Physicochemical characterization of porcine respiratory aerosol and considerations for future aerovirology

Abstract

Understanding the mechanisms which inactivate airborne viruses is a current challenge. The composition of human respiratory aerosol is poorly understood and needs to be adequately investigated for use in aerovirology studies. Here, the physicochemical properties of porcine respiratory fluid (PRF) from the trachea and lungs were investigated both in bulk solutions and in aerosols. The mass ratio of Na:K in PRF compared with cell culture media (Dulbecco's Modified Eagle Medium, DMEM), which is frequently used in aerovirology studies, was significantly lower (∼2:1 vs ∼16:1). PRF contained significantly more potassium and protein than DMEM. PRF aerosols of all samples were similarly hygroscopic to human respiratory aerosol. PRF particles could nucleate with spatially separated crystals, indicating that the protein matrix was sufficiently viscous to prevent the complete coalescence of aqueous salts prior to efflorescence. The effects of these differences in compositions on the viability of viruses are currently not well understood. The virus suspensions in aerovirology studies need to be reconsidered to adequately reflect a real-world expiration scenario.

Keywords: droplet physicochemistry; respiratory aerosol; virus viability.

Fig. 1.

(A) Na:K mass ratio, (B) protein:Na mass ratio (Pr:Na), and (C) estimated Cl:CO₃ mass ratio of porcine respiratory fluid (PRF) lung (L) and trachea (T) samples and DMEM. For DMEM, the protein mass was considered as the total mass of amino acids, and Cl:CO₃ for PRF is estimated. Error bars are from the relative standard deviation (RSD) of the inductively coupled plasma optical emission spectroscopy (ICP-OES) data. **P < 0.01.

Fig. 2.

Diametric hygroscopic growth factor (GF) as a function of relative humidity (RH) for porcine respiratory fluid (PRF) aerosols as measured by humidification tandem differential mobility analyzer (H-TDMA) for samples (A) L3, (B) L5, (C) L6, (D) T3, (E) T5, and (F) T6. Shaded regions are standard error of the TDMA inversion.

Fig. 3.

Representative energy-dispersive X-ray spectroscopy (EDS)-mapped images (Na, K, and Cl) of porcine respiratory fluid (PRF) aerosol along with reference dark-field scanning transmission electron micrographs for (A) T3, (B) L3, (C) T5, (D) L5, (E) T6, and (F) L6. The scale bar represents 100 nm.

Fig. 4.

Representative atomic force microscopy (AFM) micrographs (A–C) and aspect ratio (AR) distributions (D–F) and for porcine respiratory fluid (PRF) and DMEM particles. Panels (A) and (D) are PRF lung particles (N = 150 particles, sample L6), (B) and (E) are PRF trachea particles (N = 177 particles, sample T6), and (C) and (F) are DMEM particles (N = 101 particles).

Fig. 5.

Energy-dispersive X-ray spectroscopy (EDS)-mapped images (Na, K, Cl, Ca, and P) of DMEM aerosol along with reference dark-field scanning transmission electron micrographs. The scale bar represents 500 nm.