We could not find data on live COVID virus quantities at various locations in HVAC systems. We did find reports indicating the live virus could not travel from occupied spaces, through the HVAC system and back to the occupied space. This communication is one from an ASHRAE member on their Epidemic TASK Force. We need to know where the virus survives in HVAC systems before setting plans. The accompanying papers may assist in that requirement. Subject: RE: COVID-19 transmission consideration Hello, Please see the following response from a member of the ASHRAE Epidemic Task Force: Hello David. Thank you for your question to the ASHRAE Epidemic Task Force (ETF) and for forwarding the interesting attachments. The science surrounding the possibility and likelihood of airborne transport and transmission of COVID-19 is rapidly changing. As time goes on, more and more respected scientists are convinced that the airborne pathway for transmission is likely. For instance, in early July, 239 well-respected scientists (including some serving on the ASHRAE ETF) from 32 countries petitioned the World Health Organization (WHO) to rethink the importance of airborne transmission of the SARS-CoV-2 virus. Their letter can be found here. It prompted the WHO to change their position and recognize the airborne pathway in the ongoing pandemic. As you mentioned, there is not a lot of hard evidence showing that airborne viral particles can travel long distances and through HVAC systems. However, this lack of evidence is not proof that airborne transmission cannot happen. Sampling and culturing viable virus (any virus) from the air is extremely difficult to do. The relatively turbulent act of sampling tends to kill the virus, making true viability very difficult to ascertain. Given those limitations, it is likely viable SARS-CoV-2 viral particles are traveling further in the air than we can concretely prove. HIGHLIGHTING ADDED. After reviewing all of the literature we have available (and doing our best to keep up with new reports), the conclusions of our group of experts is that the transmission through the air is likely enough that we believe countermeasures are warranted. That is why we put together what we hope are practical suggestions for ways for owners/managers/occupants to proceed. We also tell people to use the suggestions with caution. As you suggest, difficulty maintaining the RH level is one reason to consider limiting the amount of incoming outdoor air. We also wholeheartedly agree with your assertion that more research is needed to be sure our recommendations are the best possible. However, in the throes of a global pandemic, we firmly believe we need to be intelligently proactive and not wait for absolute proof before acting. There is a significant amount of research going on currently, and there will likely be much more in the future. Eventually, we may learn more about how effective our guidance truly was during the pandemic. In the meantime, we are choosing to err on the side of caution. Regards, Steve Hammerling ASHRAE Manager of Technical Services Answers to technical inquiries are provided as a service to the public. While every effort is made to provide accurate and reliable information, they are advisory, are provided for informational purposes only, and may represent only one person’s view. They are not intended and should not be relied upon as official statements of ASHRAE. 11.191
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