1.1 ,This guide defines conventions for personal samplers of specific particle-size-dependent fractions of any given non-fibrous airborne aerosol. Such samplers are used for assessing health effects and in the setting of and testing for compliance with permissible exposure limits in the workplace and ambient environment. The conventions have been adopted by the International Standards Organization (ISO 7708), the Comité, Europé,en de Normalisation (CEN EN 481), and the American Conference of Governmental Industrial Hygienists (ACGIH) (1).2 The conventions were developed (2) in part from health-effects studies reviewed (3) by the ACGIH and in part as a compromise between definitions proposed by the ACGIH (3) and by the British Medical Research Council (BMRC) (4). Conventions are given here for inhalable, thoracic, and respirable fractions.