Aerosol sampling has evolved with changes in technology and our understanding of the importance of assessing the biologically relevant fractions of the total aerosol. During the past decade there has been international agreement on the definitions for the inhalable, thoracic, and respirable fractions and instruments have been developed to collect samples according to these conventions. These measurement techniques are now well established in the workplace and are increasingly being applied to assessments of non-occupational exposure, as the practical difficulties in obtaining samples over 24 hours have been solved. It is argued that multiple aerosol size fractions should be measured for hazardous substances--such as inorganic lead--in which inhaled material may either be absorbed in the alveoli (respirable) or cleared from the ciliated airways in the lung to the gut and then absorbed (thoracic and extrathoracic). Such measurements should improve the evaluation of the risk for inhaled lead, particularly for non-occupational exposure of children. Also, passive aerosol sampling techniques may enable measurements of non-occupational exposure to be made over several weeks and this would also help improve the reliability of the risk evaluation for aerosol toxicants with a long biological half life. However, further development of these techniques is required if they are to be practicable and biologically relevant.