average exposure.
This exposure (8.2 ppm) is slightly under
the current 8-h standard The time-weighted
average indoor and commuting exposures
in Los Angeles also are considerably higher
than in the other three cities.
4
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S-\
Figure 5. Hourly arithmetic means recorded at the urban-residential fixed-site monitoring
station in Denver
Table 2. Average Exposure of the Commuting Office Worker in Each City
Average exposure (ppm)
City
Los Angeles
Denver
Stamford
Phoenix
Morning
Commute
17.8
13.9
6.8
10.9
Office
6.9
5.1
3.0
1.9
Restaurant
7.1
6.8
5.1
3.4
Store
5.8
5.2
2.4
1.6
Evening
Commute
15:2
11.5
6.0
6.9
Estimated
for 8-h
8.2
6.3
3.6
3.1
Conclusions
The findings of this study of CO concen-
trations in Stamford, Los Angeles, Phoenix,
and Denver are summarized below:
• Except in Stamford, personal expo-
sures during commuting and resi-
dential driving were higher than fixed-
site ambient concentrations (arith-
metic mean values).
• Except in Denver, indoor exposures
were lower than fixed-site ambient
concentrations (arithmetic mean
values).
• The highest indoor exposures were
found in Denver (arithmetic mean
value of 6.1 ppm).
• The highest commuting and resi-
dential driving exposures were found
in Los Angeles (arithmetic mean values
of 16.4 ppm and 7.6 ppm, respec-
tively).
• Hourly fixed-site arithmetic mean
ambient concentrations in all four
cities were highest in the morning (7
to 8 am.) and evening rush hours (4
to 6 p.m.j. Except for a fixed monitor-
ing station located near a construction
site and busy intersection in Stamford,
where peak levels were 10 to 12
ppm, morning and evening peak levels
were approximately 6 ppm.
• In all four cities, regressing personal
exposures on simultaneous fixed-site
ambient concentrations resulted in
inconclusive linear relationships. Any
relationship that may exist between
these two types of measurements is
probably very complex and cannot
be adequately described by a simple
linear relationship.
• The highest estimated 8-h average
exposure for a commuting office
worker was found in Los Angeles
(8.2 ppm).
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The EPA authors David Holland (also the EPA contact, see below) and David
Mage are with the Environmental Monitoring Systems Laboratory, Research
Triangle Park. NC 27711.
The complete report, entitled "Carbon Monoxide Concentrations in Four U.S.
Cities During the Winter of 1981." (Order No. PB 83-224 907; Cost: $10.00.
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
David Holland can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park. NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
.Ktfct
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