At a given job or during their occupational history, many workers are exposed to more than just one carcinogen. Typical co-exposures to occupational carcinogens comprise asbestos and polycyclic aromatic hydrocarbons (PAHs), chromium and nickel compounds or radon and crystalline silica. The joint effects of these carcinogens on the development of lung cancer are largely unknown. This gap in knowledge impedes more efficient primary prevention and fair compensation of diseased workers.

Within the last decade, several epidemiological studies have been conducted across Europe and North America on risk factors for the development of lung cancer. Considerable expert knowledge has been implemented to assess exposure quantitatively in several studies. Although several studies have been large in size, the low joint prevalence of two or more occupational exposures in the general population limits the power for a detailed statistical analysis for joint effects within the scope of the individual studies.

In parallel, comprehensive exposure databases have been developed for the purpose of better exposure assessment and risk estimation. For example, the Finnish Institute of Occupational Health has compiled a European database for exposure prevalence by agent and industry (CAREX) and has developed a job-exposure matrix (FINJEM). In addition, large occupational exposure databases have been created by national bodies or within the European Union during the last two decades, resulting in enormous amounts of measurements (several million). Some of these databases contain countrywide data from multiple industries for multiple chemical agents. Examples are the U.K. Health and Safety Executive's (HSE) National Exposure Database (NEDB), the German MEGA database of the Institut für Arbeitsschutz der Deutschen Gesetzlichen Unfallversicherung (IFA), and the French COLCHIC database of the Institut National de Recherche et de Sécurité (INRS). Some of these databases contain only data for specific industries, like EXASRUB for the rubber-manufacturing industry, or for a specific agent within particular industries, like WOODEX for wood dust and DMP from the International Mineralogical Association (IMA) for silica. These databases provide valuable resources for the quantitative exposure assessment of occupational carcinogens for epidemiological studies.

Tobacco smoking is the strongest and the single most important risk factor for lung cancer in the general population. Therefore, the evaluation of the interaction of occupational carcinogens must account for the potential confounding and effect modification with tobacco smoking.

The German Social Accident Insurance (DGUV) addressed its research institute for occupational medicine, the Institute for Prevention and Occupational Medicine (IPA), to develop a research project together with the International Agency for Research on Cancer (IARC). This project, known as SYNERGY, will perform risk estimations for lung cancer to evaluate the exposure-response relationships for the single agents, their interaction with smoking and the combined exposures to more than one occupational lung carcinogen.