Home | Data and Monitoring | Emission Inventory Scope Study
Scope Study for Expanding the Great Lakes Toxic Emission Regional Inventory to include Estimated Emissions from Mobile Sources
Chapter 1 Significance of Toxic Air Emissions from Mobile Sources
Mobile sources are perhaps one category of air toxic emission sources most relevant to human activities in industrialized societies. According to the U.S. Environmental Protection Agencys (EPA) definition (EPA, 1992) for emission inventory purposes, mobile sources include highway vehicles, nonroad mobile sources, aircraft, and locomotives. Recreational marine equipment and commercial marine vessels are also classified as nonroad mobile sources. Almost every person in the United States depends on one or more mobile sources to provide transportation, information, food, and other goods.
Concern about mobile source emissions has been expressed since the 1950s. Hundreds of chemical compounds have been identified in mobile source emissions. Since the mid 1980s, a variety of studies have documented toxic air emissions from mobile sources as a major contributor to overall health risk (EPA, 1990; MPCA, 1992;, ViGYAN, 1992). However, because of the extreme complexity of estimating mobile source emissions, there still has not been a comprehensive, reliable emission inventory of air toxic pollutants from mobile sources in the United States.
EPA maintains a nationwide emission inventory for criteria air pollutants (EPA, 1995). This inventory provides the estimates of on-road vehicle emissions for every year since 1970. These emissions are based on county-level vehicle miles traveled (VMT) and emission factors. The VMT data were obtained from the Federal Highway Administrations (FHWA) Highway Performance Monitoring System (HPMS) database since 1980. However, the HPMS coverage area is less extensive than state or local traffic count programs. Moreover, the HPMS does not include private roads and uses rather small samples. Therefore, HPMS provides general data but less accurate and detailed VMT data as state or local traffic count programs do. The EPAs inventory also includes estimated 1990 base year emissions for all other types of mobile sources. The emissions for years other than 1990 since 1970 have been calculated based on the 1990 estimates by using growth factors. The 1990 emissions from aircraft, commercial marine vessels, and railroads were estimated from the area source portion of the 1985 NAPAP emission inventory by using appropriate growth factors. The basis for other non-road emissions was, in particular, population and per-source fuel usage rate data. Therefore, the emission estimates for these types of mobile sources have high uncertainty.
In addition to the coverage of criteria air pollutant emissions, the 1990 Clean Air Act Amendment (CAA) Section 112(k) inventories (EPA, 1995a) and Section 112(c)(6) inventories (EPA, 1995b) provide limited coverage of toxic air pollutant emissions for mobile sources. So far, Section 112(k) inventories contain 1990 nationwide emission estimates for seven hazardous air pollutants (HAP) under the Urban Area Source Program. Thus far, Section 112(c)(6) inventories contain estimated 1990 national emissions for five of the seven pollutants listed in section 112(c)(6). The five pollutants are polycyclic organic matter, hexachlorobenzene, alkylated lead compounds, polychlorinated biphenyl compounds, and mercury.
Although there are some local and state mobile source inventories to support 112(k) and 112(c)(6) programs, these inventories typically cover small geographic areas and are for a limited number of pollutants. Most state and local inventories for mobile source emissions have been prepared for State Implementation Plans. These inventories may only cover nonattainment areas and only for the criteria pollutants in these nonattainment areas.
Ontario, Canada, maintains a province-wide mobile source emission inventory for NOx, volatile organic compounds (VOC), CO, SOx, and particulate matter (PM). The estimates for on-road vehicles are based on the annual traveled distance of each type of vehicle and emission factors. The VMT data are estimated for variable Universal Transverse Mercator (UTM) grid size by using the Environment Canadas forecast model and Ontario Ministry of the Environment and Energy travel demand data.
Table 1-0 shows the available mobile source emission inventories in the Great Lakes States and Ontario province according to a survey conducted recently by Minnesota Pollution Control Agency. Toxic air pollutants have not been included in these emission inventories. For more information, please refer to Chapter 3.
Note: * The pollutants included in the emission inventory may vary with nonattainment areas.
Since limited individual toxic air emission information is available for mobile sources, we have analyzed emissions of criteria air pollutants, VOC, particulate matter (PM), and lead to improve our assessment of mobile source toxic air emissions. VOC and PM emissions are used to illustrate the significance of toxic air emissions because VOCs and PM are broad categories that include many individual toxic air pollutants. Lead is included because it is on the list of targeted compounds for the Great Lakes regional toxic air emission inventory.
It should be noted that the emission estimates from all sources may have varying degrees of uncertainty, depending on the quality of available source information and accuracy of the methods used to obtain the estimates in the emission inventories.
1-1. Volatile Organic Compound Emissions
VOCs are the principal components that form ozone. Ozone is a deep lung irritant. It causes damage to biological tissues and cells. Forest and ecosystem studies indicate ozone is responsible for agricultural crop yield loss in the United States and noticeable foliar damage in many crops and species of trees (EPA, 1991a). VOCs also play a major role in forming other photochemical oxidants, which are responsible for numerous chemical and physical atmospheric reactions. As discussed below, mobile sources are significant emitters of VOCs and need to be included in inventory efforts to more accurately estimate total emissions of pollutants.
The 1990 international VOC emission information was found for Canada in an EPA document titled "National Air Pollutant Emission Trends, 1900-1994" (EPA, 1995). Canada data were originally from Marc Delauriers, Pollution Data Branch, Environment Canada, Hull, Quebec (Delauriers, 1995). Table 1-1 presents a summary of the 1990 Canada VOC emission data and percent of VOC emissions for each source by source. In Canada, the transportation source category includes automobiles, light-duty gasoline trucks, heavy-duty gasoline trucks, motorcycles, propane powered vehicles, light-duty diesel trucks, heavy-duty diesel vehicles, railroads, marine, aircraft, non-road use of gasoline, tire wear. Approximately 31% of total VOC emissions was contributed from transportation.
The 1990 VOC emission data for Europe were originally prepared by Gordon McInnes, European Environmental Agency, Copenhagen, Denmark (McInnes, 1995). The compiled data by source categories for 28 European countries are presented in Table 1-2. Two categories of mobile sources were included in the table: Road Transport, and Other Mobile Sources and Machinery. The percent of mobile source emissions was calculated for the sum of these two categories. Also, an average percent of mobile source emissions was calculated along with a standard deviation. Results indicate mobile sources contribute a maximum 75 percent and a minimum 12 percent to the total country-level VOC emissions in the 28 inventoried European countries. The average contribution from mobile sources to total VOC emissions is approximately 37% with a standard deviation of 16%.
It should be noted that the emission estimation methodologies differ among countries, and the estimates for several countries are still preliminary.
The U.S. 1994 national VOC emissions were estimated for 13 source categories by EPA (EPA, 1995), but natural sources were not included. Table 1-3 and Figure 1-1 present the calculated percent of VOC for each source category. The non-road sources category includes aircraft, commercial marine vessels, railroads, and all other non-road vehicles and equipment. The mobile sources, a sum of on-road vehicles and non-road sources, are responsible for 37% of the nationwide VOC emissions. This number is consistent with international estimates of mobile source contributions to VOC emissions.
The 1994 VOC emissions for the eight Great Lakes States were estimated for the same source categories as the national VOC emissions (Solomon, 1996). Table 1-4 provides a summary. Although the amount of VOC emissions was different from state to state, a similar pattern was found among the eight Great Lakes States. Overall, the mobile sources contributed an average of approximately 39% of the VOCs emitted in the Great Lakes States with a narrow standard deviation of 3%.
Ontario VOC emissions from human activity for the early 1990s were estimated by the Ontario Ministry of Environment and Energy (1994). As shown in Figure 1-2, road vehicles and other transportation contributed about 36% of the VOC emissions in the province of Ontario, Canada.
In general, limited emission data indicate that mobile sources contribute approximately one third of the VOCs emitted to the atmosphere.
1-2. Particulate Matter Emissions
Particulate matter composes the vast majority of the air pollution that we can see. It causes substantial visibility impairment in many parts of the United States. The major adverse effects associated with particulate matter on human health are as follows: impaired breathing and respiratory symptoms, premature mortality, aggravation of existing respiratory and cardiovascular disease, damage to lung tissue, and alteration of the bodys physical and immune system defenses against inhaled particles (EPA, 1991b).
The available 1990 PM emission estimates for Canada were provided by the same source as the VOC emission estimates (Delauriers, 1995). Table 1-5 presents a summary of the data for each source category and the percent of total particulate matter (PM) emissions. Transportation was estimated to contribute 10% of the total particulate matter emissions.
The 1994 national emission estimates for fine particulate matter less than 10 microns (PM-10) were from the EPA document, "National Air Pollutant Emission Trends, 1900-1994" (EPA, 1995). The percent of PM-10 emissions for each source category was then calculated. A total of 13 source categories was examined. The non-road source category includes aircraft, commercial marine vessels, railroads, and all other non-road vehicles and equipment. The miscellaneous category includes natural sources, agriculture and forestry, other combustion (e.g. wildfires, managed burning), and fugitive dust. Table 1-6 illustrates the PM-10 emission estimates by source category. Miscellaneous sources dominated PM-10 emissions contributing 94% of the total. If miscellaneous sources are excluded, mobile sources (a sum of on-road vehicles and non-road sources) are estimated to contribute 27% of total PM-10 emissions. This is consistent with point source emissions (Figure 1-3) which were estimated to contribute 43% of the PM-10 emissions.
The 1994 PM-10 emissions for the eight Great Lakes States were also estimated for the same source categories as the nationwide PM-10 emissions by E.H. Pechan & Associates, Inc. (Solomon, 1996). Table 1-7 summarizes the emission data and presents the amount and percent of the PM-10 emissions from each examined source category. Miscellaneous sources contributed a range of 88%-94% of the PM-10 emissions. When miscellaneous sources were excluded, the contribution of mobile sources was 23% 5%. In comparison, the contribution of point sources was 45% 10% (Table 1-8).
The Province of Ontario PM emissions from human activity for the early 1990s were obtained from the Ontario Ministry of Environment and Energy (1994). Natural sources of PM were not included. As shown in Figure 1-4, transportation accounted for about 23% of the PM emissions in Ontario. On the other hand, PM emissions from point sources contributed 25% of the total in Ontario.
Mobile sources do not contribute to particulate matter emissions as significantly as they do to VOC emissions. However, their contribution to particulate matter emissions is comparable to that of point sources and should not be excluded from emission inventories.
1-3. Lead Emissions
Lead is a stable element which persists and accumulates both in the environment and in the human body. Humans can be exposed to lead through inhalation and ingestion. When lead enters the human body, it accumulates in blood, bone, and soft tissues. Also, due to its slow excretion, lead affects the kidneys, liver, nervous system, and blood-forming organs. It is particularly dangerous to fetuses, infants, and children, and can cause damage to their central nervous systems (EPA, 1991c).
The burning of leaded gasoline was the largest source of lead emissions to air from the 1920s to the mid 1970s. Much less lead is emitted by gasoline now because of the results of regulatory actions. In the early 1970s EPA began to issue regulations regarding lead in gasoline. The lead content in leaded gasoline was dramatically decreased from an average of 1.0 grams/gallon to 0.1 grams/gallon by January 1, 1986. Also, unleaded gasoline was introduced in 1975. By 1994, sales of leaded gasoline only accounted for 1 percent of the gasoline market. The 1994 nationwide lead emissions by source category is displayed in Table 1-9 and Figure 1-5. The table and figure were prepared from EPAs report on emission trends (EPA, 1995). On-road vehicles and non-road sources accounted for 32% of national lead emissions. Since January 1, 1996, leaded gasoline has been prohibited for use in highway vehicles by the mandate of the 1990 Clean Air Act Amendment. Almost all gasoline sold now in the U.S. is unleaded (containing lead less than the detection level, 0.005g/gallon) (Caldwell, 1996). It is expected that lead emissions from mobile sources will further decrease after January 1, 1996.
Since the methodology used to estimate lead emissions was very resource intensive, EPA only estimated lead emissions at the national level. This prevents us from making lead emission estimates by source category for the Great Lakes States.
1-4. Toxic Air Pollutant Emissions
The available emission data for toxic air pollutants from all sources are limited because of complexity in the development of these data. The complexity is caused by the following factors: the number of chemical compounds involved, the variety of sources emitting the compounds, the low concentrations sometimes involved, and the potential for secondary formation of compounds from others. Nationwide inventories are available for 12 toxic air pollutants (EPA, 1995):
These inventories show a mobile source contribution to the first four pollutants. For the other eight pollutants, the emissions are either non-applicable or not estimated for mobile sources. Discussions in this Section focus only on the emissions of benzene, 1,3-butadiene, formaldehyde, and EOM. The adverse health and environmental effects of these four pollutants will be fully discussed in Chapter 2.
Table 1-10 to Table 1-13 illustrate nationwide emission estimates by source categories for benzene, 1,3-butadiene, formaldehyde, and EOM. The emission estimates of the first three pollutants were from National Screening Inventory of Sources and Emissions of Candidate 112 (k) Pollutants to Support the Urban Area Source Program, Review Draft (EPA, 1995a). The EOM emission estimates were obtained from Emissions Inventory of Section 112(c)(6) Pollutants (EPA, 1995b). The percent of emissions for each source category was then calculated.
Mobile sources, including on-road and non-road, made significant contributions to total nationwide emissions of the four toxic air pollutants examined. They constituted 65% of total benzene emissions, 79% of total 1,3-butadiene emissions, 54% of total formaldehyde emissions, and 79% of total EOM emissions in 1990.
1-5. Cancer Incidence from Mobile Source Emissions
Health risk assessments of toxic air pollutants have focused on cancer as the toxic endpoint. Generally, it is easier to compare the effects of all carcinogens using the same endpoint--cancer. A number of air toxics exposure and risk assessment screening studies have been conducted by EPA and other agencies in the last decade. EPA compiled and analyzed 10 reports, 14 studies, and 2 databases in a document entitled "Cancer Risk from Outdoor Exposure to Air Toxics" (EPA, 1990). These reports and studies included a total of 65 source categories, 90 different pollutants, and covered varying geographic areas, ranging from city-specific to nationwide. Only one secondary pollutant in the atmosphere, formaldehyde, was considered in the analysis due to the absence of information. To estimate annual cancer incidence, EPA first developed estimates of the annual cancer cases per million population for each pollutant/source category combination (e.g., 1,3-butadiene emissions from mobile sources) reported in the data sources. These numbers were modified to reflect updated unit risk and emission factors. Then, EPA calculated the total nationwide annual incidence, in most instances, by multiplying the annual cancer cases per million population by the total U.S. population and summing across all pollutant/source categories. The results of EPAs analysis showed that many types of sources contribute to annual cancer incidence, but the largest contributor found in the document (EPA, 1990) was the motor vehicle source category (Figure 1-6). According to EPAs estimation, 56% of the total cancer cases were attributed to direct emissions from motor vehicles. In addition to the direct emissions, motor vehicles contributed another 2% of the cancer incidences from secondary formaldehyde. It should be noted that approximately 35% of the contribution of secondary formaldehyde was from motor vehicles. In sum, motor vehicles contributed approximately 58% of the total nationwide annual cancer incidence from exposure to outdoor air toxics (EPA, 1990). In contrast to motor vehicles, point sources were only estimated to contribute approximately 25% to the total annual cancer incidence.
Similar results were found in more current studies. The Minnesota Pollution Control Agency (MPCA) conducted a modeling study to estimate and evaluate the cancer risks from air pollution (excess cancer incidence) in the Minneapolis/St. Paul metropolitan area (MPCA, 1992). A total of 32 air toxics were selected for assessment. These air toxics were carcinogens and suspected carcinogens for which dose-response relationships had been estimated by the EPA. The population of the receptor area was approximately 1.2 million people. The ambient levels of these 32 pollutants were estimated by first compiling an emission inventory and then conducting dispersion modeling. Secondary formaldehyde formation was not considered in the study. Also, ambient monitoring data were not utilized in the study. Figure 1-7 shows the estimated excess cancer incidence by source category for this modeling exercise. Road vehicles were estimated to be responsible for 61% of the excess cancer incidence found in this study. Overall, 1% of the excess incidence was caused by point sources in the Twin Cities. However, this study did not fully assess the major point sources of carcinogenic pollutants located within the study area but outside of the receptor area. If these sources had been included in the modeling exercise, the results may be different.
ViGYAN, Inc. prepared a report for EPA, Region V in 1992 (ViGYAN, 1992). This report estimated the cancer risks associated with 30 air toxic pollutants in the Southwest Chicago area. Among these 30 air pollutants, 6 were known human carcinogens, 22 were probable human carcinogens, and 2 were possible human carcinogens. The receptor area was approximately 4- square miles, covering a total population of approximately 93,854 persons. Again, the exposure assessment was done by deriving an emission inventory and estimating atmospheric dispersion of these emissions. Background concentrations for formaldehyde and carbon tetrachloride were considered in this report. Some of the major contributors to cancer risk were examined. The findings in the report indicated mobile sources appeared to contribute about 47% of the total estimated excess cancer cases. The mobile sources here included Midway Airport, cars, trucks, buses, and trains. Figure 1-8 illustrates the estimated excess cancer cases by source category.
One point is worth noting: it was assumed in all the above studies that the emissions have remained constant for each source category over a 70-year lifetime and people spend a 70-year period in one place, outdoors. However, it is unlikely to happen in the real world. Most people may spend more time indoors than outdoors. The contribution of mobile sources to indoor air pollution has not been adequately characterized.
Although significant uncertainties are associated with the information and the analysis presented above, mobile source emissions of air toxics consistently have been shown to be a major component of air toxic-related cancer risk.
Public health risks resulting from exposure to toxic air pollutants are not limited to carcinogenicity. However, comparable data are not available for the risks other than cancer risks associated with exposure to toxic air pollutants. With current databases, we cannot determine the relative contribution of mobile sources to the non-cancer risks.
1-6. Deposition of Toxic Air Pollutants from Mobile Source Emissions to the Great Lakes
When toxic pollutants are emitted into the atmosphere, they can be transported various distances and can be deposited to aquatic ecosystems. In particular, persistent pollutants in the environment may accumulate, causing long-term exposure of fish, wildlife and humans. Such exposures have been linked to "reproductive, metabolical, neurological and behavioral abnormalities; to immunity suppression leading to susceptibility to infections and other life-threatening problems; and to increasing levels of breast and other cancers" (IJC, 1994). Unfortunately, information is scarce on the deposition of toxic air pollutants from mobile source emissions to the Great Lakes. More intensive estimation of emissions and more extended deposition studies for toxic air pollutants are expected in the future to provide information on the importance of mobile source emissions to deposition of persistent and bioaccumulative pollutants to the Great Lakes.
Available information has shown that mobile sources consistently have contributed approximately one third of the total VOC emissions in the United States, Canada, European Countries, the Great Lakes States, and the Province of Ontario. The contribution of mobile sources to particulate matter emissions is comparable to that of point sources. Mobile sources are also a significant contributor of toxic air pollutant emissions. They constituted 65% of total benzene emissions, 79% of total 1,3-butadiene emissions, 54% of total formaldehyde emissions, and 79% of total EOM emissions in the United States in 1990. The estimated cancer incidence due to exposure to emissions of motor vehicles ranges from 47 % to 61% of the total estimated cancer incidence caused by outdoor exposure to air toxics nationwide, in the Minneapolis/St. Paul metropolitan area, and in the Southwest Chicago area. However, because of the extreme complexity of estimating mobile source emissions, there still has not been a comprehensive, reliable emission inventory of air toxic pollutants from mobile sources in the Great Lakes Region. Moreover, information is scarce on the deposition of toxic air pollutants from mobile source emissions to the Great Lakes because of the lack of emission data. Therefore, the compilation of data on toxic emissions from mobile sources as well as from point and area sources in the Great Lakes States and Ontario is a key component of any strategy to reduce toxic loading to the lakes.
Caldwell, J.W., U.S. Environmental Protection Agency, Program Office, Field Operations & Support Division, Washington, DC., personal communication, 1996.
Delauriers, M., File of updated 1990 emissions from Marc Delauriers, Pollution Data Branch, Environment Canada, Hull, Quebec to Sharon Nizich, Emissions Factors and Inventory Group, Technical Support Division, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Research Triangle Park, NC. May 9, 1995.
EPA, Air Quality Criteria for Lead, EPA/600/8-83/028aF-dF.4v (NTIS PB87-142378), Office of Health and Environment Assessment, Environmental Criteria and Assessment Office, U.S. Environmental Protection Agency, Research Triangle Park, NC. 1991c.
EPA, Air Quality Criteria for Ozone and Other Photochemical Oxidants, EPA/600/8-84/020aF-eF.5v (NTIS PB87-142949), Office of Health and Environment Assessment, Environmental Criteria and Assessment Office, U.S. Environmental Protection Agency, Research Triangle Park, NC. 1991a.
EPA, Air Quality Criteria for Particulate Matter and Sulfur Oxides, EPA/600/8-82/020aF-cF.3v (NTIS PB84-15677), Office of Health and Environment Assessment, Environmental Criteria and Assessment Office, U.S. Environmental Protection Agency, Research Triangle Park, NC. 1991b.
EPA, Cancer Risk from Outdoor Exposure to Air Toxics, EPA-450/1-90-004a, b, Office of Air Quality Planning and Standard, U.S. Environmental Protection Agency, Research Triangle Park, NC. September 1990.
EPA, Emission Inventory of Section 112(c)(6) Pollutants: Polycyclic Organic Matter (POM), 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)/2,3,7,8-Tetrachlorodizenzofuran (TCDF), and Polychlorinated Biphenyl Compounds (PCBs), Final Report and background data, Radian Corporation, Research Triangle Park, NC. March, 1995b.
EPA, National Air Pollutant Emissions Trends, 1900-1994, EPA-454/R-95-011, p 3-13, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Research Triangle Park, NC. October 1995.
EPA, National Screening Inventory of Sources and Emissions of Candidate 112(k) Pollutants to Support the Urban Area Source Program, Benzene, 1,3-Butadiene, Perchloroethylene, Trichloroethylele, Methylene Chloride, Carbon Tetrachloride, and Formaldehyde, Review Draft Report and background data, U.S. Environmental Protection Agency, Research Triangle Park, NC. April, 1995a.
EPA, Procedures for Emission Inventory Preparation, Volume IV: Mobile Sources, EPA-450/4-81-026d (Revised), p 2, Emission Planning and Strategies Division, Office of Mobile Sources and Technical Support Division, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Research Triangle Park, NC. 1992.
IJC, Seventh Biennial Report on Great Lakes Water Quality (Washington, DC, Ottawa, ON, Windsor, ON, and Detroit, MI), International Joint Commission, 1994.
McInnes, G., European Environmental Agency, Copenhagen, Denmark, Computer files of updated 1990 emissions, to Sharon Nizich, Emissions Inventory Branch, Technical Support Division, Office of Air Quality Planning and Standard, U.S. Environmental Protection Agency, Research Triangle Park, NC. May 2, 1995.
MPCA, Estimation and Evaluation of Cancer Risks from Air Pollution in the Minneapolis/St. Paul Metropolitan Area, Minnesota Pollution Control Agency, Air Quality Division, St. Paul, MN, March 1992.
Ontario Ministry of Environment and Energy, Air Quality in Ontario, 1993, ISSN 0840-9366, PIBS 3305E01, December 1994.
Ontario Ministry of Environment and Energy, Air Quality in Ontario, 1993, ISSN 0840-9366, PIBS 3305E01, December 1994.
Solomon, D., 1994 Tier 1 and Tier 2 Annual Emissions for Selected States, E.H. Pechan & Associates, Inc., Durham, NC. January 23, 1996.
ViGYAN, Inc., Estimation and Evaluation of Cancer Risks Attributed to Air Pollution in Southwest Chicago: Summary Report, prepared for U.S. Environmental Protection Agency, Region V Under EPA Contract No. 68-D0-0018, Work Assignment No.6, January 1992.
Table 1-0. Available mobile source emission inventories in the Great Lakes States and Ontario
Table 1-1. 1990 Canada VOC emission estimates by source category (short tons)
Table 1-3. 1994 National VOC emission estimates by source category
Table 1-5. 1990 Canada emission estimates for total particulate matter by source category (short tons)
Table 1-6. 1994 National PM-10 emission estimates by source category (thousand short tons)
Table 1-8. 1994 PM-10 emission estimates by source category excluding miscellaneous category for the eight Great Lakes states
Table 1-9. 1994 National lead emission estimates by source category
Table 1-10. 1990 National benzene emission estimates by source category
Table 1-11. 1990 National 1,3-butadiene emission estimates by source category
Table 1-12. 1990 National formaldehyde emission estimates by source category
Table 1-13. 1990 National extractable organic matter (EOM) emission estimates by source category