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Chapter 3

WHO IS MOST AT RISK?

Most of the human studies on the health risks of diesel exhaust looked exclusively at healthy, adult men. To extrapolate from male worker studies to the general population may not adequately protect women, children, and the elderly. Furthermore, worker studies provide little information about health effects in people with chronic illnesses or depressed immune systems. We do know something about the susceptibility of some of these groups from research on the health effects of fine particle pollution.


Children

Children represent the largest subgroup of the population susceptible to the effects of air pollution.81 Compared with adults, children spend more time outdoors, particularly at midday and during the afternoons when air quality is poorest, and engage in more vigorous physical activity.82 As a result, children average a higher breathing rate, and receive greater relative doses of any pollutants in the air. At rest, an infant's metabolic rate and air intake is about twice that of an adult.83 A forty-five pound child inhales over 9,000 liters of air per day.84

Children also have narrower airways and their lungs are still developing. Irritation caused by air pollutants that would produce only a slight response in an adult can result in potentially significant obstruction in the airways of a young child.85 Furthermore, children have more frequent respiratory and other illnesses, perhaps due to incompletely developed immune protection.

Elevated levels of particulate pollution have been linked with an increased incidence of respiratory symptoms in children.86 In an ongoing study comparing air pollution in six U.S. cities and the respiratory health of individuals living in those cities, the frequencies of coughs, bronchitis, and lower respiratory illnesses in preadolescent children were significantly associated with increased levels of acidic fine particles.87 Illness and symptom rates were twice as high in the community with the highest air pollution concentrations compared with the community with the lowest concentrations. Rates of chronic cough, bronchitis, and chest illness during one school year were positively associated with particulate pollution.88 One study suggested that though all children are at risk for increased respiratory symptoms due to particulate pollution, children with preexisting respiratory conditions (wheezing, asthma) are at greater risk.89 In a diary study of 625 Swiss children between birth and five years of age, respiratory symptoms were associated with particulate concentrations, while the duration of symptoms was associated with levels of nitrogen oxide. Symptoms included coughing, upper respiratory episodes, and breathing difficulty.90

Hospital admission for respiratory illness is strongly associated with particulate air pollution and the association is stronger for children than adults. During months with peak particulate pollution levels, average hospital admissions for respiratory illness in children nearly tripled, whereas for adults comparable hospital admissions increased by 44 percent.91 Several studies have demonstrated that children living near major roadways have poorer lung function than children living in cleaner areas.92 The same studies showed that girls were more affected than boys. Lung function in both sexes was correlated with estimated levels of diesel exhaust measured in the schools.93


The Elderly

Substantial scientific evidence suggests that the elderly and those with pre-existing heart and lung disease are at greatest risk of premature mortality due to particulate air pollution. Several important studies have shown that those over 65 years of age are at greater risk of requiring emergency room services on days with higher particulate pollution.94 In addition, the relationship between particle exposure and death was about three times greater in the elderly.95 Because cardio-pulmonary disease is more common in older people, and cardiovascular and pulmonary function declines with age, the elderly are likely to have heightened sensitivity to particle exposure. To the extent that the elderly and chronically ill are slower to remove particles from deep lung tissues, they are likely to have greater risk from diesel exhaust exposure, because the time needed to clear small particles from the lungs appears to increase the risk of tumor development.96


Exposures Add Up Over a Lifetime

Cancers induced by diesel exhaust involve a latency period of a number of years between damaging exposure and development of cancer: risk increases with increasing duration of exposure.97 Exposure to diesel exhaust for nearly every human begins at birth and lasts throughout our lifetime. Neither animal, nor worker studies on diesel exhaust adequately capture this feature of the general public's exposure. Most of the animal studies involving diesel exhaust inhalation begin exposure with "adolescent" rats.98 For these reasons, direct extrapolation from traditional animal exposure studies as well as extrapolation from worker studies are likely to underestimate the risk to the public, whose exposure to ambient diesel exhaust begins in early childhood and lasts for many decades.


Focus #3: "Clean" Diesel is Still Dirty

Some industry advocates argue that 1996 model year and later diesel engines using new diesel fuel are "clean" diesel and are not a health threat. However, diesel engines-new and old-continue to pose cancer threats. In fact, recent studies suggest that, despite a substantial reduction in the total weight of particulate matter, the total number of particles in emissions from the more advanced 1991-model diesel engine is 15 to 35 times greater than the number of particles from the 1988 engine when both engines were operated without emission control devices. Thus, newer diesel may be emitting smaller particles but not fewer particles.99 Furthermore, these smaller particles are more likely to penetrate deeper in the lungs and to be trapped and retained. "Clean" diesel may not decrease risk from diesel exhaust exposure.

In addition, even the "cleaner," post-1996 diesel engines emit more smog forming nitrogen oxides and particulates than comparable alternative fueled engines. Diesel engines certified in 1998 emitted 60 percent more smog forming nitrogen oxides and 50 percent more particulates than similarly sized natural gas engines.100

A recent study comparing emissions from a new diesel engine running on older diesel fuel, and on a reformulated diesel fuel (required in California since 1993), revealed that the newer fuel only slightly reduces emissions of nitrogen oxides and particulates, and that more than 95 percent of the particle emissions are very fine (less than 1 micron in size). Dioxins were detected in diesel emissions, both with the older and newer fuel. Finally, levels of toxics such as benzene, toluene, 1,3-butadiene, and polycyclic aromatic hydrocarbons (PAHs) were essentially unchanged by use of the newer diesel fuel.101



Notes

81. Centers for Disease Control, Populations at Risk from Air Pollution - United States, 1991, Morbidity and Mortality Weekly Report, vol. 42, no. 16, April 30, 1993.

82. California Air Resources Board, Study of Children's Activity Patterns: Final Report, September 1991, pp. 66a-67.

83. International Programme on Chemical Safety, Principles for Evaluating Health Risks From Chemicals During Infancy and Early Childhood: The Need for a Special Approach, Environmental Health Criteria 59,World Health Organization, 1986.

84. Cal EPA, Technical Support Document for Exposure Assessment and Stochastic Analysis - Public Review Draft, December 12, 1996.

85. Lipsett, "The Hazards of Air Pollution to Children." Environmental Medicine, S. Brooks et al., eds., St. Louis: Mosby, 1995.

86. Timonen KL, Pekkanen J. Air pollution and respiratory health among children with asthmatic or cough symptoms.Am J Respir Crit Care Med 1997 Aug;156(2 Pt 1):546-552.

87. Ware, J. H. et al., "Effects of Ambient Sulfur Oxides and Suspended Particles on Respiratory Health of Preadolescent Children," Am. Rev. Resp. Dis., vol. 133, 1986, pp. 834-842.

88. Dockery, D. et al., "Effects of Inhalable Particles on Respiratory Health of Children," Am. Rev. Respir. Dis., vol. 139, 1989, pp. 587-594; Schwartz, J. et al., "Acute Effects of Summer Air Pollution on Respiratory Symptom Reporting in Children," Am. J. Respir. Crit. Care Med., vol. 150, 1994, pp. 1234-42.

89. Pope and Dockery, "Acute Health Effects of PM10 Pollution on Symptomatic and Asymptomatic Children. Am. Rev. Respir Dis., vol.145, 1992, pp. 1123-1128.

90. Braun-Fahrlander, C. et al., "Air Pollution and Respiratory Symptoms in Preschool Children," Am Rev. Respir. Dis., vol. 145, 1992, pp. 42-47.

91. Pope, A., "Respiratory Disease Associated with Community Air Pollution and a Steel Mill, Utah Valley," Am. J. of Pub. Health, vol. 79, May 1989, pp. 623-628.

92. Pekkanen J, et al. Effects of ultrafine and fine particles in urban air on peak expiratory flow among children with asthmatic symptoms. Environ Res 1997;74(1):24-33.

93. Brunekreef B, et al. Air pollution from truck traffic and lung function in children living near motorways., Epidemiology; 8(3):298-303, 1997; Oosterlee A, et al. Chronic respiratory symptoms in children and adults living along streets with high traffic density. Occup Environ Med 1996 Apr;53(4):241-247.

94. Schwartz J. Air pollution and hospital admissions for the elderly in Detroit, Michigan. Am J Respir Crit Care Med 1994 Sep;150(3):648-655; Schwartz J, Short term fluctuations in air pollution and hospital admissions of the elderly for respiratory disease. Thorax 1995 May;50(5):531-538.

95. Schwartz J, Dockery DW, Increased mortality in Philadelphia associated with daily air pollution concentrations. Am Rev Respir Dis;145(3):600-604, 1992.

96. Bovornkitti S, Limlomwongse L, Environment and the aging lung. Respirology 1997 Sep;2(3):169-172.

97. Steenland, et al., Case-control study of lung cancer and truck driving in the Teamster Union, Am J Pub Hlth; 80: 670-674, 1990; Bhatia R, et al. Diesel Exhaust Exposure and Lung Cancer. Epidem 9:84-91, 1998.

98. In most of the studies summarized, the age of the rodents at the beginning of exposure ranged from 8 to 17 weeks. Because two years (104 weeks) is the average lifespan of a rat, a 17-week old rat is essentially comparable in age to a human adolescent.

99. Bagley, Susan T., Kirby J. Baumgard, Linda D. Gratz, John H. Johnson, and David G. Leddy. Characterization of Fuel and Aftertreatment Device Effects on Diesel Emissions; Health Effects Institute; Research Report Number 76; September 1996; p. i.

100. California Air Resources Board. 1998 Model Year Heavy-Duty On-Road Engine Certification Listing Update. April 9, 1998. El Monte, California. Pp. 1-2.

101. Arey, J, et. al., Evaluation of Factors That Affect Diesel Exhaust Toxicity (Draft Final Report). Submitted to the California Air Resources Board, January 2, 1998.

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