Since 1985, a number of North American researchers have evaluated the association between worker exposure to metalworking fluids (MWFs) and cancer mortality or respiratory morbidity. The studies have used different methods to measure the MWF aerosol concentration and to evaluate the exposures to the specific components of the MWF aerosol (bacteria, endotoxin, elements, metals, ethanolamines, polyaromatic hydrocarbons). This diversity of approaches makes comparison of study results difficult and has impeded the development of an occupational exposure limit (OEL) for metalworking fluids. This article summarizes the exposures measured in these North American epidemiologic studies by estimating their thoracic and inhalable MWF particulate levels. In addition, issues that must be resolved before a universal sampling and analysis method for MWF can be recommended are reviewed, including: the use of gravimetric versus extractable analysis; whether the analytical limit of detection can support a lower occupational exposure limit; if the volatile components of mineral oils should also be collected; and whether there are components of specific concern in MWFs that should be regulated separately. Finally, recommendations for future directions in MWF exposure assessment and control are suggested.