Trends in passenger exposure to carbon monoxide inside a vehicle on an arterial highway of the San Francisco Peninsula over 30 years: A longitudinal study

Figures & data

Table 1. Indicators of ambient CO air quality for two counties on the San Francisco Peninsula during the first year of each survey period.

	1980	1990	2000	2010
San Mateo County
Peak 8-hr indicator (ppm)	9.4	5.9	4.4	1.9
Maximum 1-hr concentration (ppm)	13.0	12.0	9.8	3.3
Maximum 8-hr concentration (ppm)	8.3	5.9	4.4	1.7
Days above state 8-hr standard	0	0	0	0
Days above national 8-hr standard	0	0	0	0
Santa Clara County
Peak 8-hr indicator (ppm)	15.5	12.6	7.1	2.5
Maximum 1-hr concentration (ppm)	27.0	18.0	8.9	2.5
Maximum 8-hr concentration (ppm)	16.4	11.0	7.0	2.2
Days above state 8-hr standard	16	4	0	0
Days above national 8-hr standard	12	2	0	0

Note. Sources: Alexis et al. (2001); Cox, Delao, and Komorniczak (2013).

Table 2. CO emissions (tons per day, annual average) for the San Francisco Bay Area by decade.

	1980	1990	2000	2010
Stationary sources	55	65	50	38
Area-wide sources	135	160	169	182
On-road mobile	7615	4446	2203	914
Gasoline vehicles	7597	4414	2177	889
Diesel vehicles	18	32	26	25
Other mobile	529	654	539	484
All sources	8334	5325	2961	1618

Note. Source: Cox et al. (2007, 4–26).

Table 3. Descriptive statistics of measured net in-vehicle CO concentrations (ppm) for four field surveys based on 50 matched cases from each survey.

Field survey	Mean net CO concentration (ppm)	Standard deviation (ppm)	Median net CO concentration (ppm)	Range of the 50 mean net CO concentrations (ppm)
#1: 1980–1981	9.7	3.1	9.2	4.2–19.1
#2: 1991–1992	4.8	1.7	4.8	1.2–11.0
#3: 2001–2002	1.7	0.8	1.5	0.4–5.3
#4: 2010–2011	0.5	0.2	0.5	0.2–1.3

Table 4. Light-duty vehicle exhaust emission standards in grams per mile (g/mile) for carbon monoxide by model year for California and the other 49 states.

Model year	California (g/mile)	Other 49 states (g/mile)
Precontrol	84	84
1966–1967	51	84
1968–1969	51	51
1970–1971	34	34
1972–1974	34	28
1975–1976	9	15
1977–1979	9	15
1980	9	7
1981–1992	7	3.4
1993–1996	3.4 (4.2)^a	3.4 (4.2)^a,b
1997–2002	3.4 (4.2) 1.7 (2.1)^c	3.4 (4.2)^a,b
2003–	1.7 (2.1)	1.7

Notes. Sources: U.S. Department of Transportation (1999, 26); California Air Resources Board (1997, 3–3, 3–9, 3–11); Godish (2004, 280); Johnson (1988, 44); National Research Council (2003, 104); https://www.dieselnet.com/standards/us/ld_ca.php

^aThe emission standards in parentheses are phase-in standards for the 1994 model year in the United States and for the 1993 and 1994 model years in California. The first standard applies to the vehicle’s first 5 yr or 50,000 miles, and the second standard applies for 10 yr or 100,000 miles. Manufacturers must certify a minimum of 40% of their vehicles to the phase-in standard or to the more stringent standards of test procedures.

^bFederal Tier 1 CO standards, which were phased in beginning 1994, include a cold-temperature standard of 10 g/mile measured at 20°F rather than at 75°F under the Federal Test Procedure. California included an identical cold-temperature CO requirement beginning with the 1996 model year.

^cMore stringent exhaust emission CO standards were phased in on low-emission vehicles in California during this period.

Figure 1. One-minute average net CO concentrations for four matched trips in early June of each survey period.

Table 5. Four traffic indicators by field survey.

Traffic indicator	Survey #1: 1980–1981	Survey #2: 1990–1991	Survey #3: 2001–2002	Survey #4: 2010–2011
Number of cases	50	50	50	50
Trip duration (min)
Mean ± 95% C.I.^a	39.6 ± 1.0	35.6 ± 1.2	38.9 ± 1.7	36.3 ± 1.5
SD	3.6	4.2	6.1	5.1
Range	31.2–48.0	29.0–49.0	26.9–53.2	29.6–53.0
Trip speed (mph)
Mean ± 95% C.I.^a	18.0 ± 0.5	20.2 ± 0.6	18.6 ± 0.8	19.9 ± 0.7
SD	1.7	2.2	2.8	2.6
Range	14.8–22.7	14.4–24.4	13.3–26.3	13.4–23.9
Percent of total trip time stopped at traffic lights
Mean ± 95% C.I.^a	27.4 ± 1.5	22.8 ± 1.8	29.4 ± 1.8	28.6 ± 1.8
SD	5.3	6.2	6.4	6.3
Range	17.0–42.2	11.6–41.0	17.7–42.2	19.5–46.9
Surrounding vehicle count at signalized intersections (vehicles)
Mean ± 95% C.I.^a	18.1 ± 1.8	20.9 ± 1.7	26.4 ± 2.1	23.2 ± 1.6
SD	6.3	6.1	7.4	5.7
Range	6.2–32.0	11.7–31.4	2.1–42.9	9.2–34.5

Notes. Source: Ott, Switzer, and Willits (1993) for Survey #1.

^aC.I. is the confidence interval.

Table 6. Model parameters obtained by fitting the lognormal model to the net in-vehicle CO frequency distributions using linear regression.

Field survey	Arithmetic mean (ppm)	Arithmetic SD (ppm)	Coefficient of variation (ν)	Geometric mean (μg)^a(ppm)	Geometric SD (σg)	R^2
#1: 1980–1981	9.7	3.2	0.3	9.2	1.4	0.980
#2: 1991–1992	4.9	2.0	0.4	4.5	1.5	0.932
#3: 2001–2002	1.7	0.71	0.4	1.6	1.5	0.910
#4: 2010–2011	0.51	0.21	0.4	0.47	1.5	0.986

Note. ^a In the lognormal model, the geometric mean is equivalent to the median.

Figure 2. Logarithmic-probability plots of net in-vehicle CO concentrations for four field surveys.  The dots are the measured in-vehicle mean net CO trip concentrations on each field survey, including the outliers, and the straight lines are the lognormal models fitted to these data, along with their geometric means (μ_g) and their geometric standard deviations (σ_g).

Figure 3. A time-series plot of the mean net in-vehicle CO exposures for the 50 trips matched from each of the four field surveys of El Camino Real to reduce the influence of traffic and weather conditions.

Figure 4. Mean net in-vehicle CO exposure measured on El Camino Real versus the annual average CO exhaust emission rates for four field surveys compared with dashed line fitted by linear regression.

Figure 5. VMT-weighted average CO running exhaust emission rates for Santa Clara and San Mateo counties, the San Francisco Bay Area Region, and the state of California for the 1990 to 2010 period.

Table 7. In-vehicle CO concentrations and the estimated annual average CO exhaust emission rates by county and survey period.

			Second-order polynomial model		Third-order polynomial model
Survey period	Mean net in-vehicle  CO concentration for study site^a (ppm)	Percentage reduction relative to 1980–1981	Average CO exhaust emission rate (g/mile)	Percentage reduction relative to 1980	Average CO exhaust emission rate (g/mile)	Percentage reduction relative to 1980
San Mateo County
1980–1981	9.7		44.4		51.9
1991–1992	4.8	50.5	23.4	47.3	23.4	54.9
2001–2002	1.7	82.5	6.9	84.5	6.9	86.7
2010–2011	0.5	94.8	3.0	93.2	3.0	94.2
Santa Clara County
1980–1981	9.7		42.6		48.5
1991–1992	4.8	50.5	23.4	45.1	23.4	51.8
2001–2002	1.7	82.5	6.9	83.8	6.9	85.8
2010–2011	0.5	94.8	3.0	93.0	3.0	93.8

Note. ^aThe means are identical for each county, because the study site spans a portion of each county and the net in-vehicle CO concentration in the table represents the mean value for the entire trip on the study site.

Table 8. Ambient CO concentrations (ppm) at nearest fixed site monitor by field survey^a.

	Survey #1: 1980–1981	Survey #2: 1990–1991	Survey #3: 2001–2002	Survey #4: 2010–2011
Monitor precision	1.0	1.0/0.1^b	0.1	0.1
Hourly CO concentration during field surveys
Average	1.30	1.31	0.61	0.41
SD	0.55	0.57	0.36	0.27
Range	1–3	0.7–4.2	0.3–2.0	0.2–2.0
Number of observations	46	49	46	48

Notes. Sources: California Air Quality Data—Selected data available for download.

http://www.arb.ca.gov/aqd/aqdcd/aqdcddld.htm for 1980–81 survey period (accessed July 2002). https://www.arb.ca.gov/aqmis2/display.php?report=SITE31D&site=2125&year=2017&mon=09&day=11&hours=all&statistic=HVAL&ptype=aqd&param=CO for 1990–91, 2001–02 and 2010–11 survey periods (accessed June 2012). Personal communication with James Cordova, Air Quality Meteorologist, Bay Area Air Quality Management District, July 2002, for 1991–1992 and 2001–2002 survey periods.

^aThe intersection of El Camino Real and Page Mill Road was the midpoint of the study site.

^bThe Bay Area AQMD reported that the precision of the monitor improved from 1 ppm to 0.1 ppm starting January 1, 1991.

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