Summary The Québec prevention network has several hundred people who visit establishments regularly and take samples for air quality testing. These samples are analyzed at the IRSST. From 2001 to 2008, under the service agreement with the CSST and its network (MSSS and ASP), annual reports and an eight-year summary report have identified substances found in high concentrations in several industries. Taking into account the wide range of economic activities in the regions of Québec, this report identifies the substances found in the highest concentrations in each administrative region of the Ministère de la Santé et des Services sociaux (MSSS), since most of the samples came from MSSS occupational health officers. Given that about 359,120 environmental analysis results have been produced for these clienteles, it was possible to exploit the data so as to extract a detailed portrait of the main pollutants found in high concentrations in each MSSS administrative region. This should make it possible, for the first time, to read the analysis results from a new angle and thus facilitate data interpretation for MSSS officers, because it will be easier for them to find specific results for establishments in their region. It is already well documented that the volume of samples varies widely from one region to the next, and that the Montréal and Montérégie regions are the ones usually requesting the greatest number of tests—which is normal, given the high numbers of workers and MSSS officers in those regions. To present the information on a comparable basis, we chose to draw up a summary table for each region showing the ten industrial classes (with their four-digit CAEQ codes) in which the chemical/industrial class pairs with the highest concentrations appear, in decreasing order of concentration. It is thus possible to identify the most probable situations of potential occupational overexposure, by substance and by associated economic activity. The study was conducted with the aim of providing researchers and MSSS officers with information that can support new research and intervention avenues focused on the trends in each region. Some 55,200 test results met the extraction criteria and revealed the chemical/industrial class pairs most likely to require additional targeted preventive actions in the industries sampled. Analysis of these results showed that most of the substances still posing a major challenge in terms of prevention are dusts and the metals they contain. Beryllium, welding fumes, lead, quartz, other unclassified dusts, hardwood and softwood dusts and aluminum are examples of substances that have been measured in high concentrations in several regions. Nevertheless, some organic substances such as styrene still require efforts to control worker exposure.