Effect of relative humidity on the aerodynamic diameter and respiratory deposition of fungal spores

Exposure to airborne fungal spores may cause respiratory symptoms. The hygroscopicity of airborne spores may significantly affect their aerodynamic diameter, and thus change their deposition pattern in the human respiratory tract. We have investigated the change in aerodynamic diameter of five different fungal species as a function of relative humidity. Liquid and dry dispersion methods were explored for the aerosolization of the fungal spores. A new system that produces non-aggregated spore aerosol directly from a moldy surface was designed and found suitable for this study. The spores were aerosolized from a mold growth on agar by ducting dry air over the surface, and spore chains in the flow were broken up by passing the entire flow through a critical orifice. Size-spectrometric measurements with an Aerodynamic Particle Sizer showed that the aerodynamic diameter of the tested fungal spores does not change significantly when the relative humidity increases from 30% to 90%. A more distinct spore size increase was found at a relative humidity of 100%. The highest change of the aerodynamic diameter was found with Cladosporium cladosporioides: it increased from 1.8 μm to 2.3 μm when the relative humidity increased from 30% to 100%. The size increase corresponds to an approximate doubling of the particle volume. In order to estimate the effect of hygroscopic growth on the respiratory deposition of spores, the mean depositions in the human respiratory tract were calculated for fungal spores with various size changes due to hygroscopic growth. A recently developed model of the International Commission of Radiological Protection was used for the respiratory deposition calculations. We found that the 27% increase in Cladosporium size results in a 20–30% increase in the respiratory deposition of these spores. We conclude that most fungal spores are only slightly hygroscopic and the hygroscopic increase does not significantly affect their respiratory deposition. Our calculations show that for bioaerosol particles, ranging from 0.1 to 10 μm in diameter, the greatest change in the respiratory deposition due to hygroscopic size changes occurs in the particle size range of 0.5–2 μm.