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Children's Exposures to Diesel Exhaust

Summary of Findings

  1. Diesel Buses: Each day, nearly 600,000 school buses transport 24 million students to schools in the U.S. Within Connecticut, nearly 387,000 children ride 6,100 school buses, and most are powered by diesel fuel.
  2. Children’s Time on Buses: The time spent on buses by individual students varies between 20 minutes and several hours per day. For one child, a half-hour ride to school, and a half-hour ride home each day amounts to 180 hours per school year—90 full 24- hour-days over 12 years of school. Annually, U.S. children spend 3 billion hours on school buses. Connecticut children spend 50 million hours on buses each year.
  3. Background Particulates: Connecticut background fine particulate matter levels (PM2 . 5) a re near or above national standards, when averaged over 24 hours. Childre n’s exposure to diesel exhaust from school buses constitutes an additional exposure beyond backgro u n d l e vels of particulates re p o rted from current monitoring efforts.
  4. Background Ozone: Connecticut is not in compliance with current federal ozone standards. In 2001, portions of the state exceeded the 8-hour limit on 26 days, and the 1-hour limit was e xceeded on 9 days. Ozone is known to exacerbate asthma, and is normally highest in the afternoon, when childre n’s exposure to diesel particulates fro m school bus rides is also likely to be high. NOx p recursors to ozone have increased over the past 10 years. In 2001, nearly 109 million people lived in 272 counties where federal ozo n e limits we re exc e e d e d.2
  5. Carcinogenicity of Diesel Exhaust: Diesel exhaust is classified as a probable human carcinogen by many governmental authorities, including the International Agency for Research on Cancer (WHO), the U.S. National Toxicology Program, the U.S. Environmental Protection Agency, and as a known carcinogen by the State of California. The California South Coast Air Quality Management District recently estimated that nearly 71% of the cancer risk from air pollutants in the area is associated with diesel emissions. Diesel exhaust includes benzene, 1,3-butadiene, and soot, all classified as known human carcinogens. Nearly 33 studies have explored the association between diesel exhaust exposure and bladder cancer. A recent meta analysis of this literature found increased risk between 18-76%. These findings are based primarily upon studies of truck drivers, railroad workers, bus drivers and shipyard workers.
  6. Diesel Exhaust Contains 40 Hazardous Air Pollutants: In addition, diesel exhaust contains both carbon particulates and 40 chemicals that are classified as “hazardous air pollutants” under the Clean Air Act.
  7. Particulates and Respiratory Diseases: Exposure to particulates has been associated with: increased mortality among those with cardiopulmonary diseases; exacerbation of symptoms for asthma, bronchitis, and pneumonia; decreased lung function; and retarded lung development. It has also been correlated with increased hospital admissions and emergency room visits for respiratory illnesses.
  8. Children’s Susceptibility: Children may be especially susceptible to adverse respiratory effects following exposure to fine-diameter particulate matter (PM2.5) emitted from diesel engines. Nearly 94% of diesel particulates have diameters less than 2.5 micrometers (um).4 The average diameter of diesel particulates is 0.2 m i c ro m e t e r s. Smaller particles are able to penetrate children’s narrower airways reaching deeply within the lung, where they are more likely to be retained. Higher rates of respiration among child ren may lead to their higher exposure, when measured per unit of their b o d y weight.
  9. No Known Safe Exposure to Diesel Exhaust: There is no known safe exposure to diesel exhaust for children, especially those with asthma or other chronic re s p i r a t o ry disease. T h e re is no single standard for acceptable cancer risk from diesel exhaust in the U.S.
  10. Asthma Prevalence: Nationally, 4.8 million children have asthma. More than 44,500 Connecticut school children have the disease.
  11. Asthma Costs: Asthma costs an average of $500 per child per year for medications, physician care, and hospital treatment. Annual direct medical costs are estimated to be nearly $22 million for Connecticut school students alone. This estimate does not account for other costs that often include school absenteeism, lost parental work while caring for ill children, psychological effects, and abnormal social development.
  12. Children’s Exposure to Particulates on Buses : Children were exposed to airborne p a rticulate concentrations in tested buses that we re sometimes 5-15 times higher than b a c k g round levels of PM2 . 5.
  13. Variability Within Buses: Particulate and black carbon levels vary within individual buses over time. The most important influences on variability include: bus idling behavior, queuing practices, bus ventilation via windows, and outdoor concentrations on bus routes. Particulate and carbon concentrations did not vary by sampling location within diesel buses, e.g., front vs. rear. Engine model, age of engine, number of miles since last overhaul, maintenance cycles, location of bus engine (front, next to driver, or rear), elevation change, passenger load, and climate may all influence levels of interior pollutants and children’s exposure.
  14. Exhaust From Other Traffic: The intensity and type of traffic along bus routes significantly affects air quality on buses. Buses following diesel-powered vehicles, including other buses, h a ve increased levels of carbon and particulate concentrations within passenger compartments. Pa rticulate levels rose rapidly within the passenger cabin when buses pulled behind other diesel vehicles in traffic. No buses tested had air filtration equipment capable of re m oving the fine particles detected in the buses.
  15. Idling Buses: Idling buses tested had higher concentrations of particulates and carbon than moving buses. Higher concentrations occurred when idling buses had open windows when compared with buses with closed windows. There is a current Connecticut Department of Environmental Protection (DEP) regulation, DEP 22a-174-18 (a)(5), that limits idling time to 3 minutes, yet it is neither monitored nor enforced.
  16. Queued Idling Buses: Queued idling buses had the highest levels of particulates and black carbon measured. Idling buses tend to accumulate diesel exhaust which may be retained during the ride, depending upon bus ventilation rates. Particulate and carbon concentrations rise rapidly once idling begins.
  17. Length of Bus Route: The length of bus routes affects the magnitude of childre n’s e x p o s u re to air pollutants in the interior compartment. Time in transit between home and school spent by Connecticut students varied between 20-180 minutes per day in the towns sampled. The longest routes may occur in the rural parts of the state, especially in large regional school districts.
  18. Lower Emissions From Natural Gas Buses: Natural gas buses studied emitted 60- 98% less carbon than diesel-powered buses.
  19. Findings Are Likely to Underestimate Exposure: Exposures to carbon and particulates found in this study were measured in environments with exceptionally low traffic and few other sources of pollution. Most children are exposed to additional pollution from traffic and other residential, commercial and industrial activities. These findings therefore are likely to underestimate levels of fine particulates and carbon found in more urban areas and routes with higher traffic intensity.
  20. Additional Sources of Particulate Exposure Threaten Children: Residential use of tobacco products, wood stoves, candles, kerosene heaters, and poorly ventilated cooking stoves are for many children additional sources of exposure to carbon-based particulates and organic gases that result from combustion. Federal and state monitoring efforts fail to account for these exposures despite the fact that most people spend more than 80% of their time indoors. Most epidemiological studies that associate PM10 levels with adverse respiratory health effects consider particles measured by outdoor stationary monitoring facilities, neglecting indoor air exposures.
  21. School Buses Are Exempt From Emissions Testing: School buses are currently exempt from routine emissions testing in Connecticut.5 There is no federal requirement that all state governments monitor school bus emissions, although some states require testing.
  22. Federal Particulate Standards Exceeded: EPA estimates that in 2000, 11 million U.S. children lived in areas that exceeded one or more federal air quality standard. Nearly 3.5 million children lived in areas where the particulate standards were exceeded in 1998. Within Connecticut, bus exposures when combined with background outdoor particulate levels may elevate children’s average daily exposure beyond the current federal 24-hour PM2.5 standard.
  23. Absence of Passenger Cabin Air Quality Standards: Current law does not regulate air quality within buses.
  24. Federal Monitoring vs. Personal Monitoring: Federal law and regulation permit the testing of air quality by means of fixed monitors. In Connecticut, 13 fixed monitors measure PM2.5. This sampling design fails to capture the local variability and severity of air pollution in the state. National standards permit averaging particulates over 24- hour periods. These practices ensure that shorter episodes of intense pollution—such as those experienced in bus rides—are neither recognized nor regulated by the state or federal government.
  25. Tougher Federal Diesel Standards Delayed Until 2006: Tougher new diesel emissions standards will not be phased in until 2006. This delay poses respiratory health threats to Connecticut citizens, who now experience air pollution at levels above acceptable federal standards for ozone. Compliance with current standards does not ensure health protection. EPA estimated that the new standards would result in 8,300 fewer premature deaths, 17,600 fewer cases of childhood acute bronchitis, and 360,000 fewer asthma attacks. These estimates demonstrate the scale of respiratory health threat EPA believes exist under current conditions.
  26. Federal Particulate Standards: The exposures identified in this study will not be affected by the tougher federal PM standards adopted in 1997 (which are different from the diesel standards described in 26 above), since monitoring to determine compliance with the PM standards is done outdoors.
  27. Bus Parking Yards: Bus parking and maintenance facilities have the potential to create localized particulate air pollution that far exceeds ambient outdoor levels reported from State monitoring efforts. Pollution may routinely migrate to adjacent properties, as buses are left idling, or during periods of peak use—early mornings and afternoons. If vehicles are parked near schools, both outdoor and indoor school air quality may be diminished.
  28. Bus Drivers: Bus drivers’ exposure to motor vehicle and diesel exhaust is significantly higher than children’s, due to longer periods of time spent on buses.


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