Janssen et al. In the reviewed studies, ambient meas- urements of various BCP metrics were used. Although motorized traffic was an important source of BC in most of these studies, they included the impact of all combustion sources on BCP concentrations, including coal and wood burning, shipping emissions, and indus- trial sources. In a review of source apportion- ment studies for fine PM EC, Schauer (2003) found that the combined contribution of diesel- and gasoline-powered vehicles ranged from 74% to 98%; the contribution from biomass burning ranged from 0.7% to 25%; and the contribution from other sources, from 0.5% to 17%. The derived risks therefore rep- resent those for BCP as a general indicator of combustion particles, not exclusively traf- fic. Issues therefore remain when these risk estimates are applied to specific combustion sources such as traffic or wood burning. We however hold that BCP more closely resem- bles the harmful components in these air pol- lution mixtures than does general PM2.5. Associations between individual elements and mortality or morbidity could be explained by the health effects of that element or the health effects of a pollution mixture of which the element is an indicator. Therefore, BCP could be serving as an indicator for the larger category of primary combustion particles, which, in addition to BCP, can include trace metals and hydrocarbons such as polycyclic aromatic hydrocarbons, any or all of which could be acting to cause adverse health effects. Our analysis assumes that these other compo- nents are equally decreased relative to reduc- tions in BCP when measures are taken that reduce emissions of combustion particles. This assumption will be more valid for measures that do not affect engine characteristics, such as a restriction of the number of vehicles, com- pared with measures that affect particle com- position, such as speed reduction or changes in engine types or fuel mixtures. Furthermore, because BCP is a marker for tailpipe emissions, it is less suitable to evaluate the health benefits of traffic-oriented abatement measures that are expected to result in reductions in nontailpipe emissions, from brake linings, crank cases, tire wear, and so forth, which may be uncorrelated with reductions in BCP. In 2003, a WHO working group rec- ommended reevaluation of BS as part of the reconsideration of the WHO air quality guidelines and consideration of the addition of photometric analysis of BCP on the PM2.5 filters (WHO 2003). Our review supports this recommendation. We foresee application of a BCP indicator in evaluation of current levels of traffic-related air pollution, wood smoke, or other combustion particles and policies aimed at reducing these sources. Of the dif- ferent methods to measure BCP, probably the best method would be EC measurements using 1698 standard methodology (Cavalli et al. 2010). However, it is beyond the scope of this article to make recommendations about the methods that should be used to measure BCP. In summary, we do not promote BCP as an alternative marker for PM mass because this would disregard the effects of coarse particles or particles from other sources. Nonetheless, our review shows that BCP is a valuable additional air quality indicator that would be particularly useful to evaluate health risks of air pollution dominated by primary combustion emissions, as well as benefits of traffic abatement measures. Ref Rencs Adams HS, Nieuwenhuijsen MJ, Colvile RN, Older MJ, Kendall M. 2002. 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