Public Health and Components of Particulate Matter: The Changing Assessment of Black Carbon

Since the promulgation of the U.S. Clean Air Act in 1967, scientists have debated the toxicity of chemical species present in airborne particles. Periodic reviews of the National Ambient Air Quality Standards (NAAQS) for particulate matter (PM) have focused on mass-based standards, despite evidence of the potential health hazards associated with particle composition. Since the 1970s, scientists have studied sulfate, a major component of airborne particles, as the primary toxic species. With a commitment to broadening pollutant aerosol research in the 1990s (e.g., National Research Council [NRC], 2004), health-related investigations have identified other species of concern, including carbon and certain metals. With confidence in the ability to measure operationally both black carbon or elemental carbon (BC/EC) and organic carbon (OC), a number of health studies have begun to focus on these species (e.g., Nichols et al., 2013; Levy et al., 2012; Janssen et al., 2011; Mauderly and Chow, 2008). The 44th Annual Critical Review, “Public Health and Components of Particulate Matter: The Changing Assessment of Black Carbon” (Grahame et al., 2014), provides a comprehensive survey and evaluation of recent health-related investigations related to BC in ambient air.

A. Gwen Eklund Critical Review Committee Chair

Black carbon, more commonly termed “soot,” has been the prime indicator of atmospheric pollution for centuries (Brimblecombe, 1978; Brimblecombe and Bowler, 1992). The opacity of black smoke plumes has been used for more than 100 years to regulate emissions, and the method is still written into many operating permits (e.g., U.S. Environmental Protection Agency [EPA], 2009). As early as the 1950s, scientists began to study the complex nature of carbon as a mixture of BC and OC directly from sources, and OC from oxidation of organic vapors in the atmosphere (e.g., Went, 1960; U.S. Public Health Service, 1958; Cadle, 1973; Grosjean and Friedlander, 1980; Brimblecombe and Bowler, 1992). British Smoke (a qualitative indicator of BC as part of total mass concentration) measurements reached perhaps the order of milligrams per cubic meter (mg/m³) with sulfur oxides during intense pollution episodes in 1950s London (e.g., Hill, 1936; Wilkins, 1954). When chemical analysis of filter samples began in earnest in the 1950s, potentially carcinogenic polycyclic organic compounds were associated with soot (e.g., NRC, 1972). In the 1980s, the U.S. Environmental Protection Agency (EPA) included carbon in the “the sulfur oxide particulate complex” (e.g., Science Advisory Board [SAB], 1975). Later, extensive study of the toxicology of laboratory animals was investigated for diesel exhaust, which contained substantial amounts of BC (e.g., McClellan et al. 1986). Quantification of BC (or EC) and OC was largely ignored until the 1980s, when measurement methods were developed (Wolff and Klimisch, 1982; Watson et al., 2005) and indicated that carbon concentrations are similar to those of sulfate, especially in the eastern United States.

As noted in the 2014 Critical Review, efforts to investigate the health consequences of ambient exposure to BC and OC have been undertaken in earnest since the mid-1990s. These studies range from extended monitoring and species research on particulate carbon to exposure and epidemiologic studies as well as toxicological experiments (e.g., U.S. EPA, 2012). Current knowledge recognizes the physicochemical complexity of aerosol carbon, including: (1) characterization of BC in the ultrafine particle (<0.1 μm) size range, with surface coatings of organics and salts distinct from pure EC; (2) the extraordinary range of chemicals imbued in OC, of which less than ˜20% have been identified, and their linkage to BC levels; and (3) the continual evolution of fresh BC emissions with temperature changes, dilution, and atmospheric oxidation (Robinson et al., 2010). As a result, knowledge about BC exposure and effects has expanded. At the same time, major changes in combustion technologies, especially for transportation, have appeared, reducing BC (e.g., Lloyd and Cackette, 2001; Chow et al., 2001; Hesterberg et al., 2011). Uncontrolled fugitive sources of carbon, including fossil fuels and biomass fuels used for cooking and heating, are also important exposure elements (e.g., Xing et al., 2013; Blanchard et al., 2013).

The 2014 Critical Review presents a portfolio of epidemiological and toxicological evidence from studies regarding the health risks of PM carbon exposure indicated by BC concentrations. The collective inference from these studies addresses the U.S. EPA criteria for establishing health risk (Vedal, 1997). These include: (1) consistency of observed associations; (2) coherence between epidemiology and other studies; (3) biological plausibility—linkage with biological mechanisms of action; (4) biological gradient for a dose-response function; and (5) experimental evidence of changes in exposure and health effects. Grahame et al. (2014) argue that contemporary BC studies address and support adverse health risk, based on each of these criteria, and these studies indicate the need to focus priorities on BC and other compounds associated with it in ambient aerosols as an important health risk. These studies point to the need to go beyond a particulate mass indicator for health based air quality regulation (Pope and Dockery, 2006; Chow et al., 2006; Janssen et al., 2011). One counterpoint to this conclusion is drawn from other historic studies—for example, airborne sulfate risk—that lack sufficient evidence to change from mass-based ambient standards. Priority setting of exposure risk based on multipollutant considerations may be in future reform of air quality management paradigm (e.g., Hidy and Pennell, 2010; Chow and Watson, 2011). In this respect, the recent International Agency for Research on Cancer (IARC, 2013) conclusion that fine particles by total mass are carcinogenic seems like a step backward.

Air & Waste Management Association (A&WMA) members and their guests are invited to read, attend, and comment on the 2014 Critical Review at the A&WMA’s 107th Annual Conference & Exhibition to be held in Long Beach, CA. The presentation of the review and the discussants commentary is planned for Wednesday morning, June 25, 9:00–11:50 a.m. PDT. The invited discussants are:

David Diaz-Sanchez and Daniel Costa, U.S. Environmental Protection Agency.

Daniel Greenbaum, Health Effects Institute.

Ronald Wyzga, Electric Power Research Institute.

Michael Kleinman, University of California, Irvine.

John Watson, Desert Research Institute.

The discussants will provide different views on aspects of BC, its environmental toxicity, and the potential for incorporation of this species in new air quality regulations. They may agree with or challenge the authors about their evaluation of the literature. Comments also will be solicited from the floor and from written submissions to the Critical Review Committee Chair. The Chair will synthesize these comments for publication in the October issue of the Journal of the Air & Waste Management Association (JA&WMA). Members are encouraged to suggest topics for future reviews and seek membership on the Critical Review Committee to participate actively in this important element of A&WMA science reporting. If you are interested in joining the committee, please send an e-mail to [email protected].

Critical Review Committee

A. Gwen Eklund, Chair

George Hidy, Immediate Past Chair (2009–2012)

Judith Chow, Past Chair (2001–2008)

Sam L. Altshuler

Patricia A. Brush, Technical Council Chair

Prakash Doraiswamy

Marcel Halberstadt

Michael Kleinman

Naresh Kumar

Luis Diaz-Robles

Peter Mueller

Thomas Overcamp

Eric Stevenson

Abhilash Vijayan

John Watson, Past Chair (1994–2000)