Setting of occupational exposure limits (OEL) for bitumen is currently on the agenda of regulatory bodies internationally. Bitumen is mainly used in paving and rooWng applications. Its chemical composition is variable and depends on the composition of the underlying crude oil and the manufacturing process. Bitumen consists mainly of aliphatic compounds, aromatic hydrocarbons, heterocyclic compounds containing nitrogen, oxygen, and sulfur atoms, and small amounts of polycyclic aromatic hydrocarbons (PAH). Depending on the type of bitumen and its laying temperature, workers are exposed to diVerent quantities of various bitumen constituents, with varying concentrations of vapors and aerosols. The complex composition is one of the reasons why investigations, which may serve as a basis for occupational standard setting, are scarce. Another reason is the diYculty to organize Weld studies with a suYciently large number of exposed persons, when the working places and associated working conditions are highly variable. Therefore, investigations into the health eVects of occupational bitumen exposure have been remained a major scientiWc challenge. Bitumen for long has been regarded as possible carcinogen because of its contents of polycyclic aromatic hydrocarbons (PAH). For instance, the German MAK commission had classiWed bitumen as a suspected carcinogen (K3) already in the 1980s, and a health-based OEL was not derived (Greim 2001). Instead, the German tripartide Committee for Hazardous Substances (AGS) had set a TRK value (Technical Guidance Concentration), which reXects the technically manageable exposure. A classiWcation of bitumen as carcinogenic was seemingly conWrmed by positive carcinogenicity results of a long-term bioassay using dermal application (Sivak et al. 1997). In Germany, these Wndings led to lowering of the TRK value, which was Wnally suspended by end of 2004. Industry, however, argued that the route of administration in the dermal bioassay did not reXect the occupational exposure. Therefore, new studies were initiated to further clarify the situation. A long-term inhalation carcinogenicity study in rats turned out to be negative (Fuhst et al. 2007), and substantial eVorts were made to gain sustainable data on human health from exposed persons. Missing data on possible respiratory eVects were of particular interest. So far, only few studies have characterized the eVects of bitumen exposure on nonmalignant responses of the respiratory tract, and especially data on irritative eVects of bitumen under current exposure conditions in humans remained limited. This situation led to the initiation of a new occupational study in mastic asphalt workers exposed to vapors and aerosols of bitumen under current exposure conditions. The principal aim of this large-scale Human Bitumen Study in Germany was to assess irritative eVects of vapors and aerosols on the airways, using a cross-shift design in a wellcharacterized group of bitumen-exposed workers. Furthermore, it appeared worthwhile to use the study to determine possible genotoxic eVects in blood cells. With the examination of 500 workers, the new Human Bitumen Study is one of the largest investigations of bitumen-exposed workers under current working conditions. The results of these studies are important, if new OELs will be considered. The project is an example of successful collaboration of diVerent disciplines, namely occupational hygienists and physicians, pneumologists, immunologists, epidemiologists, and toxicologists. In this supplemental issue of G. Schluter (&) Wuppertal, Germany e-mail: gerhard.schlueterGS1@gmx.net