Carbon Black and Soot: Two Different Substances

Carbon blacks are manufactured under controlled conditions for commercial use primarily in the rubber, painting, and printing industries. In contrast, soots are unwanted byproducts from the combustion of carbon-based materials for the generation of energy or heat, or for the disposal of waste. Unfortunately, the terms carbon black and soot often have been used interchangeably; however, carbon black is physically and chemically distinct from soot. Greater than 97% of carbon black consists of elemental carbon arranged as aciniform particulate. Depending on the type of soot, the relative amount of carbon (< 60% of the total particle mass), the type of particulate carbon, and particle characteristics (size, shape, and heterogeneity) can vary considerably. For both carbon black and soot, other elements and chemical compounds are associated with the particulate carbon. Total inorganics (ash) represent < 1% of the carbon black particle mass. Organic compounds can be extracted from particle surfaces (solvent extractable fraction [SOF]), and for carbon black, also are < 1% of the particle mass. Soots have much greater percentages of ash, SOF, or both, than carbon black. There has been concern about the adsorbed organic compounds because of potential biologic activity. For carbon black, the SOF is strongly adherent to carbon surfaces and is not released by biological fluids. The types of organic compounds consist primarily of unsubstituted polycyclic aromatic hydrocarbons and are not as biologically potent as those compounds present in soot. Thus, carbon black is distinctly different from soot, and when discussing potential health effects, care must be taken to differentiate between the two types of carbon-based particles.