Abstract The list of substances and working conditions linked to a high risk of cancer continues to grow. To establish priorities in research needs and orient preventive action in the field of occupational cancer and carcinogenic substances, it is important to have data on worker exposure to these carcinogens. First the scope of the problem must be determined and an overall portrait drawn up in terms of both exposure to carcinogenic substances or conditions and the incidence of occupational cancer. This report presents the results of the first stage in a project at the Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST) aimed at documenting the exposure of Québec workers to carcinogenic substances or conditions. It offers an important contribution to Québec knowledge about exposure to carcinogens; its observations should be useful to anyone interested in this topic. Exposure estimates were compiled for 38 carcinogenic substances listed in Schedule I of the Regulation respecting occupational health and safety (designation C1, C2 or C3) and in the known or probable carcinogens list published by the International Agency for Research on Cancer (groups 1 and 2A). The number of workers potentially exposed to each carcinogen was obtained by applying the percentages of exposed workers in a given industry, calculated from various data sources, to the number of people working in that industry in Québec according to the 2006 census data. The information on exposure was based on laboratory tests performed by the IRSST for the Réseau public de la santé au travail, the results from a number of special projects carried out by the IRSST, data from Santé-Québec's survey on health and wellbeing (Enquête sociale et de santé 1998), Health Canada data on occupational radiation exposure, and exposure data compiled as part of the CAREX Canada project conducted by the University of British Columbia. For some carcinogens, the exposure data came from two French sources: the SUMER survey of occupational physicians by France's Ministère du travail, and the MATGÉNÉ job exposure matrices developed by the Institut de veille sanitaire. According to these calculations, the ten substances or conditions to which the greatest number of Québec workers are exposed are as follows: solar radiation (6.6%), night work or rotating shifts including night work (6.0%), diesel exhaust (4.4%), wood dust (2.9%), polycyclic aromatic hydrocarbons (excluding diesel exhaust) (2.0%), benzene (1.7%), silica (1.5%), lead (1.3%), artificial ultraviolet rays (1.1%) and mineral oils (1.0%). In several industries, over 20 different carcinogens are present; these industries include manufacturing, construction, other services except public administration, utilities, professional, scientific and technical services, and administrative, support, waste management and remediation services. Among the manufacturing industries with exposure to multiple carcinogens are non-metallic mineral products, transportation equipment, primary metals, chemicals and paper. Based on these percentages, it is estimated that at least 230,300 Quebecers are exposed to solar radiation and more than 150,000 to diesel exhaust in their jobs. Over 50,000 are exposed to carcinogens in manufacturing, transportation and warehousing, agriculture, forestry, hunting and fishing, and health care and social assistance. Exposure to polycyclic aromatic hydrocarbons (PAHs), diesel exhaust, benzene and solar radiation affects most of the industries with a young labour force, including retailing, arts, entertainment and recreation, and accommodation and food services. A breakdown of the data according to sex shows that more women are exposed to carcinogens in health care and social assistance (ionizing radiation, night work, artificial UV rays and solar radiation). Men are present in greater proportions in agriculture, forestry, hunting and fishing, mineral extraction, oil and gas, construction, utilities, manufacturing and transportation and warehousing; these industries are characterized by exposure to solar radiation, wood dust, night work, silica, diesel exhaust, mineral oils and lead. Despite their limitations, the estimates are useful indicators of the extent of Québec workers' potential exposure to carcinogenic substances, mainly because this is the first portrait of its type based on data aggregated from various sources. Because cancers take several years to develop and it can be difficult to establish a link between a cancer and a given occupational exposure, the best strategy is prevention. The preventive approach for exposure to carcinogens is the same as for any occupational hazard: anticipation, identification, assessment and control (through elimination at source, substitution and reduction of exposure), as well as informing and educating employers and workers about carcinogenic substances.