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Assessment of SARS-CoV-2 infection risks in indoor environments
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Assessment of SARS-CoV-2 infection risks in indoor environments

Study: Transmissibility of coronavirus and its variants from infected subjects in indoor environments. Image Credit: Elizaveta Galitckaia / Shutterstock

A recent study is under review at Scientific Reports journal. It was posted to the Research Square* preprint server, researchers evaluated severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection risk for indoor scenarios.

Study: Transmissibility of coronavirus and its variants from infected subjects in indoor environments. Image Credit: Elizaveta Galitckaia / ShutterstockStudy: Transmissibility of coronavirus or its variants in infected subjects indoors. Image Credit: Elizaveta Galitckaia / Shutterstock


SARS-CoV-2 infection risks in the indoor environment are not only increased by super spreading events but also due to low and moderate risk events. It is most likely that the aerosol route, which includes droplets and their residues is the main route of transmission of coronavirus 2019 (COVID-19).

Multiple models have quantified SARS-CoV-2’s indoor risk. The earlier single-hit models did not consider droplet inhalation to be a discrete process. This made it difficult to link the viral load in infected people with the likelihood of lung deposition in the susceptible individuals. A double Poisson model estimates how likely it is that an individual will inhale at least one droplet in order for one virion (SARS-CoV-2) to deposit in their lungs.

Because it is difficult to accurately evaluate the evaporation of droplets containing non volatile salts, the risk models used in the past assumed that the final diameter of the droplets was constant. However, the current study model treated the dispersion mechanism by the ventilation-dependent turbulent diffusion process.

This is the missing link in the study that examines how an enhanced viral load can increase disease transmissibility via aerosol routes following a viral loads to aerosol mechanics coupling.

About the study

In the current study, researchers used a standard inhalation infection risk problem to demonstrate the applicability of the droplet evaporation-settling-dispersion model.

They used a 50 m3Room with an air-exchange ratio (AER) below 0.5 h -1 and released 1200, 100, 6.2, and 1.7 droplets of 4.2 µm, 9 µm, 14.6 µm, and 18.8 µm, respectively, during a coughing event. They were given a SARS-CoV-2 load 5×10. 6Maximum 5 x 10 10RNA copies/mL at 0.1 h -1 inactivation rate.

Lifetime of droplets in a typical indoor environment

Droplets last for a lifetime in an indoor environment.

Researchers established the link between the viral load of SARS-CoV-2 and its degree of dissemination by calculating the exposure required to reach single-risk for a given expiratory events, including breathing, speech and coughing. They also considered factors such as mask use, indoor air quality, and ambient temperature, relative humidity (RH), which could all influence viral propagation rates. They also considered expiratory emission characteristics like droplet size distribution and frequency, virion concentration in the emitted droplets, as well as virion concentration.

Apart from ambient conditions, the equilibrium droplet size is affected by the solid content of the saliva/droplet. Researchers assumed a solid content 8 g/L and accurately modelled the evaporation, along with the other processes. Importantly, the researchers did not believe that droplets were instantly mixed in the room environment. They also modeled the effects of ventilation-induced Turbulence to simulate droplet dynamics in the room.

Different SARS-CoV-2 variations infect different parts. Omicron, for example, infects the bronchus and multiplies more quickly than the Delta variant. The researchers considered both pulmonary and bronchial deposition events.

They compared the risk predictions of the current study model with those of Nicas et al. It is notable that Nicas et al. In 2005, Nicas and colleagues presented the first comprehensive model for studying the aerosol route to infection.

Research findings

For 10 minutes, the viral load in indoor environments increased to 2×10 8RNA copies/mL to 2×10 10SARS-CoV-2 infection rate increased rapidly from 1 to 50 percent due to an increase in RNA copies/mL. For viral loads less than 10, the infection risk was below 1% 8RNA copies/mL during one hour indoors

The viral load surpassed 10 10The model predicted a gradual shift in risk to a higher value and a saturating. SARS-CoV-2 Delta, Omicron and other variants of SARS-CoV-2 produce a higher viral loads than the older wild SARS-CoV-2 varieties. The study model supported the observed increased transmissibility. Because variant severity is related to a variant’s infection, the present study showed that infection risk depended on viral load.

The model also considered the effect of ventilation rate upon indoor infection risks. The AER was increased from 0.55 h -1Up to 10 h -1For a 10-minute exposure, the single-hit chance decreased by approximately one. The reason is that ventilation kills viruses in the indoor environment. However when viral load increases, enhanced ventilation decreases because particles smaller than 10 microns are also at risk.

The SARS-CoV-2 infection threat was not affected by the ambient RH. A higher RH was associated with larger droplet sizes and lower lifetimes, which in turn led to a lower risk of infection. The observed variation of droplet lifetime with RH was significant only for particles in the range of 20–80 µm. Large-sized particles were reduced to 1/5th their original size due to gravity and evaporation.


The study demonstrated the importance of using masks, air cleaners and external ventilation in mitigating SARS CoV-2 infection risk. The study model is also useful for evaluating technology deployment’s cost-effectiveness. The most dominant parameter was the viral load, which was correlated with the variant type. The analysis revealed that viral loads and aerosol physics principles governed viral deposition within the lungs.

Overall, the study provided valuable insight into the subject of risk assessment of airborne diseases.

*Important notice

Research Square publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

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