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Air circulation for SARS-CoV-2 infection control

Friday, May 28, 2021

COVID-19

Air circulation for SARS-CoV-2 infection control

Indoor air changes and their potential impact on SARS-CoV-2 transmission

Increasing air changes and air filtration per hour is a simplistic and important concept that can be deployed to reduce the risk of SARS-CoV-2 and other respiratory infections from airborne transmission indoors and in the far world. Sound building management, including higher ventilation and enhanced filtration, is a fundamental but often overlooked part of risk reduction strategies that could benefit beyond the current pandemic.

Allen JG, Ibrahim AM. indoor air changes and their potential impact on SARS-CoV-2 transmission. Jama. published online April 16, 2021. doi: 10.1001 / jama.2021.5053

Commentary

This paper describes what is associated with indoor air circulation and SARS-CoV-2. SARS-CoV-2 has been found to be associated with time spent indoors in the majority of outbreaks involving three or more people, and there have been actual cases of infection. Therefore, it is important to control the indoor environment to reduce the risk of airborne transmission, so measures such as masking, physical distance, ventilation and filtration are necessary. However, in terms of ventilation and filtration, many buildings are not designed for infection control, so this is a step that needs to be strived for in order to meet the recommended goals.

For small indoor spaces, the recommended goals include outdoor air ventilation with recirculated air passing through filters with at least a minimum efficiency rating of 13 (MERV 13) or air passing through portable air purifiers with HEPA (high efficiency particulate air) filters. However, since the dose response of SARS-CoV-2 will be unknown, these measures have been proposed despite the lack of evidence.

In the first place, these countermeasures are proposed because SARS-CoV-2 transmission occurs primarily from exhaled aerosols, and small aerosols can remain airborne for 30 minutes to several hours and travel well beyond 6 feet.

And these proposals are based on the fundamentals of exposure science and inhalation dose risk reduction: the higher the ventilation and filtration rates, the more rapidly they are removed, thus reducing exposure and time. And these suggestions are very similar to those used in hospitals, citing observational epidemiological studies of airborne transmission and low ventilation for measles, tuberculosis, rhinovirus, influenza, and SARS, explaining that ventilation plays an important role in the transmission of infectious diseases.


So, how much ventilation should we have?

There is a ventilation standard for indoor spaces called ASHRAE, which aims to achieve a basic level of acceptable indoor air quality with diluted "biological effluents," but this is not an infection control standard. And in terms of air exchange rates, there is a standard called ACH, which is frequently used in medical settings, but the minimum total required ACH for a typical process is 0.35, which means that a school or similar scale would need to be designed with a higher factor of about 10. In other words, the higher the ACH requirement, the more likely it is that the building will be designed for infection control, similar to a medical facility.

Next, ACHe, which is the removal of respiratory aerosols by "air filtration," is considered and added to the ACH from outside air.

The clean air delivery rate (CADR) of an air purifier or other device is determined by the effectiveness of the filtration and the amount of air that passes through its filter, and becomes the term used to describe the amount of clean air delivered to a space, its effectiveness, etc.

The HEPA filters found in these devices capture 99.97% of aerosols at 0.3 μm, so filter performance and other factors evaluate their effectiveness at the lowest aerosol value of 0.3 μm.

What is explained as a practical consideration is that increasing the air exchange rate comes with trade-offs, such as the additional cost of moving more air and heating or cooling this large volume of air. And even with improved ventilation and filtration of indoor air, there is no significant impact on near-contact propagation, which still requires controls such as "wear a mask". Second, the usefulness of air exchange per hour in a volumetric flow approach to ventilation is most useful in small rooms, where ceiling heights are generally less than 12 feet. Then, the air exchange rate is useful for outbreaks during a pandemic, and these will need to be expanded in limited locations.

In places where masks are not worn, such as restaurants, air changes per hour need to be increased and people who are not eating need to wear masks. And even if you have been drinking, it is advisable for people in the restaurant to stay six feet away.

In summary, the strategies of ventilation according to the size of the space, wearing masks to prevent close contact transmission, and keeping a distance are necessary to counteract virus transmission.

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