10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Diesel Engine Exhaust Particles: Science, Regulations and Technological Developments Leading to Cleaner Emissions
ROGER MCCLELLAN, Private Consultant
Abstract Number: 1508 Working Group: Health Related Aerosols
Abstract This presentation will review health issues associated with diesel exhaust emissions over the past century. Emphasis is given to research by the author and colleagues over the past four decades that have influenced regulations and technological developments leading to contemporary diesel engines with exhaust emissions with minimal health impact. The compression-ignition, internal combustion engine, known as the Diesel engine for its inventor, Rudolph Diesel, is a key contributor to modern industrial society. Concern developed post-WWII for exhaust impairing visibility and causing respiratory tract and eye irritation. In the 1970s, concern increased over the potential for diesel exhaust particle exposures to cause lung cancer with the finding that extracts of respirable diesel exhaust particles contained polycyclic hydrocarbons and were highly mutagenic in microbial assays. The Lovelace Inhalation Toxicology Research Institute (ITRI) took a lead role in conducting lifespan studies with rats exposed to exhaust from circa 1980 diesel engines. They found that chronic exposure to 3500 or 7000 µg/m3 of diesel exhaust particles caused lung cancer, findings soon confirmed by investigators in Germany, Switzerland and Japan. Later, studies showed exposure to carbon black particles, with no absorbed hydrocarbons, produced a similar increase in lung cancer raising questions as to the relevance of the rat findings for assessing human hazards. These findings were key to the International Agency for Research on Cancer (IARC) (1987) classifying diesel exhaust as a “probable human carcinogen.” Heightened concern for exhaust exposure causing adverse health effects led to issuance of stringent emission standards. The IARC finding also stimulated new epidemiological studies. The Diesel Exhaust in Miners Study (DEMS) conducted by scientists at the US National Institute of Occupational Safety and Health and National Cancer Institute studying 12,000 workers in non-metal mining operations through 1997 found an increased lung cancer hazard in workers exposed to diesel exhaust pre-1982. DEMS investigators used estimates of Respirable Elemental Carbon (REC) extrapolated from CO measurements as markers of exposure. DEMS findings were key to IARC (2012) classifying diesel exhaust as a “human carcinogen.” The author and colleagues replicated the original DEMS analyses and, then, extended the original analyses. The extended analyses used alternative REC exposure estimates developed based on Horse Power (HP) of diesel equipment used, REC emissions per HP and mine ventilation. Use of these exposure metrics and control for radon exposure resulted in diminished estimates of the association between diesel exhaust exposure and lung cancer. Moreover, the effect was only significant in a mine with limited ventilation and use of large diesel-powered haul units. To meet the stringent emission standards introduced post-2000, major advances were made in diesel engine technology and fuels. These advances resulted first, in reduced particulate emissions and, later, reduced emissions of nitrogen oxides. ITRI (2015) conducted lifespan studies with rats exposed to diesel exhaust from new-technology heavy duty engines equipped with an exhaust after-treatment system that reduced particulate emissions to near zero meeting EPA 2007/2010 standards. No excess lung cancer was observed. Modest respiratory tract lesions associated with NO2 exposure were observed, as expected, since the engines used were not equipped with NO2 reduction systems. These results illustrate the positive impact of improved engine design, ultra-low diesel fuel, exhaust after-treatment systems and electronic controls to eliminate the lung cancer hazard that may have existed for old technology diesel engines. In conducting risk assessments for diesel exhaust exposure, it is important to recognize the recent substantial changes in particulate exhaust emissions associated with improved technology. This is an example of the role of aerosol science as an important enabling science in developing diesel technology with reduced potential for health impacts.