This paper provides an overview of airborne methylene diphenyl diisocyanate (MDI) concentrations in workplaces across North America and Europe. A total of 7649 samples were collected between 1998 and 2020 by producers of MDI during product stewardship activities at customer sites, primarily using validated OSHA or ISO sampling and analysis techniques. As would be expected from the low vapor pressure of MDI, 80% of the concentrations were less than 0.01 mg/m (1 ppb) and 93% were less than 0.05 mg/m (5 ppb). Respiratory protection is an integral part of Industrial Hygiene practices; therefore, its use was studied and summarized. While covering a variety of MDI applications, a large number of samples was obtained from composite wood manufacturing facilities, offering specific insight into potential exposures associated with different process sections and job types in this industry sector. Given the potential presence in industrial processes of MDI-containing dust or aerosols, future work should place increased emphasis on also investigating dermal exposure. The data reported in this paper provide valuable information for product stewardship and industrial hygiene purposes throughout the MDI-processing industry.