REHVA COVID-19 guidance document, March 17, 2020
(updates will follow as necessary)
How to operate and use building services in order to prevent the spread of thecoronavirus disease (COVID-19) virus (SARS-CoV-2) in workplaces

Introduction
In this document REHVA summarizes advice on the operation and use of building services in areas
with a coronavirus disease (COVID-19) outbreak, in order to prevent the spread of COVID-19
depending on HVAC or plumbing systems related factors. Please read the advice below as interim
guidance; the document may be complemented with new evidence and information when it becomesavailable.
The suggestions below are meant as an addition to the general guidance for employers and building
owners that is presented in the WHO document ‘Getting workplaces ready for COVID-19’. The text
below is intended primarily for HVAC professionals and facility managers, but may be useful for e.g.
occupational and public health specialists.
In the following the building related precautions are covered and some common overreactions areexplained. The scope is limited to commercial and public buildings (e.g. offices, schools, shopping
areas, sport premisies etc) where only occasional occupancy of infected persons is expected; hospitaland healthcare facilities (usually with a larger concentration of infected people) are excluded.

Disclaimer:This REHVA document is based on best available evidence and knowledge, but in many aspects’
corona virus (SARS-CoV-2) information is so limited or not existing that previous SARS-CoV-1 evidence1
has been utilized for best practice recommendations. REHVA excludes any liability for any direct,
indirect, incidental damages or any other damages that would result from, or be connected with theuse of the information presented in this document.

1In the last two decades we are confronted with three coronavirus disease outbreaks: (i) SARS in2003-2004 (SARS-CoV-1), (ii) MERS in 2012 (MERS-CoV) and Covid-19 in 2019-2020 (SARS-CoV-2). In
the present document our focus is on the last aspect of SARS-CoV-2 transmission. When it isreferred to the SARS outbreak in 2003-2004 we will use the name of SARS-CoV-1 virus at that time.
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Transmission routes
Important for every epidemic are the transmission routes of the infectious agent. In relation to COVID19 the standard assumption is that the following two transmission routes are dominant: via large
droplets (droplets/particles emitted when sneezing or coughing or talking) and via surface (fomite)contact (hand-hand, hand-surface etc.). A third transmission route that is gaining more attention
from the scientific community is the faecal-oral route.
The faecal-oral transmission route for SARS-CoV-2 infections is implicitly recognized by WHO, see
their latest technical briefing of March 2, 2020. In this document they propose as precautionary
measure to flush toilets with closed lid. Additionally, they suggest avoiding dried-out drains in floors
and other sanitary devices by regularly adding water (every 3 weeks depending on climate) so thatthe water seal works properly. This is in line with an observation during the SARS 2003-2004 outbreak:open connections with sewage systems appeared to be a transmission route in an apartment buildingin Hong Kong (Amoy Garden). It is known that flushing toilets are creating plumes containing dropletsand droplet residue when toilets are flushed with open lids. And we know that SARS-CoV-2 viruseshave been detected in stool samples (reported in recent scientific papers and by the Chineseauthorities). In addition, a comparable incident was recently reported in an apartment complex (MeiHouse). Therefore, the conclusion is that the faecal-oral transmission routes can’t be excluded astransmission route.
Via air there are two exposure mechanisms:
1. Airborne transmission through large droplets (> 10 microns), which are released and fall tosurfaces not further than about 1-2 m from the infected person. Droplets are formed fromcoughing and sneezing (sneezing forms many more particles typically). Most of these large
droplets fall on nearby surfaces and objects – such as desks and tables. People could catch theinfection by touching those contaminated surfaces or objects; and then touching their eyes, noseor mouth. If people are standing within 1-2 meter of an infected person, they can catch it directly
by breathing in droplets sneezed or coughed out or exhaled by them.
2. Airborne transmission through small particles (< 5 microns), which may stay airborne for hours
and can be transported long distances. These are also generated by coughing and sneezing andtalking. Small particles (droplet nuclei or residue) form from droplets which evaporate (usuallywithin milliseconds) and desiccate. The size of a coronavirus particle is 80-160 nanometre2 andit remains active at common indoor conditions up to 3 hours in indoor air and 2-3 days on room
surfaces (unless there is specific cleaning). Such small virus particles stay airborne and can travel
long distances carried by airflows in the rooms or in the extract air ducts of ventilation systems.
Airborne transmission has caused infections of SARS-CoV-1 in the past; currently there is noreported evidence yet specifically for Corona disease (COVID-19) infection via this route. Thereis also no reported data or studies to rule out the possibility of the airborne-particle route. One
indication for this: Corona virus SARS-CoV-2 has been isolated from swabs taken from exhaust
vents in rooms occupied by infected patients. This mechanism implies that keeping 1-2 m distancefrom infected persons might not be enough and increasing the ventilation is useful because ofremoval of more particles.

21 nanometer = 0.001 micron
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Figure 1. WHO reported exposure mechanisms of COVID-19 SARS-CoV-2 droplets (dark blue colour). Light blue colour: airborne
mechanism that is known from SARS-CoV-1 and other flu, currently there is no reported evidence specifically for SARS-CoV-2(figure: courtesy Francesco Franchimon).

With SARS-CoV-2 the airborne route – infection through exposure to droplet nuclei particles – has notbeen demonstrated but may exist when certain conditions are met (i.e. opportunistic airborne)
according to Chine national Health Commission (unpublished result).
Conclusion in relation to the airborne transmission route:At this date we need all efforts to manage this pandemic from all fronts. Therefore REHVA proposes,
especially in ‘hot spot’ areas to use the ALARA principle (As Low As Reasonably Achievable) and to
take a set of measures that help to also control the airborne route in buildings (apart from standard
hygiene measures as recommended by WHO, see the ‘Getting workplaces ready for COVID-19’document).

Practical recommendations for building services operation
Increase air supply and exhaust ventilation
In buildings with mechanical ventilation systems extended operation times are recommended. Change
the clock times of system timers to start ventilation a couple of hours earlier and switch off later
than usual. Better solution is even to keep the ventilation on 24/7, possibly with lowered (but notswitched off) ventilation rates when people are absent. Considering a springtime with small heating
and cooling needs, the recommendations above have limited energy penalties, while they help toremove virus particles out of the building and to remove released virus particles from surfaces.
The general advice is to supply as much outside air as reasonably possible. The key aspect is theamount of fresh air supplied per person. If, due to smart working utilization, the number of employees
is reduced, do not concentrate the remaining employees in smaller areas but maintain or enlarge thespacing among them in order to foster the ventilation cleaning effect.
Exhaust ventilation systems of toilets should always be kept on 24/7, and make sure that underpressure is created, especially to avoid the faecal-oral transmission.

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Use more window airing
General recommendation is to stay away from crowded and poorly ventilated spaces. In buildings
without mechanical ventilation systems it is recommended to actively use operable windows (muchmore than normally, even when this causes some thermal discomfort). Window airing then is the only
way to boost air exchange rates. One could open windows for 15 min or so when entering the room(especially when the room was occupied by others beforehand). Also, in buildings with mechanicalventilation, window airing can be used to further boost ventilation.
Open windows in toilets with passive stack or mechanical exhaust systems may cause a contaminatedairflow from the toilet to other rooms, implying that ventilation begins to work in reverse direction.
Open toilet windows then should be avoided. If there is no adequate exhaust ventilation from toilets
and window airing in toilets cannot be avoided, it is important to keep windows open also in otherspaces in order to achieve cross flows throughout the building.
Humidification and air-conditioning have no practical effectTransmission of some viruses in buildings can be limited by changing air temperatures and humidity
levels. In the case of COVID-19 this is unfortunately not an option as the SARS-CoV-2 virus is quite
resistant to environmental changes and is susceptible only for a very high relative humidity above
80% and a temperature above 30 ˚C, which are not attainable and acceptable in buildings for otherreasons (e.g. thermal comfort).
Small droplets under interest (0.5 – 10 micron) will evaporate fast under any relative humidity (RH)level. Nasal systems and mucous membranes are more sensitive to infections at very low RH of 10-20%, and this is the reason for which some humidification in winter is sometimes suggested (up to alevel of about 30%). This indirect need for humidification in the COVID-19 case is not relevant however
given the incoming climatic conditions (from March onwards we expect indoor RH higher than 30% in
all European climates without humidification).
Thus, there is no need to change humidification systems’ setpoints. Considering the springtime thatis about to start, these systems should not be in operation anyhow.
Heating and cooling systems can be operated normally as there are no direct implications on COVID19 spread. Usually, any adjustment of setpoints for heating or cooling systems is not needed.
Safe use of heat recovery sections
Under certain conditions virus particles in extract air can re-enter the building. Heat recovery devices
may carry over virus attached to particles from the exhaust air side to the supply air side via leaks.
In rotary heat exchangers (including enthalpy wheels) particles deposit on the return air side of theheat exchanger surface after which they might be resuspended when heat exchanger turns to the
supply air side. Therefore, it is recommended to (temporarily) turn off rotary heat exchangers during
SARS-CoV-2 episodes.
If leaks are suspected in the heat recovery sections, pressure adjustment or bypassing can be anoption in order to avoid a situation where higher pressure on extract side will cause air leakages tosupply side.
Virus particle transmission via heat recovery devices is not an issue when a HVAC system is equippedwith a twin coil unit or another heat recovery device that guarantees 100% air separation between
return and supply side.
No use of recirculation
Virus particles in return ducts can also re-enter a building when centralized air handling units areequipped with recirculation sectors. It is recommended to avoid central recirculation during SARSCoV-2 episodes: close the recirculation dampers (via the Building Management System or manually).
In case this leads to problems with cooling or heating capacity, this has to be accepted because it ismore important to prevent contamination and protect public health than to guarantee thermal
comfort.

Page 6 | 4Sometimes air handling units and recirculation sections are equipped with return air filters. Thisshould not be a reason to keep recirculation dampers open as these filters normally do not filter out
particles with viruses effectively since they have standard efficiencies and not HEPA efficiencies.
When possible, decentralized systems such as fan coil units that use local recirculation, also should
be turned off to avoid resuspension of virus particles at room level (esp. when rooms are usednormally by more than one occupant). Fan coil units have coarse filters which practically do not filterout particles with viruses. If not possible to turn off, these units are to be included into cleaning
campaigns, because they might collect particles as any other surface in the room.
Duct cleaning has no practical effect
There have been overreactive statements recommending to clean ventilation ducts in order to avoid
SARS-CoV-2 transmission via ventilation systems. Duct cleaning is not effective against room-to-room
infection because the ventilation system is not a contamination source if above guidance about heatrecovery and recirculation is followed. Viruses attached to small particles will not deposit easily inventilation ducts and normally will be carried out by the air flow anyhow. Therefore, no changes areneeded to normal duct cleaning and maintenance procedures. Much more important is to increase
fresh air supply, avoid recirculation of air according to the recommendations above.
Change of outdoor air filters is not necessary
In COVID-19 context, it has been asked should the filters to be replaced and what is the protection
effect in very rare occasions of outdoor virus contamination, for instance if air exhausts are close toair intakes. Modern ventilation systems (air handling units) are equipped with fine outdoor air filtersright after the outdoor air intake (filter class F7 or F8 3 or ISO ePM1) which filtrate well particulatematter from outdoor air. The size of a coronavirus particle of 80-160 nm (PM0.1) is smaller than thecapture area of F8 filters (capture efficiency 65-90% for PM1), but many of such small particles will
settle on fibres of the filter by diffusion mechanism. SARS-CoV-2 particles also aggregate with larger
particles which are already within the capture area of filters. This implies that in rare cases of virus
contaminated outdoor air, fine outdoor air filters provide a reasonable protection for a low
concentration and occasionally spread viruses in outdoor air.
From the filter replacement perspective, normal maintenance procedures can be used. Clogged
filters are not a contamination source in this context, but they reduce supply airflow which has anegative effect on indoor contaminations itself. Thus, filters must be replaced according to normal
procedure when pressure or time limits are exceeded, or according to scheduled maintenance. Inconclusion, we do not recommend changing existing outdoor air filters and replace them with othertype of filters nor do we recommend changing them sooner than normal.
Room air cleaners can be useful in specific situations
Room air cleaners remove effectively particles from air which provides a similar effect compared toventilation. To be effective, air cleaners need to have at least HEPA filter efficiency. Unfortunately,
most of attractively priced room air cleaners are not effective enough. Devices that use electrostatic
filtration principles (not the same as room ionizers!) often work quite well too. Because the airflow
through air cleaners is limited, the floor area they can effectively serve is normally quite small,typically less than 10 m2. If one decides to use an air cleaner (again: increasing regular ventilation
often is much more efficient) it is recommended to locate the device close to the breathing zone.
Special UV cleaning equipment to be installed for the supply air or room air treatment is also effective
as killing bacteria and viruses but this is normally only a suitable solution for the equipment for healthcare facilities.

3 An outdated filter classification of EN779:2012 which is replaced by EN ISO 16890-1:2016, Air filters forgeneral ventilation - Part 1: Technical specifications, requirements and classification system based upon
particulate matter efficiency (ePM).

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Toilet lid use instructions
If toilet seats are equipped with lids it is recommended to flush the toilets with closed lids in orderto minimize the release of droplets and droplet residues from plumes in the air. It is important thatwater seals work all time. Therefore, organise that building occupants are instructed to use the lids.
Feedback
If you are specialist in the issues addressed in this document and you have remarks or suggestions forimprovements, feel free to contact us via info@rehva.eu. Please mention ‘COVID-19 interim
document’ as subject when you email us.
Colophon
This document was prepared by a group of REHVA volunteers in the period March 6-15th 2020. Members of theexpert group are:Prof. Jarek Kurnitski, Tallinn University of Technology, Chair of REHVA Technology and Research Committee
Atze Boerstra, REHVA vice-president, managing director bba binnenmilieu
Francesco Franchimon, managing director Franchimon ICM
Prof. Livio Mazzarella, Milan Polytechnic University
Jaap Hogeling, managing director ISSO international projectFrank Hovorka, REHVA president, director technology and innovation FPI, ParisProf. em. Olli Seppänen, Aalto University
The draft document of the paper was reviewed by Prof. Yuguo Li from the University of Hongkong and Prof.
Shelly Miller from the University of Colorado Boulder.

LiteratureThis document is partly based on a literature survey, the scientific papers and other documentsthat were used can be found in this document:
https://www.rehva.eu/fileadmin/user_upload/REHVA_COVID19_guidance_document_Bibliography.pdf

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