A job-exposure matrix (JEM) is a tool used to assess exposure to potential health hazards in occupational epidemiological studies. Essentially, a JEM comprises a list of levels of exposure to a variety of harmful (or potentially harmful) agents for selected occupational titles. In large population-based epidemiological studies, JEMs may be used as a quick and systematic means of converting coded occupational data (job titles) into a matrix of possible exposures, obviating the need to assess each individual's exposure in detail.