2
Air Quality, Emissions, and Health Impacts Overview

OVERVIEW OF POLLUTANTS AND STANDARDS

The primary goal of air quality management is the protection of public health and welfare. The governing legislation that exists today is the federal Clean Air Act (CAA), which was established in 1963 (PL 86-493) and substantially amended in 1967, 1970, 1977, and 1990. Several amendments to those CAAs also occurred between full CAA reviews and formal legislative revisions, particularly during the 1960-1980 period. The CAA provides the regulatory framework for air quality management, including mobile-source emissions. The management framework has five goals: mitigate ambient concentrations of criteria pollutants (described below), limit exposure to hazardous air pollutants (HAPs), protect and improve visibility in pristine areas, reduce emissions that cause acid deposition, and curb the use of stratospheric ozone-depleting chemicals (NRC 2004). The reduction of mobile-source emissions plays a key role in attaining those goals and is dealt with explicitly in the CAA, as discussed in Chapter 3 of this report. Regulation of mobile-source emissions is aimed predominantly at mitigating criteria pollutants; however, these sources also emit pollutants that contribute to air toxic exposures (also called hazardous air pollutants or HAPs), acid deposition, visibility degradation, and greenhouse gas concentrations.

The benefits from the CAA and its amendments have been substantial. The economic benefits to public health from improved air quality have outweighed the overall costs required to implement all mitigation strategies (OMB 2004). EPA estimated that the benefits of implementa-



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State and Federal Standards for Mobile-Source Emissions 2 Air Quality, Emissions, and Health Impacts Overview OVERVIEW OF POLLUTANTS AND STANDARDS The primary goal of air quality management is the protection of public health and welfare. The governing legislation that exists today is the federal Clean Air Act (CAA), which was established in 1963 (PL 86-493) and substantially amended in 1967, 1970, 1977, and 1990. Several amendments to those CAAs also occurred between full CAA reviews and formal legislative revisions, particularly during the 1960-1980 period. The CAA provides the regulatory framework for air quality management, including mobile-source emissions. The management framework has five goals: mitigate ambient concentrations of criteria pollutants (described below), limit exposure to hazardous air pollutants (HAPs), protect and improve visibility in pristine areas, reduce emissions that cause acid deposition, and curb the use of stratospheric ozone-depleting chemicals (NRC 2004). The reduction of mobile-source emissions plays a key role in attaining those goals and is dealt with explicitly in the CAA, as discussed in Chapter 3 of this report. Regulation of mobile-source emissions is aimed predominantly at mitigating criteria pollutants; however, these sources also emit pollutants that contribute to air toxic exposures (also called hazardous air pollutants or HAPs), acid deposition, visibility degradation, and greenhouse gas concentrations. The benefits from the CAA and its amendments have been substantial. The economic benefits to public health from improved air quality have outweighed the overall costs required to implement all mitigation strategies (OMB 2004). EPA estimated that the benefits of implementa-

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State and Federal Standards for Mobile-Source Emissions tion of the CAA between 1970 and 1990 were $5-50 trillion greater than the costs (EPA 1997). EPA (1997) estimated that these benefits include about 100,000 to 300,000 fewer premature deaths per year and 30,000 to 60,000 fewer children each year with intelligence quotients below 70. In addition, regulations to improve air quality have helped propel the development of the emission-control industry. Criteria Pollutants The six criteria pollutants are identified as those reasonably anticipated to endanger public health or welfare and those whose presence results from numerous or diverse mobile and stationary sources (CAA section 108(A) (1)). The federal CAA Amendments of 1970 directed EPA to set National Ambient Air Quality Standards (NAAQS) for criteria pollutants and to review the NAAQS at intervals of not more than 5 years and to update them as needed (CAA section 109). Standard concentrations for each pollutant are set at two levels: a primary standard that protects the public health and a secondary standard that protects public welfare, including effects on visibility and agriculture.1 NAAQS were first established in 1971 on the basis of the current scientific knowledge of the effects of the pollutants on health and welfare. The NAAQS of 1971 included carbon monoxide (CO), sulfur dioxide (SO2), nitrogen dioxide (NO2), total photochemical oxidants, total suspended particles (TSP), and hydrocarbons (HC). Over time, a standard for lead (Pb) was added, TSP was revised to a standard for PM10,2 a standard for ground-level ozone replaced the oxidants standard, and the standard for HC was removed. The most recent NAAQS were promulgated in 1997 when a standard was added for fine PM (PM2.5),3 and the standard for ozone was lowered 1   The term primary and secondary is also used in some instances in air pollution to differentiate between pollutants emitted directly by sources and pollutants formed in the atmosphere. For example, particulate matter emitted from a factory is called primary particulate matter, and particulate matter formed in the atmosphere from sulfur emissions from the same source is called secondary particulate matter. 2   PM10 refers to a subset of particulate matter collected by a sampling device with a size-selective inlet that has a 50% collection efficiency for particles with an aerodynamic diameter of 10 micrometers (μm). 3   PM2.5 refers to a subset of particulate matter collected by a sampling device with a size-selective inlet that has a 50% collection efficiency for particles with an aerodynamic diameter of 2.5 μm.

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State and Federal Standards for Mobile-Source Emissions and changed from a 1-hr average basis to an 8-hr average basis, effectively making the standard more stringent. Mobile sources contribute to ambient concentrations of all criteria pollutants. Air Quality Standards Some of the earliest, most severe, and most persistent air pollution episodes in the United States have been in California, particularly in the Los Angeles basin, and parts of California continue to have the most severe air pollution in the country. State legislation to control air pollution was passed as early as 1947, well before the federal CAA of 1963, and California has often preceded the federal government in establishing air quality and emissions standards. California’s Department of Public Health set ambient standards as early as 1959, and later the California Air Resources Board (CARB), formed in 1969, became the agency authorized by California law to set state ambient air quality standards. The federal CAA authorizes states to adopt ambient air quality standards to protect public health that are more protective than the EPA standards. California is among several states that have adopted separate ambient standards. California’s standards are more stringent than the NAAQS, and the state has adopted standards for additional criteria pollutants. In contrast to the federal standards, California’s ambient standards do not have attainment deadlines. The current federal and California standards are presented in Table 2-1. Nonattainment Areas The CAA mandates that ambient concentrations of criteria pollutants be monitored in urban and rural areas throughout the United States. EPA then determines whether the monitored regions attain the NAAQS based on statistical analysis of monitored data. Figure 2-1 shows nonattainment counties as of September 2005, indicating how many criteria pollutants are in nonattainment in each county. Figure 2-2 shows nonattainment counties for PM2.5 and ozone (with concentrations averaged over 8 hr) and, when compared with Figure 2-1, shows that the majority of nonattainment counties in the United States violate either the ozone or PM2.5 NAAQS. The figures show that nonattainment of criteria pollutants is especially problematic in much of California and the Northeast region

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State and Federal Standards for Mobile-Source Emissions TABLE 2-1 Federal and California Ambient Air Quality Standards Pollutant Averaging Time Federal Standards California Standardsa Primary Primary Secondary Ozone 1 hr 0.12 parts per million (ppm) (235 μg/m3) Same as primary 0.09 ppm (180 μg/m3)   8hr 0.08 ppm (157 μg/m3) Same as primary 0.070 ppm (137 μg/m3) PM10 24 hr 150 μg/m3 Same as primary 50 μg/m3   Annual arithmetic mean 50 μg/m3 Same as primary 20 μg/m3 PM2.5 24 hr 65 μg/m3 Same as primary No separate state standard   Annual arithmetic mean 15 μg/m3   12 μg/m3 CO 8 hr 9 ppm (10 mg/m3) None 9.0 ppm (10 mg/m3), Lake Tahoe 6 ppm (7 mg/m3)   1 hr 35 ppm (40 mg/m3)   20 ppm (23 mg/m3) NO2 Annual arithmetic mean 0.053 ppm (100 μg/m3) Same as primary     1hr     0.25 ppm (470 μg/m3) SO2 Annual arithmetic mean 0.030 ppm (80 μg/m3)       24 hr 0.14 ppm (365 μg/m3)   0.04 ppm (105 μg/m3)   3 hr   0.5 ppm (1,300 μg/m3)     1 hr     0.25 ppm (655 μg/m3) Pba 30-day average     1.5 μg/m3   Calendar quarter 1.5 μg/m3 Same as primary   Visibility 8 hr No federal standards   Extinction coefficient of 0.23 per kilometer; visibility of 10 miles or more

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State and Federal Standards for Mobile-Source Emissions Sulfates 24 hr No federal standards   25 μg/m3 Hydrogen sulfide 1hr No federal standards   0.03 ppm (42 μg/m3) Vinyl chloridea 24 hr No federal standards   0.01 ppm (26 μg/m3) aCARB has identified lead and vinyl chloride as “toxic air contaminants” with no threshold level of exposure for adverse health effects determined. These actions allow for the implementation of control measures at levels below the ambient concentrations specified for these pollutants. Source: Adapted from CARB 2003a.

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State and Federal Standards for Mobile-Source Emissions FIGURE 2-1 Counties designated “nonattainment” for NAAQS as of September 2005. Guam—Piti and Tanguisson Counties are designated nonattainment for the SO2 NAAQS. Partial counties, those with part of the county designated nonattainment and part attainment, are shown as full counties on the map. Source: EPA 2005a. of the United States. Many counties in the United States are nonattainment designated for both ozone and PM2.5 concentrations. Trends On average, monitored concentrations for all criteria pollutants have decreased throughout the nation since 1970. A National Research Council (NRC 2004) assessment of air quality management credits the CAA with these substantial emissions reductions despite growth in population, energy use, and vehicle activity. The exceedances of the NAAQS, however, differ in relative severity from location to location and over time, depending on emissions sources, prevailing meteorology, and ef-

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State and Federal Standards for Mobile-Source Emissions FIGURE 2-2 Counties designated nonattainment for PM2.5 and/or ozone with concentrations averaged over 8 hrs. Designations for ozone were made in June and September 2004. Designations for the PM2.5 were made in April 2005. Source: EPA 2005b. fectiveness of regulation. Ambient Pb concentrations, for example, have been dramatically reduced throughout the nation since the 1980s as a result of a national policy to remove Pb from gasoline. Because of effective regulation of CO emissions, especially from mobile sources, the number of CO nonattainment areas has been reduced from many to only a few, representing another area of success due to the CAA (NRC 2003; Holmes and Russell 2004). In contrast, ground-level ozone remains above the NAAQS in many areas despite decades of precursor reductions. Figure 2-3 shows the number of days per year that ozone concentrations exceeded the NAAQS from 2001 to 2003. The figure shows that areas in California exceed the NAAQS most frequently and that exceedances are common in Texas, the Midwest, and the entire Northeast region. Figures 2-4 and 2-5 presents historical air quality trends in exceedances of the 1-hr maximum ozone concentration and the 8-hr maximum ozone concentration in Los Angeles and New York. Both New York and Los Angeles are approaching the 1-hr NAAQS as a result of air quality improvement programs. The new standard for ozone, however, places both locations in a much more diffi

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State and Federal Standards for Mobile-Source Emissions FIGURE 2-3 Frequency with which the NAAQS for ozone (with concentrations averaged over 8 hr) was exceeded 2001-2003. Source: Witherspoon 2004. FIGURE 2-4 Trends in maximum ozone concentrations averaged over 1 hr for New York and Los Angeles airsheds from 1978 to 2004. Source: EPA 2004a; CARB 2005a.

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State and Federal Standards for Mobile-Source Emissions FIGURE 2-5 Trends in maximum ozone concentrations averaged over 8 hr for New York and Los Angeles airsheds from 1994 to 2004. Source: EPA 2004a; CARB 2005a. cult position. Meeting this standard will require more aggressive ozone precursor control strategies for each region. That position is typical of other nonattainment areas on both the east and the west coasts of the United States. More generally, further progress in improving air quality in the United States is likely to be challenging, especially in meeting the new NAAQS for PM2.5 and ozone and in addressing issues such as regional haze, HAPs, and greenhouse gas emissions (NRC 2004; Chameides et al. 2005). Mobile-source emissions standards, the focus of this report, are promulgated primarily to meet NAAQS, but such standards will also affect other air quality issues, such as mobile-source HAPs and greenhouse gas emissions. For example, EPA analyses show that mobile-source emissions standards programs already in place will yield significant reductions of mobile-source HAPs (EPA 2000a). GROUND-LEVEL OZONE AND FINE PARTICULATE MATTER Ground-level ozone and fine PM currently account for the majority of nonattainment areas in the United States and will be the focus of many

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State and Federal Standards for Mobile-Source Emissions FIGURE 2-6 Schematic of the atmospheric processes involved in the formation of O3 and secondary PM. Major precursors are shown in the boxes with thick sides. Secondary particle components are shown in the boxes with thin, solid sides. Source: NARSTO 2004. Reprinted with permission; copyright 2004, Cambridge University Press. future mitigation efforts. General characteristics of these two pollutants relevant to mobile-source management are presented here. Figure 2-6 shows a schematic representation of ozone and secondary PM formation and the relationships among the atmospheric processes involved in their formation. Later sections expand on the contribution of mobile sources to the concentrations of ozone and fine PM. Ozone Some criteria pollutants accumulate in the atmosphere due to direct emissions and are characterized as primary pollutants. Ozone, in contrast,

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State and Federal Standards for Mobile-Source Emissions is characterized as a secondary pollutant because it is formed almost entirely in the atmosphere. Ozone formation is a highly nonlinear process that depends on sunlight intensity, meteorology, and the emissions and transport of its two major precursors, NOx and HCs. Understanding tropospheric ozone chemistry is a key to understanding how ozone formation depends on concentrations and emissions of its major precursors.4 A detailed description of this chemistry can be found elsewhere (Seinfeld and Pandis 1998; Finlayson-Pitts and Pitts 1999), and the following is meant to give a brief overview of the relationship of ozone to NOx and HC concentrations. Ozone is formed when NO2 disassociates in the presence of sunlight to form NO and a single reactive oxygen atom (O), which can then combine with molecular oxygen (O2) to produce ozone. However, NO can remove ozone by reacting with it to recreate NO2 in a cycle that by itself would not necessarily result in ozone accumulation. Ozone accumulates when NO is converted to NO2 by alternate pathways, thereby eliminating an ozone sink (reaction with NO) and creating a new ozone source (more NO2). The alternate NO to NO2 conversion pathways are driven by HCs and reactive, short-lived species called radicals. The hydroxyl radical (OH) can react with HCs to form new organic (carbon-containing) radicals and inorganic (noncarbon-containing) radicals through multistep reactions with such species as oxygen. These new radicals convert NO to NO2 while regenerating more organic and inorganic radicals in a self-propagating process, including regeneration of OH that can then oxidize a new HC. The NO2 is then available to form ozone as described above. Ozone accumulates when conditions favor this recycling of radicals and NOx. Organic radicals and NO2 can be removed from the system by termination reactions that result in formation of less-reactive or stable compounds, thereby reducing their ability to promote ozone formation. Ozone formation slows or reverses when conditions favor these termination processes. NOx and thousands of different HC species may participate in this process. Because ozone formation is driven by sunlight, as are other important radical reactions, ozone formation (and concentrations) is typically at a maximum in the afternoon and at a minimum before sunrise; however, ozone concentrations also depend on the temporal patterns of emissions and concentrations of available NOx and HCs. 4   The discussion in this report focuses on air pollution of the troposphere or lower (ground-level) atmosphere.

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State and Federal Standards for Mobile-Source Emissions FIGURE 2-17 Historical trends in nonroad sources of (a) CO, (b) NOx, and (c) ROG emissions in California by equipment class, 1975-2004. Source: CARB 2005b.

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State and Federal Standards for Mobile-Source Emissions LINK BETWEEN EMISSIONS AND AIR QUALITY Factors Influencing Variability in Air Quality Mixing, transport, chemical reaction, and physical removal of emitted pollutants directly influence the ambient concentrations to which populations are ultimately exposed. These processes are complex, given the dynamic nature of the atmosphere and the numerous gas and particle air pollutants with different physical and chemical properties. They can also vary significantly from region to region, leading to different ambient pollution levels in regions that have similar emissions. Meteorology and topography influence air pollutant concentrations through effects on vertical mixing, wind speeds, temperature, humidity, and emissions. Atmospheric inversions occur when the temperature of the atmosphere increases with altitude, greatly reducing vertical mixing in the atmosphere. Combined with low wind speeds, inversions prevent air circulation because colder air is trapped near the ground by warmer air above. The California South Coast Air Basin, Fairbanks (Alaska), and Denver (Colorado) are examples of urban areas where vertical and horizontal transport is limited by topography and meteorology (NRC 2003). Such conditions allow pollutants to accumulate and enhance chemical and photochemical transformations due to longer residence times of precursors. In regions with limited ventilation, exceedances of the NAAQS can occur at emissions concentrations that generally do not lead to NAAQS exceedances in other regions. Another phenomenon that can cause regional differences in ambient air pollution concentrations is the long-range transport of pollutants. The northeastern United States experiences increased air pollutant concentrations in part because of long-range precursor and pollutant transport from industrialized regions in the Midwest. Regions affected by long-range transport of pollutants might need to reduce local emissions to a greater extent to meet the NAAQS than areas not affected by regional transport. Spatial and Temporal Variability in Mobile-Source Emissions In addition to the factors discussed above, variability in emissions inventories may affect variability in air quality between and within regions. On-road and nonroad mobile sources are significant contributors to emissions inventories in all major urban areas. Figure 2-18 shows that

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State and Federal Standards for Mobile-Source Emissions FIGURE 2-18 Representative emissions inventories (tons/day) from areas that are in nonattainment of the ozone NAAQS, based on 1-hr averaged concentrations. The California South Coast (SC) and Houston (HG) have average daily inventories. Eastern Massachusetts (EM) and District of Columbia (DC) areas have “ozone season” daily inventories. See Table 2-2 for definitions of emission-source categories in legend. Sources: MADEP 2002; TNRCC 2002; MWCG 2003; SCAQMP 2003. the fraction of total emissions due to mobile sources varies significantly from region to region. For the four regions reported in the Figure, the fraction of total emissions due to mobile sources ranges from 45% to 65% for VOCs and 45% to 89% for NOx. Factors influencing regional differences in mobile emissions include population, activity patterns, regional characteristics of the fleet (the distribution of vehicle types and ages), and control programs in place. The spatial variation of some primary pollutants within a single region can follow the spatial pattern of emissions, including mobile-source emissions. Figure 2-19 shows how CO, black carbon (similar to elemental carbon), and particle number vary in close proximity to a major highway (Zhu et al. 2002). Such primary pollutants emitted from motor vehicles will not have uniform concentrations across a region, and spots with higher concentrations of ambient pollutants can occur at discrete locations. These locations often are in places with high vehicle traffic, although topographical and meteorological conditions also play a role. Mobile-source emissions also vary by day of week and throughout the day, which influences ambient concentrations of primary pollutants,

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State and Federal Standards for Mobile-Source Emissions FIGURE 2-19 Total particle number, black carbon (similar to elemental carbon), and CO concentrations versus downwind distance from a freeway. Source: Zhu et al. 2002. Reprinted with permission; copyright 2002, Air & Waste Management Association. such as CO, and secondary pollutants, such as ground-level ozone. In areas influenced by commuter traffic, emissions from light-duty vehicles typically peak during the morning and afternoon rush hours. Minimal ozone concentrations in urban areas on weekday mornings are common and are attributed to commuter traffic NO emissions that chemically remove ozone, as described previously. Ozone has also been observed to be as high or higher on weekend days than on weekdays in some urban areas, which is known as the weekend ozone effect. (See recent review by Heuss et al. [2003], and analyses by Pun et al. 2003, and Fujita et al. [2003], Blanchard and Tanenbaum [2003, 2005]). These analyses have also found that NOx and, in some cases, CO concentrations are significantly lower on weekends than weekdays. One explanation for reduced precursor concentrations on weekends is reduced emissions on weekends, particularly mobile-source NOx (Chinkin et al. 2003). Because NOx can result in both ozone formation and removal as well as secondary PM

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State and Federal Standards for Mobile-Source Emissions formation, there is some debate as to the effectiveness of NOx emissions reduction for ozone and PM control (Croes et al. 2003; Lawson 2003). HAZARDOUS AIR POLLUTANTS HAPs (also called air toxics) are compounds or compound groups that may cause serious health effects, even at low concentrations. Unlike criteria pollutants, HAPs do not have NAAQS. In the 1990 CAA, Congress mandated that EPA regulate a list of approximately 190 HAPs. The CAA amendments further direct EPA to assess the need and feasibility for emissions standards of HAPs from mobile sources and to regulate these emissions as necessary (CAA section 202(l)). In 2001, EPA issued a rule that identifies 21 HAPs associated with mobile sources, including benzene, 1,3-butadiene, formaldehyde, acetaldehyde, and total PM from diesel exhaust. The EPA rule projects that current mobile emissions standards and fuel formulations that address criteria pollutants are sufficient mitigation strategies for HAPs. Thus, there are no separate federal standards for mobile-source HAPs; however, reducing HAP benefits regulation of criteria pollutants and their precursor emissions from mobile sources. Ambient concentrations of HAPs have not been monitored for as long as criteria pollutants. Ambient concentrations of benzene, an important mobile-source HAP, from 95 urban locations decreased by 47% on average from 1994 to 2000 (EPA 2003a). It should be noted that indoor sources also contribute to exposure of some HAPs. AIR QUALITY HEALTH EFFECTS FROM EXPOSURE TO MOBILE SOURCES Effects on health resulting from exposure to air pollutants depend on a large number of variables, including the contaminants present during the exposure, the toxicity of the contaminants, their concentrations and durations of exposure, the dose, and the health status of the person exposed. Exposure to mobile-source air pollutants typically occurs within the context of exposure to a host of air pollutants from a host of sources, and health effects are probably due to exposure to the mixture and not to any one contaminant or source. However, health effects related to exposure to several key constituents of the mobile-source emis-

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State and Federal Standards for Mobile-Source Emissions sions mixture are well understood. There also is research correlating air pollution health impacts to proximity to major roadways, although an assessment of the research is beyond the scope of this committee. Carbon Monoxide The toxic properties of CO have been extensively characterized and are directly related to the ability of CO to competitively bind to the heme group of hemoglobin. The binding mechanism of CO is identical to that of O2, but the affinity of CO for heme is 234 times as great as that of O2. As a result, when CO is present, it is more likely to bind and remain attached to hemoglobin, creating the potential for hypoxia if the concentrations are high enough (Townsend and Maynard 2002). The acute toxic effects of CO are well known and range from headache and shortness of breath at low percentage of hemoglobin bound to CO (%COHb) to death when the %COHb reaches 50% to 90%. Typical ambient concentrations of CO generally do not produce acute toxic effects; however, in the NAAQS nonattainment area of Southern California, a relationship between chronic exposure to CO and birth outcomes was reported (Ritz et al. 2002). CO at ambient concentrations is also hypothesized to affect the health of persons suffering from cardiovascular disease. For persons with preexisting heart disease, exposure to CO at low concentrations has been shown to have an impact on cardiac function (Allred et al. 1989). Although the evidence is inconsistent, studies of the relationship between exposure to ambient air pollutants and cardiovascular disease have suggested an association between some outcomes and CO (Morris et al. 1995; Schwartz and Morris 1995). CO is unique among the criteria air pollutants because of its significance for both ambient air quality management and public safety. Outdoor exposures to CO at very high and lethal concentrations were reported from motor boat exhaust (CDC 2004), and from farm equipment (CDC 1997). Control of CO through new-vehicle emissions standards has had a significant collateral public-safety benefit through the reduction of accidental CO poisoning (Cobb and Etzel 1991; Shelef 1994; Marr et al. 1998). For example, Mott et al. (2002) discussed how exposure to motor vehicle CO emissions results in a substantial number of accidental deaths and estimates the number of accidental CO poisonings that have been prevented as a result of more stringent CO emissions standards.

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State and Federal Standards for Mobile-Source Emissions Oxides of Nitrogen Evidence for health effects associated with exposure to NO2 remains inconclusive. The health effects of exposure to NOx are largely related to exacerbation of symptoms of respiratory disease. Short-term exposures (for example, less than 3 hr) to low concentrations of NO2 might result in changes in airway responsiveness and lung function in persons with preexisting respiratory illnesses. Such exposures might also increase respiratory illnesses in children. Long-term exposures to NO2 might result in increased susceptibility to respiratory infection and might cause irreversible alterations in lung structure (EPA 2004d). Hydrocarbons A broad range of VOCs have been identified in both diesel and gasoline emissions. Individual VOCs found in the emissions mixture are themselves toxic. Although health effects of exposure to the entire mixture have not been characterized, the toxicities of many of the components of the mixture are well understood. The health effects of a few mobile-source organic compounds that are considered HAPs are described below. 1,3 Butadiene 1,3-Butadiene is generated by incomplete combustion of gasoline and diesel fuel. Butadiene is a mildly irritating gas that can cause neurological symptoms at high exposure concentrations. Epidemiological studies of workers exposed to butadiene in rubber plants found an increased risk for cardiovascular disease (ATSDR 1993). Animal studies found developmental and reproductive effects related to inhalation exposure (EPA 2002a). Butadiene exposures were also associated with the development of leukemia and other lymphomas in both epidemiological and animal studies, and EPA has classified butadiene as a human carcinogen (EPA 2002a).

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State and Federal Standards for Mobile-Source Emissions Benzene Both on-road and nonroad gasoline-powered mobile sources contribute benzene to the ambient air. Benzene is a known human carcinogen and is associated with the development of leukemia. Neurological symptoms of inhalation exposure to benzene include drowsiness, dizziness, headaches, and unconsciousness in humans (EPA 2005e). Formaldehyde Formaldehyde is a nearly colorless gas with a pungent, irritating odor even at concentrations below 1 ppm. Formaldehyde is an eye, skin, and respiratory tract irritant. Inhalation of vapors can produce narrowing of the bronchi and an accumulation of fluid in the lungs. The systemic effects of formaldehyde may include metabolic acidosis, circulatory shock, respiratory insufficiency, and acute renal failure. Formaldehyde is a potent sensitizer at high concentrations and a probable human carcinogen (ATSDR 2004). Benzo(a)pyrene Acute effects of benzo(a)pyrene at increased concentrations potentially include red-blood cell damage, resulting in anemia and a suppressed immune system. Long-term exposure to benzo(a)pyrene might result in developmental and reproductive effects. It was classified as a carcinogen by the International Association of Cancer Research (IARC 1973). Particulate Matter Epidemiological studies over the last several years consistently demonstrated a statistical relationship between exposure to particles and cardiopulmonary morbidity and mortality (Pope et al. 1991; Dockery et al. 1993; Samet et al. 2000; Pope et al. 2002; Metzger et al. 2004). Toxicological studies exploring specific mechanisms of injury from exposure to PM supported the epidemiological findings (Ghio et al. 2000; Framp-

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State and Federal Standards for Mobile-Source Emissions ton et al. 2002; Utell et al. 2002; Devlin et al. 2003). For persons suffering from chronic obstructive pulmonary disease (COPD), which includes chronic bronchitis, emphysema, and some cases of chronic asthma, exposure to PM can induce inflammatory responses resulting in exacerbation of symptoms. Adverse physiological effects in individuals diagnosed with cardiovascular disease have been associated with increased PM exposure. These effects include increased blood pressure, cardiac arrhythmias, increased oxidative stress and inflammation, and progression of atherosclerosis. Pope et al. (2002) also found a statistically significant increase in the risk of developing lung cancer associated with exposure to particulate air pollution. Studies examining health effects related to exposure to PM from mobile sources have primarily focused on diesel exhaust. Diesel exhaust, which includes hundreds of organic compounds and includes particles in the ultrafine size range, was designated as probable human carcinogen by IARC (1989) and EPA (2003c). Some studies suggested that ultrafine particles, which do not contribute substantially to the total particle mass, might carry a greater risk per weight than particles of other sizes (HEI 2002). Several specific components of mobile-source particles were shown to be toxic. A large number of polycyclic aromatic hydrocarbons (PAHs) have been identified in the exhaust of both diesel- and gasoline-powered vehicles, including benzo(a)pyrene. Additional carcinogenic compounds, such as dioxins, have also been identified in the exhaust stream from both diesel and gasoline vehicles. Measurements of TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) ranged from about 2 picograms (pg) of toxicity equivalent quotient per kilometer (TEQ/km) to 5,100 pg TEQ/km for a diesel light-duty vehicle (HEI 2002). A recent study of exhaust particles plus semi-VOCs showed no difference in the toxicity of gasoline and diesel exhaust from light-duty vehicles, and more potent on an equivalent mass basis from high-emitter vehicles than normal emitters (Seagrave et al. 2002). Ozone Although ozone is not a component of mobile-source emissions, components of the emission mixture result in the formation of ozone. Ozone is the product of photochemical processes involving VOCs and NOx, and is formed in greatest abundance during summer months when sunlight is strongest. Ozone is a known respiratory irritant and can cause

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State and Federal Standards for Mobile-Source Emissions inflammation of the airways resulting in breathing restrictions. For persons suffering from respiratory illnesses, such as COPD and asthma, exposure to ozone can be especially problematic. Significant association between daily variations in ambient ozone concentrations and adverse health outcomes, such as lung function decrements, aggravation of preexisting disease, increases in hospital admissions and emergency room visits for respiratory symptoms, and increases in mortality have been found by a large number of epidemiological studies (Thurston and Ito 1999). A recent study has also found an association between short-term changes in ozone and short-term mortality for 95 large urban areas in the United States, although the potential for statistical confounders is a concern (Bell et al. 2004). CONCLUSIONS Substantial progress has been made over the past few decades in reducing air pollutant emissions from many sources, including mobile sources. However, some locations in the Unites States continue to experience ambient concentrations of criteria pollutants above the NAAQS. Further improvements in air quality will be needed, particularly to attain the recently adopted standards for fine particulate matter and ozone. Evidence presented in the chapter suggests that mobile-source emissions contribute to poor air quality and have important health effects. The following conclusions are drawn based on the evidence: Mobile sources, both on-road and nonroad, are major sources of precursors for ground-level ozone and PM2.5. Estimates of emissions suggest that mobile-source emissions are composed of approximately 50% anthropogenic NOx and VOC emissions inventories and approximately 75% anthropogenic CO inventories. On-road light-duty vehicles are still the largest contributor to total mobile emissions, even though their emissions have been decreasing despite increases in vehicle activity. Nonroad emissions have remained relatively constant since 1970. As a result, the relative amount of nonroad to on-road emissions has been increasing over this period. There are many uncertainties in the methods and data used to estimate mobile- source emissions and thus in the estimates themselves. The technical, research, and regulatory community continue to fill data gaps, thereby improving the methods.

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State and Federal Standards for Mobile-Source Emissions The contribution of mobile-source emissions to air pollution varies from area to area. Ambient concentrations of pollutants depend on several factors, such as the relative mix of sources, the extent of pollutant transport, the meteorology, and the topography of the area. The result is that different levels of controls on mobile and nonmobile sources are typically required for different areas in the Unites States.