COVID-19 | REHVA GUIDANCE Dont discount UV and high-efficiency filters Professor William Bahnfleth comments on REHVAs Covid-19 guidance document and compares it with ASHRAE recommendations I PROFESSOR WILLIAM BAHNFLETH Professor of architectural engineering at Penn State University m glad to see the possibility of airborne transmission acknowledged clearly in REHVAs guidance document on Covid-19. However, I think there would be debate about what it says about humidity. The document says humidification and air conditioning have no practical effect, and concludes that there would have to be an unacceptably high humidity to have an effect on the virus. That conclusion is similar to what you will find in the section on temperature and humidity in the ASHRAE Position Document on Airborne Infectious Diseases, which declines to make any broad recommendations about humidity. However, it does cite a number of studies that have found viral infection rates to be higher at lower humidity and suggests that practitioners may choose to take that into consideration on a case-by-case basis. On the other hand, experts such as Dr Stephanie Taylor advocates the 40-60% RH range,1 partly because SOMETHING WAS MISSING FROM OUR AIR HANDLING UNITS RANGE. A PERFECTLY AVERAGE PRODUCT. PIONEERING NEW AIR TECHNOLOGY 15817_NUAIRE_NEPTUNE_CIBSE_HP_AD.indd 1 of the other consideration that dry mucosa may be more vulnerable to infection and that, at low humidity levels, respiratory droplets evaporate more quickly to particle sizes capable of remaining airborne for extended periods. There are many factors to consider when evaluating the advice on recirculation. Certainly, closing it off will reduce transfer of airborne pathogen containing aerosols from one space to another. It will also, assuming supply airflow rates dont change, greatly increase the amount of outdoor air being brought in to reduce exposure by dilution. Forcing the system into 100% outside-air mode without any recirculation may result in the need to condition a large quantity of cold, very dry air in the winter or hot, very moist air in the summer. This may have consequences for comfort, microbial growth, and occupant susceptibility. The guidance dismisses filtration on the grounds that the filters typically found in such systems are not of When we say average, you need to understand that its still market leading, its just that it sits nicely in between our other products. The introduction of Neptune completes our family of AHU units giving you more choice. Average has never been so appealing. Take a look for yourself at nuaire.co.uk/neptune 28/02/2020 14:31 28 April 2020 www.cibsejournal.com CIBSE Apr20 pp26-28 Coronavirus.indd 28 20/03/2020 17:06