Summary Nanometric particles that are unintentionally released into the workplace are potentially toxic to workers. They can easily settle in the respiratory system and are distinctive because of their large specific surface area and high potential for causing pulmonary inflammation. The purpose of this study was to characterize unintentionally released nanoparticles (URNPs) found in six workplaces on the basis of a broad range of indicators. Concentrations were assessed according to numerical and mass metrics using an array of direct-reading instruments (DRIs). Integrated measurements were also taken, based on the type of contaminant specific to each workplace. These measurements included (i) respirable and submicron carbon (elemental and organic) fractions, as well as respirable combustible dust from diesel exhaust fumes (DEFs) found in an underground mine (M1), in a truck repair garage (M2) and in maintenance in an underground transit system (M3); (ii) gravimetric measurements and concentrations of 12 metals (aluminum, cadmium, chromium, cobalt, copper, iron, magnesium, manganese, nickel, lead, vanadium, zinc) in fumes and metallic dust released in a foundry (M4), as well as in a machine shop (welding, grinding and cutting) (M5); (iii) paraffin wax (C18-C36) from fumes released in a wax-moulding shop (M6). In parallel, measurements for the purpose of microscopic characterization were also taken in the six workplaces. For the measurements taken by DRIs, the daily numerical concentrations in the six workplaces ranged between 12,900 and 228,600 particles/cm³ and the mass concentrations between 0.01 and 3.22 mg/m³. In terms of number of particles, the underground mine was the environment with the highest concentrations, while the wax-moulding shop had the highest mass concentrations. In environments where DEFs were found, daily concentrations of elemental carbon (EC) ranging from 0.002 to 0.503 mg/m³ were measured with DRIs. For the integrated measurements, the concentrations of total carbon (TC) measured in this study were lower than the Quebec regulated level of 0.4 mg/m³ stipulated in the Regulation Respecting Occupational Health and Safety in Mines, with the exception of a level of 0.7 mg/m³ recorded in workplace M1. A comparison of the metal concentrations from workplaces M4 and M5 with the recommendations of the American Conference of Governmental Industrial Hygienists (ACGIH) found that all the concentrations were 10% below recommended levels, with the exception of one measurement of manganese (respirable fraction) in the foundry (M4). The paraffin wax concentrations measured were below the occupational exposure limit of 2 mg/m³ set by the ACGIH for the fumes this substance releases. Workers exposed to DEFs (M1, M2 and M3) are exposed to mostly nanometric-size airborne particles whose mass concentration is largely in the submicron fraction. In the presence of foundry fumes (M4), workers are exposed to airborne particles that are mostly nanometric in size and whose mass concentration is chiefly in the submicron fraction for chromium, cobalt, copper, iron, manganese, lead, vanadium and zinc. The workers in the machine shop (M5) are exposed to fumes and dust particles from machining, most of which are nanometric in size, but some of the processes they use generate larger, micrometric particles. The contribution of larger particles to the mass concentration is significant in this environment and, as a result, the mass concentration is to be found in the inhalable fraction, especially for chromium, copper, iron and nickel. Workers in the wax shop (M6) are exposed to fumes that are chiefly nanometric in size and whose mass concentration is mostly in the submicron fraction. Our innovative strategy enabled us to characterize the URNPs released in the different workplaces with respect to both numerical and mass concentrations. Microscopy studies on particle samples from the microscope grid taken with a Mini Particle Sampler® were used to characterize the particles collected based on their morphology and chemical composition.