Janssen et al. Possible candidates for such an indicator are measures of black carbon particles [BCP; e.g., BS, black carbon (BC), absorption coef- ficient (Abs), and elemental carbon (EC)] and organic carbon (OC), particle number con- centration, particle surface area, and combus- tion-specific PM components. The extent of available data to support the health relevance of these measures varies widely; most of the information is available for BS. Evidence is also available from both toxicological and epidemiological studies of health effects of ultrafine PM (Knol et al. 2009), but the costs and complexity of monitoring and concerns about the validity of central-site monitor- ing to estimate personal exposure to ultrafine particles, which is characterized by particle number concentrations rather than particle mass, probably limit the feasibility of particle number as an additional metric. A WHO (2003) working group recom- mended reevaluating BS as an indicator of traffic-related air pollution, but a systematic comparison using PM versus BCP indicators to estimate health effects is still lacking. Grahame and Schlesinger (2010) reviewed the evidence of effects of BC on cardiovascular health end points and concluded that it may be desirable to promulgate a BC PM2.5 standard. However, no systematic comparison with PM10 or PM2.5 mass was included. Conversely, Smith et al. (2009) noted that although the results of their time-series meta-analysis suggest larger effects per unit mass of sulfate than of BS; this dis- tinction was less clear in the few studies that directly compared estimated effects of both indicators. This observation indicates the need to critically compare studies that have meas- ured PM mass as well as BCP. BCP would be a useful indicator in addi- tion to particle mass if a) health risks associated with BCP are quantitatively or qualitatively different from those associated with PM mass on a mass unit basis, b) the spatial contrast related to the vicinity of combustion sources and the impact of emission reductions is larger than for PM mass, and c) BCP and particle mass are not—or at least not usually—highly correlated in time or space. In this article we evaluate the value of BCP as an indicator of the adverse health effects of combustion particles in addition to PM mass. We performed a systematic review and meta-analysis of epidemiological stud- ies that measured both PM mass and BCP and estimated the potential impact on life expectancy of a traffic abatement measure using the pooled concentration–response functions for BCP and PM2.5. Although we focused on traffic-related particles, we refer to combustion particles in general because health effects estimates were based on measurements of BCP from all combustion sources, not exclusively traffic. 1692 Materials and Methods Measurement methods for BCP. BCP as a metric of combustion-derived PM may be determined by optical methods or thermal- optical analysis. Optical methods used to measure BS, BC, and the Abs of PM are all based on the blackness of a filter sample. For BS the amount of reflected light is converted to total PM mass, whereas for BC it is con- verted to EC mass. Although BS is expressed in micrograms per cubic meter, there is no consistent relationship to PM mass because conversion of the optical measurement to mass units depends on location, season, and type of combustion particles (Hoek et al. 1997). BS has been used in Europe since the 1920s, and although it has been phased out since the introduction of PM10 as the new regulatory particulate metric, some countries, including the Netherlands and the United Kingdom, continue to measure BS in selected locations (Bloemen et al. 2007). EC is determined by thermal-optical analysis in a multistep process, typically resulting in a measurement of OC as well. There are several different protocols to measure EC, and results may differ by up to a factor of 2 (HEI 2010). Extreme care is thus necessary when comparing data on EC from different studies. Concentrations for EC averaged over 24 hr are available for regional and urban monitoring sites through the U.S. IMPROVE (Interagency Monitoring of Protected Visual Environments) network and the U.S. Environmental Protection Agency Chemical Speciation Network, but there are no national monitoring networks for EC in Europe. Here we use “BCP” as a generic term for any of the different metrics (BS, EC, BC, or Abs) but refer to the study-specific metric when describing individual studies. The different optical measurements for BCP (BS, BC, and Abs) are highly correlated (Quincey 2007; Roorda-Knape et al. 1998). Although thermally determined EC and optical measures of BC are also highly corre- lated, the quantitative relation between them varies between countries, cities, and type of location (e.g., regional, urban, traffic), high- lighting the need for site-specific calibrations (Cyrys et al. 2003; Schaap and van der Gon 2007). Differences between EC measurement methods add to this variation. To facilitate comparisons among studies that used different measures of BCP, we derived a BS-to-EC con- version factor based on the average increase in EC associated with a 10-gμ/m 3 increase in BS reported in 11 studies with information on both measures (Adams et al. 2002; Cyrys et al. 2003; Edwards et al. 1983; Erdman et al. 1993; Janssen et al. 2001; Kinney et al. 2000; Lena et al. 2002; Schaap and van der Gon 2007). Based on this analysis, we assume by default that 10 μg/m 3 BS is equivalent to 1.1 μg/m 3 EC [Supplemental Material, Table A1 (http://dx.doi.org/10.1289/ ehp.1003369)]. In addition, we conducted sensitivity analyses using conversion fac- tors over the range of the estimates from the individual studies (0.5–1.8 μg/m 3 EC per 10 gμ/m 3 BS). Systematic review of health effects of BCP compared with PM mass. Literature search. We conducted a search for peer-reviewed literature in PubMed (National Library of Medicine 2007) for epidemiological studies that evaluated the health effects of (a meas- ure of) PM mass as well as health effects of (a measure of) BCP. We used the following key words: (British smoke or black smoke or black carbon or elemental carbon or EC or soot or absorbance or absorption coefficient) AND [particles or particulate matter or par- ticulates or particulate air pollution or fine partic* or “PM10” or “PM2.5” or “PM(2.5)” or “PM(10)” or sulfate* or sulphate*] AND (mortality or cohort or hospital or emergency). We scanned all abstracts and retrieved papers that potentially included effect estimates for PM mass as well as BCP. For acute health effects, we considered only time-series stud- ies on daily mortality and hospital admissions or emergency department visits because these are generally more similar in design and are therefore more likely to allow meta-analyses than are studies on, for example, symptoms or biomarkers. For health effects due to long-term exposure, we considered only cohort studies because they have provided the most relevant data for setting air pollution standards. For the time-series studies, we also used the Air Pollution Epidemiology Database (APED; St George’s, University of London, London, UK) to identify suitable studies. This database comprises standardized estimates extracted from ecological time-series studies identified by systematic review that meet cer- tain quality criteria, with the last retrieval per- formed on 15 May 2009 (Smith et al. 2009). We searched APED for estimates related to effects of PM10, PM2.5, PM with aerodynamic diameter ≤ 13 (PM13), total suspended par- ticulates (TSP), or sulfate as well as BS, BC, or EC. We identified 40 papers on time-series studies on daily mortality or hospital admis- sions that included area-specific estimates for both PM and BCP, and 17 papers on cohort studies. The APED search identified six papers that were not identified in the PubMed search, but four were excluded because more recent estimates from the same city were available. The search identified four papers published after the last APED systematic review in May 2009. For the time-series studies on daily mor- tality and hospital admissions, we excluded five papers on TSP because more recent data, including effect estimates for PM10, were avail- able for most of the cities. Also, we excluded ovlume 119 | number 12 | December 2011 • Environmental Health Perspectives