Review Black Carbon as an Additional Indicator of the Adverse Health Effects of Airborne Particles Compared with PM10 and PM2.5 Nicole A.H. Janssen,1 Gerard Hoek,2 Milena Simic-Lawson,3 Paul Fischer,1 Leendert van Bree,4 Harry ten Brink,5 Menno Keuken,6 Richard W. Atkinson,3 H. Ross Anderson,7 Bert Brunekreef,2,8 and Flemming R. Cassee1 1National Institute for Public Health and the Environment, Bilthoven, the Netherlands; 2Institute for Risk Assessment Sciences, Utrecht University, Utrecht, the Netherlands; 3Division of Population Health Sciences and Education and the Medical Research Council–Health Protection Agency Centre for Environment and Health, St. George’s, University of London, London, United Kingdom; 4Netherlands Environmental Assessment Agency, Bilthoven, the Netherlands; 5Energy Research Center of the Netherlands, Petten, the Netherlands; 6Netherlands Applied Research Organization, Utrecht, the Netherlands; 7Medical Research Council–Health Protection Agency for Environment and Health, King’s College, London, United Kingdom; 8Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, Utrecht, the Netherlands Background : Current air quality standards for particulate matter (PM) use the PM mass concentration [PM with aerodynamic diameters ≤ 10 mμ (PM10) or ≤ 2.5 mμ ( PM2.5)] as a metric. It has been suggested that particles from combustion sources are more relevant to human health than are particles from other sources, but the impact of policies directed at reducing PM from combus- tion processes is usually relatively small when effects are estimated for a reduction in the total mass concentration. o Bjectivs : We evaluated the value of black carbon particles (BCP) as an additional indicator in air quality management. Methods : We performed a systematic review and meta-analysis of health effects of BCP compared with PM mass based on data from time-series studies and cohort studies that measured both expo- sures. We compared the potential health benefits of a hypothetical traffic abatement measure, using near-roadway concentration increments of BCP and PM2.5 based on data from prior studies. r sulte : Estimated health effects of a 1-gμ/m 3 increase in exposure were greater for BCP than for PM10 or PM2.5, but estimated effects of an interquartile range increase were similar. Two-pollutant models in time-series studies suggested that the effect of BCP was more robust than the effect of PM mass. The estimated increase in life expectancy associated with a hypothetical traffic abatement measure was four to nine times higher when expressed in BCP compared with an equivalent change in PM2.5 mass. c onclusi : BCP is a valuable additional air quality indicator to evaluate the health risks of air quality dominated by primary combustion particles. k ey words : air quality management, black carbon, combustion particles, health effects, particu- late matter, review. Environ Health Perspect 119:1691–1699 (2011). http://dx.doi.org/10.1289/ ehp.1003369 [Online 2 August 2011] Particulate matter (PM) is a heterogeneous mixture varying in physicochemical properties depending on meteorological conditions and emission sources [World Health Organization (WHO) 2006]. Current air quality standards for PM use the mass concentration of PM [PM with aerodynamic diameters ≤ 10 μm (PM10) or ≤ 2.5 μm ( PM2.5)] as a metric, supported by health studies showing robust associations between ambient PM mass con- centrations and a wide array of adverse health effects (WHO 2006). However, it is likely that not every PM component is equally important in causing these health effects (WHO 2007). Combustion-related particles are thought to be more harmful to health than PM that is not generated by combustion (Krzyzanowski et al. 2005; WHO 2007). In urban areas, road traffic is a major source of combustion PM [Health Effects Institute (HEI) 2010]. In a systematic review of the literature, Krzyzanowski et al. (2005) concluded that transport-related air pollution contributes to an increased risk of death, particularly from cardiopulmonary causes, and that it increases the risk of respiratory symptoms and diseases that are not related to allergies. In a more recent review of traffic-related air pollution, HEI (2010) concluded that there was suf- ficient evidence to support a causal relation- ship between exposure to traffic-related air pollution and exacerbation of asthma, and suggestive evidence of a causal relationship with onset of childhood asthma, nonasthma respiratory symptoms, impaired lung func- tion, total and cardiovascular mortality, and cardiovascular morbidity. Combustion particles also derive from a variety of sources other than motorized road traffic, including wood and coal burn- ing, shipping, and industrial sources, and these sources may contribute significantly to ambient combustion particle concentrations (WHO 2006). There is increasing concern that current mass-based PM standards are not well suited for characterizing health risks of air pollution near sources of combustion particles, such as motorized traffic on major roads or in wood-smoke–dominated com- munities. Furthermore, emission reduction measures such as the use of particle traps or the introduction of environmental zones are Environmental Health Perspectives • volume 119 | number 12 | December 2011 thought to be effective in reducing exposure to traffic-related air pollution, but the estimated impact of such measures is relatively small when expressed in relation to a reduction in the PM mass concentration (Lefebvre et al. 2011; Millstein and Harley 2010; Tonne et al. 2008). Nitrogen dioxide (NO2) is a regulated component of air pollution that is also used as an indicator of traffic-related air pollution in health impact assessment and air quality man- agement (Tonne et al. 2008). However, NO2 is not a suitable indicator to evaluate the effect of traffic abatement measures on exposure to combustion particles because some abatement measures, such as filters on diesel fueled vehi- cles, may increase NO2 levels (Millstein and Harley 2010). In addition, spatial gradients near roadways are less pronounced for NO2 than for black smoke (BS) and particle num- ber because of high background concentra- tions of NO2 (Krzyzanowski et al. 2005). This is less of a concern for nitric oxide (NO) and nitrogen oxides (NOx), which is the sum of NO and NO2, but these components are not regulated currently, and they do not appear to be toxicologically important at current ambient levels. These considerations led us to consider whether another PM metric might better reflect the health effects of combustion-related air pollution than PM mass or provide an additional indicator of the effectiveness of air quality management plans aimed at reduc- ing exposure to particles from combustion sources. We have deliberately used the term “additional indicator” because we do not claim that all health effects associated with PM mass in previous studies can be attributed to a marker of combustion particles. Address correspondence to N.A.H. Janssen, National Institute for Public Health and the Environment (RIVM), P.O. Box 1, 3720 BA Bilthoven, the Netherlands. Telephone: 31 302744027. Fax: 31 302744451. E-mail: nicole.janssen@rivm.nl Supplemental Material is available online (http:// dx.doi.org/10.1289/ehp.1003369). The study was funded by the Dutch Ministry of Infrastructure and the Environment. The authors declare they have no actual or poten- tial competing financial interests. Received 21 December 2010; accepted 2 August 2011. 1691