Exhausted by Diesel
How America's Dependence on Diesel Engines Threatens Our Health
DIESEL: HEAVY USE AND HIGH EXPOSURES
Nationally, trucks are moving an increasingly large portion of freight, and most heavy trucks are fueled by diesel. From 1950 to 1985 the miles driven by trucks in the United States increased by 235 percent and tons carried by trucks increased by 169 percent.22 Trucks are now carrying a heavier load while traveling farther, increasing their overall emissions. In 1995, over half a million diesel trucks and over 20,000 diesel buses were sold in the United States, representing a doubling of annual sales since 1980.23 A total of around six million heavy trucks, tractor-trailers, and buses were registered for use in the United States in 1995.24 That same year, diesel vehicles nationwide consumed 37 billion gallons of diesel fuel.25 California's 1995 diesel fuel consumption was roughly 2.1 billion gallons.26 California and other western states have experienced higher than average growth in motor vehicle travel and have also experienced increased total vehicle emissions.27
Diesel exhaust is a major contributor to various types of air pollution, including smog-forming oxides of nitrogen (NOx) and fine particles (PM2.5). In 1996, diesel exhaust accounted for over one quarter of the 23,393,000 tons per year of NOx pollution produced nationally.28 In California, an estimated 26 percent of particles (PM10) from fuel combustion sources in outdoor air come from diesel engines.11 Exhaust from heavy-duty diesel engines contains between 100 to 200 times more small particles than gasoline engine exhaust.29
Figure 1: 1996 Diesel Exhaust Contribution to National NOx Emissions
Source: Office of Air Quality Planning Standards. National Air Pollutant Emission Trends, 1900-1996.
United States Environmental Protection Agency. Appendix A. December 1997.
The California Air Resources Board estimates, based on a 1995 emissions inventory, that approximately 27,000 tons of diesel exhaust particles are emitted into California's air annually.30 As Figure 2 illustrates, on-road mobile sources (heavy-duty trucks and buses, together with far smaller numbers of light-duty cars and trucks) are the major contributors, emitting 58 percent (approximately 15,680 tons per year) of total diesel exhaust particle emissions in California. Other mobile sources (mobile equipment, ships, trains, and off-road vehicles) contribute about 9,820 tons per year (37 percent). Stationary sources such as diesel generators, drilling equipment, and pumps contribute the remaining 1,400 tons per year (5 percent).
Figure 2: Sources of Diesel Particle Emissions in California, 1995
Source: California Air Resources Board, Emission Inventory 1995, Technical Support Division, October 1997.
Further, a study published by the Health Effects Institute reports that more than 98 percent of the total number of particles in diesel exhaust are less than 1 micron in size.31 Small particles, such as those in diesel exhaust, are particularly hazardous because they penetrate deeper into the recesses of the lungs, and tend to remain in the lungs and surrounding lymph nodes rather than being cleared efficiently from the body32 (see Focus #1 below).
The California Air Resources Board in its 1998 draft report on diesel estimated that the average diesel exhaust particle concentration in California outdoor air in 1995 was 2.2 micrograms/cubic meter (µg/m3).33 This estimate averages levels in urban areas and rural areas, giving extra weight to the urban areas to account for the denser population. For example, average outdoor concentrations of diesel exhaust ranged from a low of 0.2 µg/m3 in the Great Basin valleys, to a high of 3.6 µg/m3 in the South Coast Air Basin.34 Obviously people are exposed to the actual levels in their particular local environment, rather than the statewide or regional average.
The limited data available on actual measurements (rather than estimates) of diesel exposure show that in cities like Stockton, Fresno and Bakersfield -- cities with typically lower truck and bus traffic than more urban areas -- sampling from actual sites frequently measured particulate levels from motor vehicles near or above 10 µg/m3 (three times the level suggested as a statewide average).35 A majority of the sites where the Air Resources Board measured particulate levels -- and from which it estimated the statewide average diesel exhaust levels -- were sites remote from heavy diesel traffic and from businesses that rely on diesel transport.36
Other data support a conclusion that the CARB exposure models underestimate actual exposures to diesel exhaust. In a study conducted in Los Angeles in the 1980s, diesel exhaust accounted for approximately 7 percent of the fine particles emitted into the air. Average ambient levels of diesel exhaust particles ranged from 1 to 3 µg/m3 in areas with low levels of air pollution. The highest average levels of diesel particles were approximately 10 µg/m3 during winter months. Other areas may have it even worse. For example, an estimated 52.8 percent of the airborne particles found in Manhattan's streets come from diesel tailpipes.37 With total measured particles averaging roughly 50 µg/m3 on an annualized basis, that means the diesel exhaust concentration may be as high as 26.4 µg/m3 -- far higher than most American cities.
Short-term or peak exposures to diesel particulate matter, especially in urban settings such as street canyons, are usually higher than monthly or annual average concentrations.38 For example, researchers have shown that "street canyons" between high buildings in cities can concentrate diesel exhaust levels to as high as 8.8 µg/m3 from light-duty diesel vehicles alone.39 Because the Air Resources Board estimates that 96 percent of California's on-road diesel exhaust particles are emitted by heavy-duty vehicles,11 we can expect very high concentrations of diesel particles in urban streets where truck and bus traffic is high. In 1998, monitoring conducted by NRDC and the Coalition in such areas confirmed concentrations of diesel exhaust above 50 µg/m3 for a significant portion of the monitoring period.40
According to the Air Resources Board, most people spend more than 22 hours each day indoors.41 Indoor air contains diesel exhaust at levels which are affected both by outdoor concentrations and by the method of building ventilation. Modern buildings with heating, ventilation and air conditioning (HVAC) systems sometimes have particle filtration systems which can reduce diesel exhaust levels. Buildings with older or less expensive ventilation systems usually lack particle filtration. Where building inhabitants open windows and doors to ventilate the building, indoor diesel exhaust concentrations have been found to be just as high as outdoor levels.42 If loading docks or garages where diesel trucks may idle are located near an air intake for the building, exposures may be greater than expected indoors, even in a tightly sealed building.
The air we breathe contains not only gases, such as oxygen and carbon dioxide, but also numerous small particles, mostly invisible to the naked eye. These particles come from dust, fabrics, plant materials, and numerous sources of anthropogenic (human-made) pollution, such as industrial facilities and motor vehicles. All particles are not created equal. Larger particles, including the majority of naturally occurring particles, don't remain in the air for very long, and rapidly settle to the ground. Finer particles remain suspended in air for far longer, sometimes for weeks, and they can travel in winds hundreds of miles from their sources.
The larger particles don't actually get inhaled deep into the lungs. Instead they are captured by fine hairs and mucus in the nose, throat, and large airways (trachea and bronchi). These particles are then quickly cleared from the body by sneezing, coughing, or swallowing. Clearance occurs in 2-24 hours in healthy people. Small particles, less than about 10 microns in diameter (less than 1/7 the width of a human hair) are more likely to make their way past the upper airways and penetrate into the deeper portions of the lungs. Ultra-tiny particles, such as those in diesel exhaust, are even more likely to find their way into the deepest tissues of the lungs.* There the particles need to be cleared by cells of the immune system, a process which takes months or years. Some of the tiny particles are never cleared from the body, but instead accumulate in the lungs and the lymph nodes.44 Autopsy studies of people living in urban areas show significant blackening of the lungs due to accumulation of fine particles.45
The small particles which come from diesel exhaust are particularly dangerous because they are coated with a mixture of chemicals such as polycyclic aromatic hydrocarbons, nitroaromatics, benzene, dioxins, and other toxicants. The particles act like a special delivery system which places these toxic chemicals deep within our bodies. Some asthma medications use the principle of delivering a beneficial drug in a fine inhaled aerosol. Diesel exhaust is like a perversion of a drug delivery system which delivers hazardous toxicants into our lungs. The particles are retained in the body along with the toxic chemical hitchhikers which would otherwise be quickly eliminated. Thus the particles lengthen our exposures to the toxicants in diesel exhaust.
* By far the largest deposition in the deepest airways occurs for particles between .005 and 2 micron (µ) in size. Calculation by Dale Hattis, Ph.D. based on International Commission on Radiological Protection, Human Respiratory Tract Model for Radiological Protection, ICRP Publication 66, Elsevier Science Inc., Tarrytown, N. Y
31. Bagley, Susan T., et al. 1996. Characterization of Fuel and Aftertreatment Device Effects of Diesel Emissions. Research Report Number 76. Health Effects Institute, Topsfield, Massachussetts. September.
38. GR Cass, HA Gray. 1995. Regional emissions and atmospheric concentrations of diesel engine particulate matter: Los Angeles as a case study. In: Health Effects Institute. 1995. Diesel Exhaust: A Critical Analysis of Emissions, Exposure, and Health Effects (A Special Report of the Institute's Diesel Working Group). Health Effects Institute, Cambridge, MA.
39. Volkswagen, Unregulated Motor Vehicle Exhaust Gas Components, Volkswagen AG, Research and Development, Project Coordinator: Dr. K. H Lies, 3180 Wolfsburg 1, Germany, 1989, cited in ARB, Draft Diesel Exposure Assessment, February 1998, p. A-43.
42. Nazaroff, W. et al, Concentration and Fate of Airborne Particles in Museums, Environ. Sci. Technol., 24(1): 66-76, 1990; Ligocki, M.P. et al., Characteristics of Airborne Particles Inside Southern California Museums, Atmospheric Environment, 27A(5): 697-711, 1993.
44. 96% of the particles found in the lung parenchyma at autopsy in never-smoking adults are PM2.5. Churg A, Brauer M, Human Lung Parenchyma Retains PM2.5. Am J Respir Crit Care Med; 155(6): 2109-2111, 1997.
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