(Smith et al. 2009). For time-series studies, there are no large differences in temporal rela- tionships between central-site ambient con- centrations and personal exposure for BCP and PM2.5 (Janssen et al. 2005). In addition, issues related to the correlation between dif- ferent pollutants and the extent to which they can act as surrogates for the etiologic agent(s) complicate the interpretation of results from multipollutant models (Tolbert et al. 2007). Our interpretation that the results from two- pollutant models for the time-series studies suggest that BCP is a more health–relevant indicator in these studies than is PM mass is also supported by Roemer and van Wijnen (2001a, 2002), who calculated separate effect estimates with separate exposure estimates using background and traffic-influenced meas- urement stations and for the total population and people living along busy roads. Effect esti- mates for urban background BS were larger in the population living along busy roads than for the total population, suggesting that this subpopulation was more highly exposed. Indeed, effect estimates for the population living along busy roads using BS measured at traffic stations were more or less equivalent to effect estimates for the total population using BS measured at urban background stations. Further evidence of health effects of pri- mary combustion is obtained in studies that use source apportionment techniques to assess associations of particles from different sources with health. Particles from traffic or local combustion were associated with daily mortality and hospital admissions (Cakmak et al. 2009a, 2009b; Laden et al. 2000; Mar et al. 2000; Sarnat et al. 2008). Although measures of BCP are not frequently used in human controlled exposure studies, several human exposure studies using exposure to diesel exhaust have documented airway and systemic inflammation (Salvi et al. 2000), as well as responses that provide a possible mechanism for cardiac events such as myo- cardial infarction (Mills et al. 2007). Two studies illustrated the importance of particle composition: Mills et al. (2008) found little effect of 2-hr exposure to high PM2.5 con- centrations taken in Edinburgh, attributed to the high sea salt content (90%); Urch et al. (2005) found blood pressure increases in healthy subjects related to the OC content of fine PM—largely from motorized traffic—but not to total PM2.5. Spatial contrast in BCP compared with PM2.5. Higher street:background ratios for BCP compared with PM (Figure 2) are consistent with the larger impact of traffic on BCP than on PM mass concentrations. However, the studies included in our review represent a variety of settings, including dif- ferent distances to the roadside, traffic densi- ties (including vehicle types), averaging times, seasons, and meteorological conditions. These factors probably (partly) explain the variability in ratios observed between studies. The impact of traffic on BCP was also demonstrated for temporal concentration variations by Schaap and van der Gon (2007), who showed that BS levels on rural and urban locations in the Netherlands were about 50% higher on weekdays than on Sundays, whereas BS concentrations at urban traffic locations were about 100% higher on weekdays than on Sundays. Comparison of weekdays and Sundays for PM10 mass concentrations showed much smaller differences (5–15%). We estimated that, on average, 55% of the roadside increment in PM2.5 consisted of EC based on absolute dif- ferences in concentrations between street and background concentrations. Deriving an overall quantitative estimate of this percentage across studies is complicated by the different measure- ment methods used for BCP, in which differ- ences between methods for measuring EC and differences in conversion of optical measures of BCP to EC concentrations both need to be taken into account. We therefore converted BS and Abs to EC using a conversion factor based on the average of 11 identified area-specific comparison studies and used the range of these 11 values in sensitivity analyses. Our estimates compare well with previous studies (Lefebvre et al. 2011; Lena et al. 2002; Millstein and Harley 2010). In a study on the spatial variation in EC and PM2.5 in rela- tion to local truck-traffic density, Lena et al. (2002) estimated that EC represents 52% of the total PM2.5 generated by large trucks. In comparison, in a modeling study on the effects of retrofitting trucks with diesel particle fil- ters, EC accounted for 64% of total diesel PM2.5 emissions (Millstein and Harley 2010). Similarly, in a modeling study of the effect of a speed limit reduction on traffic-related EC, Lefebvre et al. (2011) estimated that EC traffic emissions account for 70% of the total PM2.5 exhaust emissions. These figures are also in the range provided by the European emission inventory guidebook for the EC fraction in PM2.5 in exhaust emissions for different vehi- cle categories (e.g., passenger cars, vans, and trucks) (Ntziachristos and Samaras 2009). The contribution of BCP in roadside increments of PM10 will be smaller because resuspended road dust, including brake and tire wear, results in a more substantial contribution to PM10 (Gietl et al. 2010; Janssen et al. 1997). Comparison of calculated health benefits of traffic abatement measures using PM2.5 or BCP. We evaluated the gain in life expectancy of a 1-gμ/m 3 decrease in PM2.5 and 0.55 gμ/m 3 EC, based upon the average contribution of EC to the increment in PM concentration in stud- ies comparing a major road and urban back- ground. This calculation can be interpreted as an indication of the potential difference in a Environmental Health Perspectives • volume 119 | number 12 | December 2011 Black carbon as additional indicator health impact assessment based upon PM2.5 or BCP for populations living along a major road. It can also be interpreted as the potential health gain for policies that reduce concentrations in approximately the same ratio as the current roadside increment, for example, a limitation of overall traffic intensity. There are few empirical data to support larger impacts of policies on BCP than on PM2.5 mass. In an evaluation of the effects of retrofitting trucks with diesel engine particle filters on air quality in Southern California, Millstein and Harley (2010), using an Eulerian photochemical air quality model, estimated a decrease in EC concentrations in 2014 of 12–14%. The estimated effect on PM2.5 mass concentrations was much smaller (< 1%). In a modeling study of the effect of a speed limit reduction (from 120 to 90 km/hr) on air qual- ity in Flanders, EC concentrations decreased up to 30% just next to the busiest highways, com- pared with an estimated reduction of at most 8.5% for PM2.5. For buffer zones of 0–100 m distance to the highways EC concentrations decreased by 9–10% (Lefebvre et al. 2011). A small monitoring study on the effects of road closures associated with the 2004 Democratic National Convention in Boston suggested slightly lower concentrations of EC and NO2 during the road closure periods at monitoring sites proximate to the closed highway segments. This decrease was not observed for PM2.5 or far- ther from major highways (Levy et al. 2006). Our finding of a larger increase in life expectancy associated with a hypothetical traf- fic abatement policy measure when expressed per achievable reduction in BCP than when expressed per an equivalent amount of PM2.5 mass illustrates that health effects of such poli- cies may be seriously underestimated when based upon PM2.5 or PM10. As an illustration, we calculated the increase in life expectancy for the population living along major roads. We did not attempt to calculate the impact for the larger population. However, although the absolute improvement of air quality will be smaller, we expect that the differences between BCP and PM mass will be similar. In a modeling study of the effect of a speed limit reduction, Lefebvre et al. (2011) estimated a decrease in EC concentrations of 0.4 μg/m 3 for buffer zones within 200 m of the high- ways, affecting about 75,000 inhabitants if the abatement measure would be implemented on all highways in Flanders and Brussels. For the buffer zone within 1,500 m of the highway the reduction was smaller, 0.17 gμ/m 3 (5%), but affecting up to 1.8 million inhabitants. Overall discussion. Our review shows that BC is associated with health effects that are not reflected quantitatively in the same way by particle mass, as indicated by the higher effect estimates per 1 μg/m 3 for BCP com- pared with PM mass. 1697