Issues: Oil & Energy

Introduction

For over a century, the trucking and transit worlds have embraced the relative efficiency and easy maintenance of the diesel engine. For just as long, pedestrians and drivers alike have complained loudly about the smell and fumes from the diesel engine.

In response to rapidly growing public health concerns about the health risks of breathing diesel fumes, engine manufacturers and developers of emission control equipment have introduced new technologies that reduce conventional pollutants. In recent years, the industry has coined the term "clean diesel" in an attempt to convince the public that new diesels meeting 1996-1998 standards are clean enough for urban America. However, with lifetime soot emissions of greater than 100 automobiles, this is clearly not the case.^[1]

In their most recent quest to gain public acceptance, the industry has been urging transit fleets in New York, Boston, Los Angeles and elsewhere to turn their attention to a new technology -- called a diesel-electric hybrid -- that combines a diesel generator with a series of electric motors to provide a reduced-emission diesel bus. Environmental, public health, and community groups are increasingly concerned about the optimistic industry announcements of the arrival of this new technology.

Unfortunately, announcements regarding diesel-electric hybrid buses fail to address the most serious health risks of diesel exhaust and a number of critical issues related to the operation, maintenance, and cost of these new buses. In this document, we summarize the health risks of diesel exhaust, raise a number of critical concerns about the current wave of diesel-electric hybrid bus demonstration projects, and offer a series of recommendations for transit operators and regulators who are trying to gauge the environmental performance of these new buses.

Defining the Problem

Diesel exhaust may be the most significant -- and most under-addressed -- public health threat in urban America today. Although diesel exhaust contains over 450 different chemical components, the main threats of diesel emissions to public health can be classified under three broad categories: fine particulate matter (PM), oxides of nitrogen (NOx), and toxic compounds. Some people are at greater risk from diesels' hazardous mix, including children, the elderly, people with respiratory conditions or who smoke, and people who exercise, work or live near diesel exhaust sources.

Particulates. Diesel exhaust is a major source of fine particles, especially in urban areas. Over half of all particulate matter in Manhattan comes from a relatively small number of diesel tailpipes. Under the same engine and load conditions, diesel engines emit 100 times more particles than gasoline engines.^[2] Roughly 80-95% of diesel PM mass is finer than 1 micron in diameter.^[3] These finer particles are small enough to evade our respiratory defense mechanisms and lodge deeply within our lungs. Numerous public health studies have found links between fine PM and a wide variety of health impacts, including increased asthma hospitalizations and emergencies, chronic bronchitis, pneumonia, heart disease, and even premature death.

There has been increasing evidence that historic efforts aimed at reducing PM10 mass from diesel engines may have actually increased the number of ultrafine and nanoparticles (those less than 0.1 microns and 0.05 microns in diameter, respectively).^[4] This is especially troubling given the emerging public health research that suggests strongly that ultrafine and nanoparticles pose significant health risks.

Nitrogen Oxides. NOx is a primary precursor in the formation of ground-level ozone, a major component of urban smog. As much as 38% of the NOx in California's South Coast Air Basin comes from diesels.^[5] Roughly one-third of the NOx in the northeastern U.S. comes from diesels.^[6] NOx also contributes to acid deposition, resulting in harm to fish and plant life in lakes throughout the country. NOx emissions adversely affect the quality of the water we drink through its contribution to nutrient pollution in the New York City reservoir system and other major waterways. And, NOx emissions can even increase the amount of particulate matter that we breathe through chemical reactions that transform NOx into particulate matter in the atmosphere.

Toxics. Perhaps the greatest diesel health threat can be linked to the unregulated toxic emissions that are found in diesel exhaust. Diesel exhaust contains over 40 chemicals that are listed by California and the U.S. Environmental Protection Agency (EPA) as toxic air contaminants, probable human carcinogens, known human carcinogens, reproductive toxicants or endocrine disrupters. Lifetime exposure to diesel exhaust at the average outdoor concentrations found in California's South Coast Air Basin may result in as many as one in every 1,200 people developing cancer due to a lifetime of exposure.^[7] These values suggest that diesel exhaust may pose a substantially greater risk than other individual toxic constituents. For example, a recent Congressional study for Los Angeles indicated a potential lifetime cancer risk of about one in 3,900 from 1,3 Butadiene, the single most important toxic evaluated in the study.^[8] In Manhattan, the cancer risk from diesel exhaust may be even greater than in Los Angeles, roughly 8 cancers among every 1,000 people exposed.^[9]

Environmental agencies are starting to take action against diesel toxics. In 1998, California formally listed particulate emissions from diesel-fueled engines as a toxic air contaminant, based on data supporting links between diesel exposure and cancer. EPA also weighed in on the subject in 1998, releasing a draft report that concluded that whole diesel exhaust was a probable human carcinogen.^[10] And in December 1998, the National Toxicology Program advisory board recommended that diesel PM be listed as "reasonably anticipated to be a human carcinogen" in the ninth edition of the Congressionally-mandated Report on Carcinogens.^[11] These conclusions echo similar findings of the National Institute of Occupational Safety and Health, and the World Health Organization's International Agency for Research on Cancer (IARC).

Evaluating the Solutions

Although early testing demonstrates the potential for promising reductions in NOx and PM10 from diesel-electric hybrid buses, emerging public health and technical evidence suggest that a more comprehensive evaluation of emissions is warranted. In particular, additional information is needed in two key areas to adequately assess the environmental performance of diesel-electric hybrid buses and determine their appropriate role in future transit bus operations.

We strongly urge the U.S. Environmental Protection Agency, the Federal Transit Administration and the appropriate state environmental, transportation and transit regulators to address the concerns outlined below.

(1) Emerging Public Health Concerns

• Ultrafine and nanoparticles. Efforts to characterize emissions savings from diesel-electric hybrid buses must evaluate the impact on ultrafine and nanoparticle emissions including, at a minimum, changes in the number and mass of these particulates relative to a traditional diesel bus baseline. If compressed natural gas (CNG) buses continue to be another basis for comparisons, this technology should also be evaluated for ultrafine and nanoparticle emissions.
  
  
• Toxics. Diesel-electric hybrid bus exhaust should be evaluated (e.g., through exhaust speciation and other methods) to determine whether emissions of toxics, such as polycyclic aromatic hydrocarbons (PAHs), benzene, 1,3-butadiene, aldehydes, and other compounds, are changed relative to traditional diesel buses. Furthermore, to the extent that public health science permits, the role that diesel-electric hybrid buses can play in reducing the toxicity of whole diesel exhaust should be studied. For comparison, CNG bus exhaust should also be tested for key listed toxic emissions.

(2) Certification and Real-World Emission Issues

• Testing details. Some early testing data compares diesel-electric hybrid bus emissions to CNG and traditional diesel bus emissions. Before declaring the diesel-electric hybrid bus to be an environmental success, regulators need to insist upon strong results from comprehensive, independent emissions tests. In addition to incorporating the hybrid test cycle and other concerns identified below, a full understanding of these results requires knowing the following key technical details for each bus type: test cycle and testing equipment used; number of buses tested and statistical significance of results; engine size, model, age, and mileage of the buses tested; fuel used during tests (compared to fuel used in the field); and emission control devices employed on the vehicle.
  
  
• Hybrid test cycle. As many industry and regulatory experts have noted, defining an appropriate test cycle for a hybrid vehicle is a significant challenge. For example, a fair test must account for the battery's state of charge before and after the test (or the potential for differing operating strategies for different driving conditions). These critical issues must be incorporated into tests used for diesel-electric hybrid bus prototypes to ensure accurate emissions information.
  
  
• Durability. Given the long lifetime of a typical transit bus, additional testing is needed to indicate how diesel-electric hybrid bus emissions change as miles accrue. The smaller diesel engines to be employed in hybrids should be durability tested, since they may not normally be used in transit or other heavy-duty applications. Furthermore, the entire hybrid system should be evaluated to determine the emissions impact as the battery pack ages, since reductions in battery energy or power capacity may alter the system operating protocol (e.g., potentially increasing diesel engine operation to offset reduced battery efficiency).
  
  
• Offcycle emissions. The recent Consent Decrees among the engine makers, EPA, and the California Air Resources Board highlight the need for greater attention to emissions occurring outside the existing federal emissions test cycle. These consent decrees resulted from an investigation that showed that NOx emissions could be as much as tripled when operating outside the federal test cycle. In the context of testing the performance of the diesel-electric hybrid bus, NOx emissions from the downsized diesel engine during high-load operation must be considered -- both over the standard transit bus test cycle and during a high-acceleration, high-speed test that accounts for potential offcycle concerns.

Notes

1. Mark, J. and L.R. Davis. 1998. Shifting Gears: Advanced Technologies and Cleaner Fuels for Transit Buses. Cambridge, MA: Union of Concerned Scientists. April.

2. Mauderly, J.L. 1992. "Diesel Exhaust" in Lippman M. (ed.) Environmental Toxicants: Human Exposures and Their Health Effects. New York: Von Nostrand Reinhold.

3. Kittelson, D.B. 1998. "Engines and Nanoparticles: A Review." Journal of Aerosol Science. Vol. 29. pp. 575-588.

4. See, for example: Bagley, Susan T., Kirby J. Baumgard, Linda D. Gratz, John H. Johnson, and David G. Leddy. 1996. Characterization of Fuel and Aftertreatment Device Effects on Diesel Emissions. Cambridge, Mass.: Heath Effects Institute. September.

5. In the wake of new evidence that real-world diesel emissions are actually much higher than expected, these official inventory estimates may need to be adjusted upwards.

6. Northeast States for Coordinated Air Use Management (NESCAUM), 1997. Heavy-Duty Engine Emissions in the Northeast. Boston, MA: NESCAUM. May.

7. Solomon, G., et al. 1998. Exhausted by Diesel: How America's Dependence on Diesel Engines Threatens Our Health. New York, NY: Natural Resources Defense Council. May.

8. "Exposure to Hazardous Air Pollutants in Los Angeles." Minority Staff Report of Committee on Government Reform, U.S. House of Representatives. March 1, 1999.

9. Solomon, et al. op cit.

10. U.S. EPA, Office of Research and Development, "Health Assessment Document for Diesel Emissions, " Review Draft, EPA/600/8-90-057C, February 1998, Chapter 5. This draft document is currently undergoing further review, and is expected to be finalized in 1999.

11. Calstart 12/18/98 "News Notes." and dieselnet.com/news/9812ntp.htm