SARS-CoV-2 transmission through indoor aerosols

A recent review published in the International Journal of Indoor Environment and Health presents the science behind the person-to-person transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in an indoor environment.

The review addresses key elements such as emissions from the respiratory tract, environmental transport and fate, and uptake by a susceptible person.

The ongoing coronavirus disease 2019 (COVID-19) pandemic, caused by SARS-CoV-2, has spread across the world, infecting about 3% of the global human population.

The SARS-CoV-2 transmission has primarily occurred in community settings, initiated by infected individuals emission of SARS-CoV-2 via the respiratory tract in the form of aerosols.

Study: Indoor aerosol science aspects of SARS-CoV-2 transmission. Image Credit: Peterschreiber media / Shutterstock

Ventilation

Ventilation and air movements in buildings are strongly associated with the transmission of infectious diseases such as measles, tuberculosis, chickenpox, influenza, smallpox, and SARS.

The review defines key concepts and terms, such as “airborne,” “aerosol,” and “particle,” drop and size classification, regions of the respiratory tract, modes of transmission, transmission scales, ventilation rates, infectious dose, disease isotropy, and superspreading transmission events.

The emissions from the respiratory tract can be grouped into three categories based on their size: small particles (0.1– 5 µm), large particles (5– 100 µm), and ballistic drops (>100 µm). Notably, the environment transport, dynamic behavior, transport distance, and fate of airborne particles strongly depend on the particle size.

Importantly, under any airflow conditions, most of the emitted particles that are smaller than 20 µm in diameter, carrying the virus, will remain suspended long enough to travel more than 1 m from the emission source. However, this can be totally arrested if the mouth and nose of the infected individual are covered.

The review provides a comparison of the effects of masking on inhalation intake rates of respiratory particles from an infectious emitter who is speaking in a classroom when the person is unmasked or wearing cloth, surgical, or a respirator mask.

NPIs

While non-pharmaceutical interventions (NPI), such as masking, social distancing, efficient ventilation, and air-filters,  reduce virus transmission, studying the mechanism reveals that the exposure to both large (5-100 microns) and small (0.5-1 microns) particles with the SARS-CoV-2 increases the risk of infection in a susceptible individual.

Mitigation of virus transmission in a room includes strategies such as 1) masks efficient to filter small particles, 2) sufficient ventilation, 3) air filtration, 4) limiting occupancy, and 5) limiting the event duration.

The reviewer observed that room-scale spreading is the main contributor to a super spreading event. However, notably, the study on the effect of ballistic drops and large particles on the exposure risk by depositing on a susceptible person’s skin and clothing is yet to be done.

Discussing pertinent information from COVID- 19 outbreak investigations, the reviewer described pivotal aspects of the indoor dynamic behavior of particles emitted from the respiratory tract.

Processes in the transmission of the viral aerosols include the variation in transport distance with the particle size, the rate of deposition to indoor surfaces, the role of water evaporation inducing particle shrinkage, and the particle motion attributable to inertia.

The reviewer also described how the particle size impacts the qualitative and quantitative emission of the particles from the respiratory tract and the regional deposition.

Interestingly, the reviewer stressed distinct behavior such as coughing and sneezing (illness symptoms) and general talking and quiescent breathing (asymptomatic).

The reviewer discussed how evaporation-induced shrinking of the respiratory particles is size-dependent. Also, the chemical composition influences this process, though research on this aspect is yet to be done.

On emission of respiratory particles and drops, the reviewer presented the emission rates from the respiratory tract during breathing, talking, and coughing.

The review discussed the asymptomatic emissions and the inclusion of varying viral loads into the emission estimates, noting that the large variability here would greatly contribute to an uneven transmission risk pattern.

Further, the review detailed the particle deposition in the respiratory tract and the various factors that influence it.

The flow, such as advective flow, gravitational sedimentation, and inertial drift, the size – inhalable (<100  µm),  thoracic (<~10  µm), and respirable (<~ 4  µm), etc. all affect the penetration into the respiratory tract and alveolar or pulmonary region. The reviewer presented a deposition fraction as a function of particle size in the overall respiratory tract.

Observing the airborne transmission in a room, the reviewer emphasized the importance of indoor environment settings for SARS-CoV-2 transmission compared to outdoor settings. Using different models, the review addressed the transfer efficiency, intake fraction for inhalable particles, and estimated infection risk.

Based on the study, the reviewer observed that the SARS- CoV- 2 might exhibit infection isotropy – wherein the risk of infection for the deposited viral RNA varies with location in the respiratory tract. In conclusion, this review highlights ‘key elements from the disciplinary perspective of indoor aerosol science and technology.’

Written by

Dr. Ramya Dwivedi

Ramya has a Ph.D. in Biotechnology from the National Chemical Laboratories (CSIR-NCL), in Pune. Her work consisted of functionalizing nanoparticles with different molecules of biological interest, studying the reaction system and establishing useful applications.