Practical Indicators for Risk of Airborne Transmission in Shared Indoor Environments and Their Application to COVID-19 Outbreaks

The ways in which the virus that causes Covid-19 is spread have been under intense public and scientific scrutiny. This article looks at what can be done to reduce airborne transmission of the virus in indoor environments

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Airborne Transmission Route of COVID-19: Why 2 Meters/6 Feet of Inter-Personal Distance Could Not Be Enough

International Journal of Environmental Research and Public Health ◽

10.3390/ijerph17082932 ◽

2020 ◽

Vol 17 (8) ◽

pp. 2932 ◽

Cited By ~ 105

Author(s):

Leonardo Setti ◽

Fabrizio Passarini ◽

Gianluigi De Gennaro ◽

Pierluigi Barbieri ◽

Maria Grazia Perrone ◽

...

Keyword(s):

Virus Transmission ◽

Field Studies ◽

University Hospital ◽

World Health ◽

Indoor Environments ◽

Airborne Transmission ◽

Surface Stability ◽

Air Samples ◽

Daily Life Activities ◽

The World

The COVID-19 pandemic caused the shutdown of entire nations all over the world. In addition to mobility restrictions of people, the World Health Organization and the Governments have prescribed maintaining an inter-personal distance of 1.5 or 2 m (about 6 feet) from each other in order to minimize the risk of contagion through the droplets that we usually disseminate around us from nose and mouth. However, recently published studies support the hypothesis of virus transmission over a distance of 2 m from an infected person. Researchers have proved the higher aerosol and surface stability of SARS-COV-2 as compared with SARS-COV-1 (with the virus remaining viable and infectious in aerosol for hours) and that airborne transmission of SARS-CoV can occur besides close-distance contacts. Indeed, there is reasonable evidence about the possibility of SARS-COV-2 airborne transmission due to its persistence into aerosol droplets in a viable and infectious form. Based on the available knowledge and epidemiological observations, it is plausible that small particles containing the virus may diffuse in indoor environments covering distances up to 10 m from the emission sources, thus representing a kind of aerosol transmission. On-field studies carried out inside Wuhan Hospitals showed the presence of SARS-COV-2 RNA in air samples collected in the hospitals and also in the surroundings, leading to the conclusion that the airborne route has to be considered an important pathway for viral diffusion. Similar findings are reported in analyses concerning air samples collected at the Nebraska University Hospital. On March 16th, we have released a Position Paper emphasizing the airborne route as a possible additional factor for interpreting the anomalous COVID-19 outbreaks in northern Italy, ranked as one of the most polluted areas in Europe and characterized by high particulate matter (PM) concentrations. The available information on the SARS-COV-2 spreading supports the hypothesis of airborne diffusion of infected droplets from person to person at a distance greater than two meters (6 feet). The inter-personal distance of 2 m can be reasonably considered as an effective protection only if everybody wears face masks in daily life activities.

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Quantitative assessment of the risk of airborne transmission of SARS-CoV-2 infection: prospective and retrospective applications

10.1101/2020.06.01.20118984 ◽

2020 ◽

Cited By ~ 5

Author(s):

G. Buonanno ◽

L. Morawska ◽

L. Stabile

Keyword(s):

Exposure Time ◽

Quantitative Assessment ◽

Individual Risk ◽

Indoor Environments ◽

Airborne Transmission ◽

Mount Vernon ◽

Retrospective Assessment ◽

Novel Approach ◽

Prospective Assessment ◽

The Individual

AbstractAirborne transmission is a recognized pathway of contagion; however, it is rarely quantitatively evaluated. This study presents a novel approach for quantitative assessment of the individual infection risk of susceptible subjects exposed in indoor microenvironments in the presence of an asymptomatic infected SARS-CoV-2 subject. The approach allowed the maximum risk for an exposed healthy subject to be evaluated or, starting from an acceptable risk, the maximum exposure time. We applied the proposed approach to four distinct scenarios for a prospective assessment, highlighting that, in order to guarantee an acceptable individual risk of 10−3 for exposed subjects in naturally ventilated indoor environments, the exposure time should be shorter than 20 min. The proposed approach was used for retrospective assessment of documented outbreaks in a restaurant in Guangzhou (China) and at a choir rehearsal in Mount Vernon (USA), showing that, in both cases, the high attack rate values can be justified only assuming the airborne transmission as the main route of contagion. Moreover, we shown that such outbreaks are not caused by the rare presence of a superspreader, but can be likely explained by the co-existence of conditions, including emission and exposure parameters, leading to a highly probable event, which can be defined as a “superspreading event”.

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Can Air-Conditioning Systems Contribute to the Spread of SARS/MERS/COVID-19 Infection? Insights from a Rapid Review of the Literature

International Journal of Environmental Research and Public Health ◽

10.3390/ijerph17176052 ◽

2020 ◽

Vol 17 (17) ◽

pp. 6052 ◽

Cited By ~ 6

Author(s):

Francesco Chirico ◽

Angelo Sacco ◽

Nicola Luigi Bragazzi ◽

Nicola Magnavita

Keyword(s):

Air Conditioning ◽

Far East ◽

Hvac Systems ◽

Rapid Review ◽

Sufficient Evidence ◽

Indoor Environments ◽

Airborne Transmission ◽

International Agencies ◽

Viral Particles ◽

Air Conditioning Systems

The airborne transmission of SARS-CoV-2 is still debated. The aim of this rapid review is to evaluate the COVID-19 risk associated with the presence of air-conditioning systems. Original studies (both observational and experimental researches) written in English and with no limit on time, on the airborne transmission of SARS-CoV, MERS-CoV, and SARS-CoV-2 coronaviruses that were associated with outbreaks, were included. Searches were made on PubMed/MEDLINE, PubMed Central (PMC), Google Scholar databases, and medRxiv. A snowball strategy was adopted to extend the search. Fourteen studies reporting outbreaks of coronavirus infection associated with the air-conditioning systems were included. All studies were carried out in the Far East. In six out the seven studies on SARS, the role of Heating, Ventilation, and Air Conditioning (HVAC) in the outbreak was indirectly proven by the spatial and temporal pattern of cases, or by airflow-dynamics models. In one report on MERS, the contamination of HVAC by viral particles was demonstrated. In four out of the six studies on SARS-CoV-2, the diffusion of viral particles through HVAC was suspected or supported by computer simulation. In conclusion, there is sufficient evidence of the airborne transmission of coronaviruses in previous Asian outbreaks, and this has been taken into account in the guidelines released by organizations and international agencies for controlling the spread of SARS-CoV-2 in indoor environments. However, the technological differences in HVAC systems prevent the generalization of the results on a worldwide basis. The few COVID-19 investigations available do not provide sufficient evidence that the SARS-CoV-2 virus can be transmitted by HVAC systems.

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Ventilation procedures to minimize the airborne transmission of viruses at schools

10.1101/2021.03.23.21254179 ◽

2021 ◽

Author(s):

L. Stabile ◽

A. Pacitto ◽

A. Mikszewski ◽

L. Morawska ◽

G. Buonanno

Keyword(s):

Classroom Environment ◽

Ad Hoc ◽

Virus Transmission ◽

Transmission Risk ◽

Mitigation Measures ◽

Indoor Environments ◽

Airborne Transmission ◽

Transmission Potential ◽

Feedback Control Strategy ◽

Ventilation Rates

AbstractReducing the transmission of SARS-CoV-2 through indoor air is the key challenge of the COVID-19 pandemic. Crowded indoor environments, such as schools, represent possible hotspots for virus transmission since the basic non-pharmaceutical mitigation measures applied so far (e.g. social distancing) do not eliminate the airborne transmission mode. There is widespread consensus that improved ventilation is needed to minimize the transmission potential of airborne viruses in schools, whether through mechanical systems or ad-hoc manual airing procedures in naturally ventilated buildings. However, there remains significant uncertainty surrounding exactly what ventilation rates are required, and how to best achieve these targets with limited time and resources. This paper uses a mass balance approach to quantify the ability of both mechanical ventilation and ad-hoc airing procedures to mitigate airborne transmission risk in the classroom environment. For naturally-ventilated classrooms, we propose a novel feedback control strategy using CO2 concentrations to continuously monitor and adjust the airing procedure. Our case studies show how such procedures can be applied in the real world to support the reopening of schools during the pandemic. Our results also show the inadequacy of relying on absolute CO2 concentration thresholds as the sole indicator of airborne transmission risk.

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Ventilation System Operation to Minimize the COVID-19 Airborne Transmission in Schools

TECNICA ITALIANA-Italian Journal of Engineering Science ◽

10.18280/ti-ijes.652-424 ◽

2021 ◽

Vol 65 (2-4) ◽

pp. 300-306

Author(s):

Luca Stabile ◽

Antonio Pacitto ◽

Giorgio Buonanno ◽

Marco Dell’Isola

Keyword(s):

Exchange Rates ◽

Ventilation System ◽

Virus Transmission ◽

Infection Risk ◽

Indoor Environments ◽

Mass Balance Equation ◽

System Operation ◽

Airborne Transmission ◽

Mechanically Ventilated ◽

Route Of Transmission

Minimizing the SARS-CoV-2 virus transmission is essential to face the COVID-19 pandemic. This is even more important for highly crowded indoor environments, e.g. schools, where the mitigation solutions based on social distancing and hand washing seem to be not effective to reduce the virus airborne transmission mode, which is the main route of transmission. To minimize the airborne virus transmission a proper ventilation is necessary. In the study, a simplified mass balance equation (box-model) was applied to school scenarios in order to determine the required conditions to maintain the infection risk below an acceptable level. In particular, the required air exchange rates for mechanically-ventilated classrooms and the adequate airing procedures for naturally ventilated classrooms were determined. Moreover, for naturally ventilated classrooms, a control strategy based on the measurement of CO2 indoor concentration was also developed.

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Airborne Transmission of Virus-Laden Aerosols inside a Music Classroom: Effects of Portable Purifiers and Aerosol Injection Rates

10.1101/2020.12.19.20248374 ◽

2020 ◽

Author(s):

Sai Ranjeet Narayanan ◽

Suo Yang

Keyword(s):

Aerosol Deposition ◽

Injection Rate ◽

Musical Instruments ◽

World Health ◽

Indoor Environments ◽

Airborne Transmission ◽

Classroom Effects ◽

Airborne Concentration ◽

Health Organization ◽

Injection Rates

The ongoing COVID-19 pandemic has shifted attention to the airborne transmission of exhaled droplet nuclei within indoor environments. The spread of aerosols through singing and musical instruments in music performances has necessitated precautionary methods such as masks and portable purifiers. This study investigates the effects of placing portable air purifiers at different locations inside a classroom, as well as the effects of different aerosol injection rates (e.g., with and without masks, different musical instruments and different injection modes). Aerosol deposition, airborne concentration and removal are analyzed in this study. It was found that using purifiers could help in achieving ventilation rates close to the prescribed values by the World Health Organization (WHO), while also achieving aerosol removal times within the Center of Disease Control and Prevention (CDC) recommended guidelines. This could help in deciding break periods between classroom sessions, which was around 25 minutes through this study. Moreover, proper placement of purifiers could offer significant advantages in reducing airborne aerosol numbers (offering orders of magnitude higher aerosol removal when compared to nearly zero removal when having no purifiers), and improper placement of the purifiers could worsen the situation. The study suggests the purifier to be placed close to the injector to yield a benefit, and away from the people to be protected. The injection rate was found to have an almost linear correlation with the average airborne aerosol suspension rate and deposition rate, which could be used to predict the trends for scenarios with other injection rates.

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Estimation of airborne viral emission: quanta emission rate of SARS-CoV-2 for infection risk assessment

10.1101/2020.04.12.20062828 ◽

2020 ◽

Cited By ~ 1

Author(s):

G. Buonanno ◽

L. Stabile ◽

L. Morawska

Keyword(s):

Viral Load ◽

Research Area ◽

Infection Risk ◽

Activity Level ◽

Indoor Environments ◽

Emission Rates ◽

Airborne Transmission ◽

Emission Data ◽

Novel Approach ◽

Before And After

AbstractAirborne transmission is a pathway of contagion that is still not sufficiently investigated despite the evidence in the scientific literature of the role it can play in the context of an epidemic. While the medical research area dedicates efforts to find cures and remedies to counteract the effects of a virus, the engineering area is involved in providing risk assessments in indoor environments by simulating the airborne transmission of the virus during an epidemic. To this end, virus air emission data are needed. Unfortunately, this information is usually available only after the outbreak, based on specific reverse engineering cases. In this work, a novel approach to estimate the viral load emitted by a contagious subject on the basis of the viral load in the mouth, the type of respiratory activity (e.g. breathing, speaking), respiratory physiological parameters (e.g. inhalation rate), and activity level (e.g. resting, standing, light exercise) is proposed. The estimates of the proposed approach are in good agreement with values of viral loads of well-known diseases from the literature. The quanta emission rates of an asymptomatic SARS-CoV-2 infected subject, with a viral load in the mouth of 108 copies mL−1, were 10.5 quanta h−1 and 320 quanta h−1 for breathing and speaking respiratory activities, respectively, at rest. In the case of light activity, the values would increase to 33.9 quanta h−1 and 1.03×103 quanta h−1, respectively.The findings in terms of quanta emission rates were then adopted in infection risk models to demonstrate its application by evaluating the number of people infected by an asymptomatic SARS-CoV-2 subject in Italian indoor microenvironments before and after the introduction of virus containment measures. The results obtained from the simulations clearly highlight that a key role is played by proper ventilation in containment of the virus in indoor environments.

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