Review : Inhalation Toxicology and Human Risk Assessment of Carbon Black

Abstract

Carbon black is a substance consisting of colloidal sized fused aggregates of elemental carbon obtained by the partial combustion of hydrocarbon sources - typically oil. Apart from its use in printing inks and paints, its major usage is as a reinforcing agent in rubber products, notably tyres. Because of its size and carbon nature, it has been used as a control dust in a number of studies which were primarily aimed at testing various hypotheses related to the possible pul monary carcinogenicity of diesel soot, over which there was some concern. Surprisingly, in the chronic rat inhalation studies, performed in separate labo ratories, the rats exposed to carbon black, as well as diesel soot, developed lung cancers and other lesions. However, in the various studies the animals also were shown to be experiencing severe 'particle overloading' of the lung - a phenomenon in which the defence scavenging cells in the alveoli are so over whelmed that they lose mobility and stop particle clearance. This in turn leads to a chain of pathophysiological events resulting in severe inflammation and chronic fibrosis. It is from and within such a background that the lung tumours develop in the parenchyma (not the bronchi) of the lungs. Other rodents and humans do not seem to develop lung tumours under similar conditions of particle overload from such biologically inert particles, thus, the lung tumours seen in such studies, may in part or all, be a rat-specific event. It is therefore argued that lung tumours induced in rats under conditions of severe overload from inert dusts such as carbon black do not provide a reliable model for human lung cancer.