The dynamics of SARS-CoV-2 infectivity with changes in aerosol microenvironment

Abstract

Understanding the factors that influence the airborne survival of viruses such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in aerosols is important for identifying routes of transmission and the value of various mitigation strategies for preventing transmission. We present measurements of the stability of SARS-CoV-2 in aerosol droplets (∼5 to 10 µm equilibrated radius) over timescales spanning 5 s to 20 min using an instrument to probe survival in a small population of droplets (typically 5 to 10) containing ∼1 virus/droplet. Measurements of airborne infectivity change are coupled with a detailed physicochemical analysis of the airborne droplets containing the virus. A decrease in infectivity to ∼10% of the starting value was observable for SARS-CoV-2 over 20 min, with a large proportion of the loss occurring within the first 5 min after aerosolization. The initial rate of infectivity loss was found to correlate with physical transformation of the equilibrating droplet; salts within the droplets crystallize at relative humidities (RHs) below 50%, leading to a near-instant loss of infectivity in 50 to 60% of the virus. However, at 90% RH, the droplet remains homogenous and aqueous, and the viral stability is sustained for the first 2 min, beyond which it decays to only 10% remaining infectious after 10 min. The loss of infectivity at high RH is consistent with an elevation in the pH of the droplets, caused by volatilization of CO 2 from bicarbonate buffer within the droplet. Four different variants of SARS-CoV-2 were compared and found to have a similar degree of airborne stability at both high and low RH.

Expiratory aerosol pH is determined by indoor room trace gases and particle size

“…After very collegial and helpful discussions with the authors of ref. 1 , we would like to express our appreciation of their work but also raise a concern about the possible overgeneralization of the alkaline pH found in their study. The pH of exhaled liquid aerosol particles is determined by complex interactions between the particle and the gas phase and is highly dependent on particle size.…”

“…The work of ref. 1 draws attention to the important role of pH in the inactivation of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in aerosol particles. After very collegial and helpful discussions with the authors of ref.…”

“…Ref. 1 exposed their particles to a humidity- and temperature-controlled airflow originating from a compressed air supply. The compressed laboratory air is most likely depleted of many of the stickier trace components, such as nitric acid (HNO 3 ) and ammonia (NH 3 ), due to wall absorption effects.…”

“…Thus, not only outdoor aerosol but also exhaled aerosol particles become acidic due to interaction with indoor air, contrary to the statements of ref. 1 . Acidification is even more rapid if synthetic lung fluid or mucus derived from primary airway cultures ( 2 ) is used instead of the cell culture minimal essential medium (MEM) applied by ref.…”

“…Acidification is even more rapid if synthetic lung fluid or mucus derived from primary airway cultures ( 2 ) is used instead of the cell culture minimal essential medium (MEM) applied by ref. 1 , as the former contains less bicarbonate buffer.…”