Grainger Editorial Staff
Influenza, COVID-19 and other viral respiratory illnesses can spread from one person to another via airborne pathogens. People who are infected may expel pathogenic material into the air by coughing, sneezing, talking or even just by breathing.
Controlling the spread of infection in a facility is absolutely critical during a pandemic, and highly desirable at all times. Facility managers and engineers can help control airborne pathogens by modifying the HVAC operations in their facilities. Building technology organization ASHRAE has published a document on infectious aerosols describing evidence-based HVAC strategies that can help lower the risk of infection. The Environmental Protection Agency (EPA) has referred building professionals to ASHRAE resources for guidance on how HVAC operations can address COVID-19.
To understand how HVAC operations can influence airborne pathogens, it's important to know that infectious material is carried through the air by particles that behave differently depending on their size.
Some infectious material is carried by larger droplets, such as those produced by sneezing or coughing. (Thus the importance of social distancing and cloth face coverings.) Most large droplets settle rapidly onto surfaces fairly close to their source—about three to seven feet away, according to ASHRAE. The organization states that HVAC systems don't have much influence on the behavior of larger droplets that transmit illness over short distances.
But pathogenic material is also carried by smaller droplets that can be aerosolized. This means that they're suspended in the air, much like a mist, remaining aloft for minutes, hours or even days depending on the particle size, and capable of traveling long distances under the right conditions. According to one study, people infected by influenza may produce pathogenic aerosols without sneezing or coughing—simply by breathing.
HVAC systems can play a role in controlling the movement of infectious aerosols, helping to dilute them, to conduct them out of occupied spaces and to eliminate them through filtration—thus helping prevent the spread of airborne illness within the facility.
During pandemics and other emergencies, the following modifications to HVAC operations may help decrease exposures to airborne pathogens, according to ASHRAE. The organization suggests considering these options for inclusion in emergency response plans in non-healthcare buildings.
The HVAC modifications described above are intended to be considered as part of an emergency response plan in non-healthcare facilities. Along with these emergency measures, ASHRAE also lists HVAC strategies to consider implementing for normal, non-emergency operations, based on a hazard assessment.
These non-emergency strategies echo many of ASHRAE's proposed emergency measures and include:
Additionally, ASHRAE recommends designing ventilation systems to promote clean airflow patterns, allowing airborne particles to leave the space. While the organization has not published detailed documentation on how to achieve this in non-healthcare facilities, they have collected guidance and resources that cover airflow patterns in healthcare environments and other topics.
ASHRAE recommends that healthcare facilities consider the following HVAC strategies:
Q: Should HVAC systems be turned off to reduce the risk of COVID-19 transmission?
A: ASHRAE states that in general disabling HVAC systems is not recommended as method to reduce transmission of the virus that causes COVID-19. It is the organization's view that HVAC systems can lower the concentration of viral material in the air and thus lower the risk of transmission, and that people in unconditioned environments may be subjected to unhealthy temperature stress.
Q: How does relative humidity in a building effect transmission of infection?
A: According to research summarized by ASHRAE, there are three ways that low humidity can makes transmission of respiratory infections more likely. While this research is not specific to COVID-19, it is applicable to respiratory infections in general. First, large infectious particles can shrink in dry air to become droplet nuclei, which are dormant but still infectious and can travel long distances. Second, many viruses are most viable in low humidity. Third, there is evidence that ambient humidity below 40% lowers immune system defenses, in part by impairing mucus membrane barriers. Detailed analysis of many of these issues can be found in "Seasonality of Respiratory Infections."
Q: Can HVAC filtration help prevent the spread of illness?
A: There is evidence that filtration can help remove infectious aerosols, according to ASHRAE. The organization suggests that upgrading HVAC filtration to MERV-13 (or the highest level achievable in the system) can be a useful step in pandemic response plans.
Q: What is a respiratory droplet?
A: People produce respiratory droplets when they cough, sneeze, talk and breathe. These droplets are composed mostly of water but can also include cells from the person who emitted them, eloctrolytes from that person's mucous or saliva, and infectious agents such as viruses and bacteria, according to research published by the World Health Organization. The smallest droplets can remain suspended in air for long periods of time, while larger droplets fall quickly to the ground.
The information contained in this article is intended for general information purposes only and is based on information available as of the initial date of publication. No representation is made that the information or references are complete or remain current. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.
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