measured environmental concentration measured environmental concentration an measured environmental concentration (mec) relates to a chemical substance found in an environmental sample. the concentration of the compound may result from direct contamination, transformation and/or metabolization of a different chemical contaminant, natural origin or a combination of these sources. mec is to be used as a reference in the context of chemical safety assessments (csa) and should be compared with the respective predicted environmental concentration (pec) and predicted no-effect concentration (pnec) in order to decide whether exposure model is valid and the compound related risk is controlled.