Aerodynamic versus physical size of spores: Measurement and implication for respiratory deposition

Abstract

Different methods available for size measurements of fungal and actinomycete spores were compared for four fungal species ( Penicillium brevicompactum, Penicillium melinii, Cladosporium cladosporioides , and Aspergillus versicolor ) and two actinomycete species ( Streptomyces albus and Thermoactinomyces vulgaris ). The physical size of spores was measured with three microscopic methods: with an optical microscope from stained (wet) slides, with an optical microscope from unstained (dry) slides and with an environmental scanning electron microscope (SEM) directly from the microbial culture. The aerodynamic diameter, d a , of airborne spores was measured with an aerodynamic particle sizer. The respiratory deposition of spores was calculated with a computer-based model. The environmental SEM measurements indicated larger size for fungal spores than the optical microscope, whereas for actinomycete spores, both microscopes gave comparable results. Optical microscopic measurements showed that the stained fungal spores were 1.1-1.2 times larger than the unstained ones, which was attributed to the different hydration status of spores. There was no clear trend in the relationship between the d a and the physical diameter measured with any of three tested microscopic methods. For example, the physical diameter of Cladosporium cladosporioides spores was larger than the d a by a factor ranging from 2.0 to 2.2, whereas the d a of Streptomyces albus spores was larger than the physical diameter by a factor of 1.3-1.5. Thus, the aerodynamic diameter of microbial spores cannot be accurately estimated solely based on the physical diameter but needs information on the density of the spores that may vary considerably. The results on the spore size were utilized to calculate respiratory deposition of spores. The errors in the size measurement were found to result in overestimation of the respiratory deposition of C. cladosporioides spores by a factor of 1.2-1.8, and underestimation of the respiratory deposition of S. albus spores by a factor of 0.6-0.7. These errors in the size measurement cause bias in the exposure assessment and in the estimation of the efficiency of control devices. More research is needed to standardize the method for particle diameter estimates applicable for airborne spores.