This study provides the first report of concentrations of OPEs in UK indoor and outdoor air. These data are used to estimate human exposure via inhalation, and its importance relative to other exposure pathways and to RfD values, in so doing, testing the hypothesis that inhalation is an important pathway of human exposure to OPEs. In addition, this study provides insights into the relative impact of indoor sources of individual OPEs, and also establishes a baseline against which any current or future measures designed to reduce environmental contamination with OPEs may be evaluated.