An investigation of occupational exposure to diesel exhaust, in terms of elemental carbon, was conducted as part of a feasibility study in the Canadian railroad industry. Both personal and area samples were collected from three major operating divisions of the railways: mechanical service, transportation, and engineering. A total of 255 elemental carbon samples have been described. The results show that all but six elemental carbon concentrations, expressed as size-selective respirable air samples taken using a 10 mm nylon cyclone, are well below the 2001 proposed American Conference of Governmental Industrial Hygienists' (ACGIH) threshold limit value (TLV) of 20 microg/m3. The concentration of diesel exhaust, expressed as elemental carbon, in the railroad industry is much lower than that in some other major industries such as mining and forklift truck operations. If the TLV is to be applicable to a broad range of workplace settings such as railroad, construction, and mining, the use of a TLV that is based on an elemental carbon measurement of size selective respirable samples, as recommended in the 2001 ACGIH proposal, would appear to be the most valid strategy for control of exposure to diesel exhaust.