Effect of Fluid Type and Microbial Properties on the Aerosolization of Microorganisms from Metalworking Fluids

Abstract

Aerosolization of mist from metalworking fluids (MWFs) has been well characterized in previous studies. Much less is known about the aerosolization of microorganisms, although airborne microbial exposures at MWF sites have been associated with occupational respiratory symptoms and diseases. In this study, the effects of fluid type, microorganism concentration in the liquid, and the microbial species on the aerosolization of microorganisms from MWFs were tested. Three microorganisms were employed to represent different size and surface characteristics: Bacillus subtilis bacterial endospores (hydrophobic particles with aerodynamic diameter = 0.9 μm), Pseudomonas fluorescens bacterial vegetative cells (hydrophilic, 0.8 μm), and Penicillium melinii fungal spores (hydrophobic, 3.1 μm). The testing was first performed using a Collison nebulizer to aerosolize microorganisms from three fluids: water, semisynthetic MWF, and soluble oil. No significant difference in the aerosolization ratio (microbial concentration in the air normalized to the microbial concentration in the liquid) was observed among the three fluids. For all tested microorganisms, the concentration in the air increased proportionally with the increase of the microbial concentration in the liquid. The aerosolization ratio of B. subtilis endospores was greater than that of P. fluorescens cells and P. melinii spores. To explore the aerosolization of microorganisms from MWFs under the conditions that are closer to industrial settings, the tests were conducted with a MWF simulator (a laboratory-scale setup simulating the mist generation during grinding process). Simulator tests showed the same trend with respect to microbial aerosolization as those performed with the Collison nebulizer. This was further confirmed by a separate experiment, in which the Collison nebulizer and the MWF simulator were tested with liquids containing polystyrene latex (PSL) particles. As a result, our study showed that hydrophobic microorganisms were easier to aerosolize from MWFs than hydrophilic microorganisms and that increasing microorganism size was likely to result in decreasing aerosolization ratio.