COVID-19 surfaces and objects and then their eyes, nose or mouth. People standing 1-2 metres from an infected person could catch it directly in droplets sneezed or coughed out. Noakes believes that drops greater than 10 microns can travel further than two metres. Some of those very big droplets will fly ballistically, but even particles up to 20 microns can be carried further than we might expect because of airflows in the room, she says. It doesnt necessarily mean theres huge additional risk, because theres probably a small concentration of virus, but we should be aware of where surfaces might be contaminated. Small particles (< 5 microns) These may stay airborne for hours and can be transported long distances. They are generated through coughing, sneezing or talking. Small particles (droplet nuclei or residue) form from droplets that evaporate (usually within milliseconds) and desiccate. The coronavirus particle is 80-160 nanometres (1 micron = 1,000 nanometres) and remains active in common indoor air conditions for up to three hours and two to three days on room surfaces. These small particles can stay airborne and travel long distances by airflows in the room or via air ducts of ventilation systems. REHVA says there is no evidence yet for Covid-19 infection via this route, but it noted that there were no studies that ruled it out. It also referred to a case where coronavirus SARS-Cov-2 was isolated from swabs taken from exhaust vents in rooms occupied by infected patients. This implies that keeping 1-2 metre from an infected person might not be enough, concluded REHVA, and that increases in ventilation may be useful, as it would remove more particles. It recommends taking a series of measures that help control the airborne route in buildings as follows: | REHVA GUIDANCE susceptible only to a very high relative humidity (RH) above 80% and a temperature above 30C, which is not acceptable for reasons of thermal comfort. The reason humidification is suggested in winter (up to a level of 30%) is because nasal systems and mucous membranes are more susceptible to infections at very low RH of 10-20%. However, from March, climatic conditions will see RH higher than 30% in all European climates, without humidification. Safe use of heat-recovery devices Virus particles in extract air can re-enter the building. Heat-recovery devices may carry over the virus attached to particles from the exhaust airside to the supply airside via leaks. In rotary heat exchangers (including enthalpy wheels) particles deposit on the return airside of the heat exchanger surface, after which they might be resuspended when the heat exchanger turns to the supply airside. Based on current evidence, REHVA recommends turning off rotary heat exchangers temporarily during SARS-CoV-2 episodes. Its document goes on to state: if leaks are suspected in the heat-recovery sections, pressure adjustment or bypassing can be an option to avoid a situation where higher pressure on the extract side causes air leakages to the supply side. Transmission via heat-recovery devices is not an issue when a HVAC system is equipped with a twin-coil (run around coil) or other heat-recovery device that guarantees air separation between return and supply side. No use of recirculation The guidance document says virus particles in return ducts can reenter a building if centralised air handling units have recirculation. It recommends avoiding central recirculation during SARS CoV-2 episodes and closing the recirculation dampers, even if there are return air filters, as the guidance says these dont normally filter out viruses. It also advises that decentralised systems, such as fan coil units that use local circulation, should be turned off to avoid resuspension of particles at room level. If they cant be turned off, they should be cleaned regularly. Increase air supply and exhaust ventilation Duct cleaning has no practical effect The general advice is to supply as much outside air as possible. Expanded operation times are recommended for buildings with mechanical ventilation. Consider keeping the ventilation on 24/7 with lower ventilation rates when people are absent. If employee numbers reduce, do not place remaining staff in smaller areas. Exhaust ventilation systems of toilets should always be left on 24/7, and relatively negative pressure must be maintained in the room air to help avoid faecal-oral transmission. Virus particles will not deposit easily in ventilation ducts and will normally be carried away by the airflow, says REHVA. No changes are needed to normal duct cleaning and maintenance procedures. Increasing the fresh-air supply and avoiding recirculation are more important. Use more window-driven natural ventilation In buildings without mechanical ventilation, the use of openable windows is recommended, even if this causes thermal discomfort. Even in buildings with mechanical ventilation, open windows can be used to boost ventilation. Open windows in toilets with passive stack or mechanical exhaust systems may cause contaminated airflow from the toilet to other rooms so, in these circumstances, it is recommended that toilet windows remain shut. If there is no adequate exhaust ventilation from toilets, and window airflow cannot be avoided, keep windows open in other spaces to achieve crossflows through buildings. Humidification has no practical effect Covid-19 is resistant to environmental changes and is Change of outdoor air filters not necessary In rare cases of virus-contaminated outdoor air, fine outdoor air filters provide reasonable protection for a low concentration, but occasionally spread viruses from outdoor air, according to the guidance. Clogged filters are not a contamination source, but should continue to be changed as part of any good-practice maintenance regime. Room-air cleaners can be useful Particles can be removed from the air, but air cleaners must have at least HEPA filter efficiency. Attractively priced room-air cleaners are not effective enough, says REHVA. As the airflow through air cleaners is limited, the floor area they can serve is normally quite small, typically less than 10m2. If used, they should be placed close to the breathing zone. Special UV cleaning equipment for supply-air or room-air treatment is effective at killing bacteria and viruses, but the guidance document says this is normally only suitable for healthcare facilities. The guidance is a live document. REHVA invites specialists to respond at info@rehva.ue A bibliography is at www.rehva.eu/activities/covid-19guidance See CIBSEs guidance at www.cibse.org/coronavirus CJ www.cibsejournal.com April 2020 27 CIBSE Apr20 pp26-28 Coronavirus.indd 27 20/03/2020 17:06