Very fine particle number and mass concentrations were mapped in an engine machining and assembly facility in the winter and summer. A condensation particle counter (CPC) was used to measure particle number concentrations in the 0.01 microm to 1 microm range, and an optical particle counter (OPC) was used to measure particle number concentrations in 15 channels between 0.3 microm and 20 microm. The OPC measurements were used to estimate the respirable mass concentration. Very fine particle number concentrations were estimated by subtracting the OPC particle number concentrations from 0.3 microm to 1 microm from the CPC number concentrations. At specific locations during the summer visit, an electrical low pressure impactor was used to measure particle size distribution from 0.07 microm to 10 microm in 12 channels. The geometric mean ratio of respirable mass concentration estimated from the OPC to the gravimetrically measured mass concentration was 0.66 with a geometric standard deviation of 1.5. Very fine particle number concentrations in winter were substantially greater where direct-fire natural gas heaters were operated (7.5 x 10(5) particles/cm(3)) than where steam was used for heat (3 x 10(5) particles/cm(3)). During summer when heaters were off, the very fine particle number concentrations were below 10(5) particles/cm(3), regardless of location. Elevated very fine particle number concentrations were associated with machining operations with poor enclosures. Whereas respirable mass concentrations did not vary noticeably with season, they were greater in areas with poorly fitting enclosures (0.12 mg/m(3)) than in areas where state-of-the-art enclosures were used (0.03 mg/m(3)). These differences were attributed to metalworking fluid mist that escaped from poorly fitting enclosures. Particles generated from direct-fire natural gas heater operation were very small, with a number size distribution modal diameter of less than 0.023 microm. Aerosols generated by machining operations had number size distributions modes in the 0.023 microm to 0.1 microm range. However, multiple modes in the mass size distributions estimated from OPC measurements occurred in the 2-20 microm range. Although elevated, very fine particle concentrations and respirable mass concentrations were both associated with poorly enclosed machining operations; the operation of the direct-fire natural gas heaters resulted in the greatest very fine particle concentrations without elevating the respirable mass concentration. These results suggest that respirable mass concentration may not be an adequate indicator for very fine particle exposure.