OBJECTIVE: Ultrafine particles (UFPs) are generated from common work processes and have thus existed for a long time. Far more prevalent than engineered nanoparticles, they share common toxicological characteristics with them. However, there is no existing retrospective assessment tool specific to UFPs, for example, for epidemiological purposes. Thus, we aimed to develop a job-exposure matrix dedicated to UFPs. METHOD: Fifty-seven work processes were identified as well as the chemical composition of UFPs emitted, following a literature review and the input of an expert panel. These work processes were associated with occupational codes as defined by the ISCO 1968 classification. The probability and frequency of UFP exposure were assessed for each combination of occupational code and process. Summarized probabilities and frequencies were then calculated for all ISCO occupational codes associated with several processes. Variations in exposure over time or across industrial sectors were accounted for in the assessment of each occupational code. RESULTS: In the ISCO classification, 52.8% of the occupational codes (n = 835) assessed were associated with exposure to UFPs, consisting mainly of carbonaceous, metallic, and mineral families (39.5%, 22 and, 15.8%, respectively). Among them, 42.6% involved very probable exposure, and at a high frequency (regularly or continuously). CONCLUSION: These results suggest that occupational exposure to UFPs may be extensive at the workplace and could concern a wide variety of workers. Pending the integration of a third parameter assessing the intensity of UFP exposure, the MatPUF JEM already constitutes a promising and easy-to-use tool to study the possible adverse health effects of UFPs at work. It may also guide prevention policies in the occupational environments concerned, including those involving engineered nanoparticles.Read less <