Persistence of Phi6, a SARS-CoV-2 surrogate, in simulated indoor environments: Effects of humidity and material properties

Abstract

The SARS-CoV-2 virus caused the COVID-19 pandemic and brought major challenges to public health. It is transmitted via aerosols, droplets, and fomites. Among these, viral transmission through fomites is not well understood although it remains a very important transmission route. This motivated us to study how fomites play a role in viral transmission within controlled indoor environments. To achieve this, we investigated viral aerosol persistence on fomites under different humidity levels to mimic the built environment. We developed a protocol to study the effect of humidity on viral infectivity using a full-scale environmental chamber. The results show that the infectivity of aerosolized Phi6 in air decreased by ≥ 1 log10 as the relative humidity (RH) increased from 25% to 75% but then increased by ≥ 1 log10 as the RH further increased to 85%, resulting in a characteristic V-shape curve which varied with exposure time. Consistently, we show that although material properties may impact viral persistence, changes in the local humidity more significantly influence viral persistence on fomites. These results provide new insights into indoor fomite-mediated viral transmission under different environmental conditions. These findings will help guide the design of more effective strategies for viral control in indoor environments.

Fig 1. Higher sampling flow rates result in higher viral recovery.

(A) Concentration of viruses recovered from air (including total viral load and infectious viral load) with different sampling flow rates (n = 3). Lower flow rates (6 L/min & 4 L/min) resulted in less viral particles recovered compared to higher flow rates (11.5 L/min & 9 L/min). Infectious viral load across the sampling flow rates did not show significant variation (p > 0.05). (B) Peak particle sizes in the particle size distribution were < 5 μm, which confirms the presence of Phi6-laden aerosols.

Fig 2. Persistence of aerosolized Phi6 in air.

(A) Effect of relative humidity (RH) on the infectivity of aerosolized Phi6 follows a characteristic V-shape which varies over time. (B) The infectious viruses recovered as a function of exposure time under tested humidity conditions. (C) Size distribution of Phi6-laden aerosols. All conditions were tested in duplicate.

Fig 3. Persistence of Phi6 on materials.

(A) Phi6 is pre-loaded on materials as conventional droplets (n = 3). (B) Phi6 is aerosolized followed by deposition on materials (n = 6). Dashed lines indicate the limit of detection. (C) RH changes within an absorptive material (gypsum wallboard as an example). (D) Deposited viral load on materials in the full-scale chamber (n = 3).

Fig 4. A schematic depicting the risk of infection from deposited virus-laden droplets and aerosols on porous fomites.