Aerosolization of microorganisms from metalworking fluids (MWFs) was studied using a laboratory-scale setup simulating grinding operations. An optical particle counter (OPC), a condensation nucleus counter (CNC), an electrical low pressure impactor (ELPI), and a photometric aerosol mass monitor were used to measure the airborne particles and microorganisms aerosolized from MWFs. The tests were performed using a serni-synthetic MWF with and without bacterial contamination (Pseudomonas fluorescens). Microbial contamination of the MWF increased the number and mass concentrations of aerosolized particles by a factor of 2 (as measured by the OPC and the photometric aerosol mass monitor, respectively). At the same time, there was an up to 50-fold increase in the concentration of fine particles (0.02-1 mu m), as measured by the CNC. The data collected with the ELPI showed that the peak of the fine particle number concentration was at 0.37 mu m. The results indicate that MWF mist may contain high concentrations of microbial fragments, which may not be detected by traditional microbial analysis methods, such as cultivation or microscopic counting. (c) 2004 Elsevier Ltd. All rights reserved.