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White Paper on Infectious Aerosols

Infectious aerosols transmission:
Mitigation measures for heating, ventilating and airconditioning (HVAC) systems in operating and new hotels
4 July 2020
Infectious aerosols transmission: Mitigation measures for heating,
ventilating and air-conditioning (HVAC) systems in operating and new hotels
EXECUTIVE SUMMARY .................................................................................................. 2
INTRODUCTION .............................................................................................................. 3
PROPOSED MITIGATION MEASURES ........................................................................... 4
CONCLUSION .................................................................................................................. 9
REFERENCES ................................................................................................................13
Heating ventilating and air-conditioning (HVAC) systems can either promote or curtail the
spread of infectious airborne aerosols. A well-designed and well-maintained HVAC system
has a significant contribution to the protection of building occupants by disrupting the
transmission path of infectious diseases and diluting the level of infection.
In April 2020, Federation of European Heating, Ventilating and Air-conditioning Associations
(REHVA) issued COVID-19 guidelines1. American Society for Heating, Refrigeration and Airconditioning Engineers (ASHRAE) also published a position document on infectious
aerosols2. In May and June 2020, both U.S.3 and European4 Center for Disease Control and
Prevention have respectively issued advisories stating that ventilation is essential in
mitigating the spread of COVID-19.
Based on the above-mentioned guidelines and published papers, this document outlines
HVAC mitigation measures applicable for hotels.
As proof of more mitigation measures become available, the recommendations can be
updated. There needs to be flexibility and practicality applied as all buildings have their
unique issues requiring different consideration.
Currently, the most effective measures are summarised as below:
Ensure ventilation is well distributed to all areas
Maximise fresh air supply
Control room relative humidity within the range of 40 - 60%
Enhance filtration for high-risk (occupant-dense) areas
In 1979, a commercial airliner with 54 persons onboard was delayed on ground (due to engine
failure) for 3 hours and the ventilation system was inoperative during the entire duration.
72 percent of passengers subsequently developed cough, fever and other symptoms of
influenza. Scientists concluded that there was airborne transmission of the H3N2 virus5. In
decades that followed, cases of outbreaks and scientific research6 continued to reinforce the
point that airborne transmission of infectious diseases cannot be ignored.
Pathogens spread through the air in the form of droplets or droplet nuclei (smaller than 10m
or 10 x 10-6 m), also known as aerosols. Such droplets are emitted from the primary host by
oral means such as coughing, sneezing or talking. Most large droplets will descend on
surfaces due to self-weight within 1-2 meters from the source (see Figure 1). Some large
droplets may reduce in size to become aerosols, due to evaporation (also known as
desiccation). These aerosols are lightweight enough to suspend in the air and remain virulent
for long periods of time ranging from several minutes to days.
Figure 1 ( a) Comparative settling times by particle diameter 7 and ( b) Illustration of transmission
of droplets and small airborne particles produced by an infected patient (courtesy of Yuguo Li)
HVAC systems play an important role to prevent the spread of infectious aerosols within the
building. While ventilation systems cannot prevent the settlement of large droplets, mitigation
measures on HVAC systems can influence the transmissibility of infectious aerosols to a
great extent.
Provide minimum ventilation and exhaust for all areas as per ASHRAE8 or EN9 standards
(For operating and new hotels)
Inadequate ventilation increases the risks of airborne transmission of infectious aerosols. A
minimum amount of outdoor air ventilation ensures that airborne contaminants do not
accumulate to harmful concentration levels. Fresh air should be made available for all areas;
bathroom exhausts should always be working. The windows of naturally ventilated premises
should be kept open where practicable. The fresh air rates should be increased above these
minimum levels wherever possible, even if only on a temporary basis.
Avoid use of energy recovery ventilation systems that leak exhaust air into outdoor air supply
(For operating and new hotels)
Energy recovery systems such as heat recovery wheels are commonly used to harness the
cooling potential of exhaust air for pre-cooling of outdoor air. The wheels rotate at low-speeds,
constantly alternating contact with exhaust air and outdoor air (see Figure 2). Under certain
circumstances, as much as 20% of polluted exhaust air side may carry over to outdoor supply
air due to leakages at the joints.
For operating hotels with heat recovery wheels, if leaks are suspected, adjustments or bypass
are possible workaround solutions. For new hotels, alternative forms of energy recovery
systems such as heat pipes should be considered.
Figure 2: Leakages through heat recovery wheel
Maintain kitchen’s negative air pressure difference within acceptable limits
(For operating and new hotels)
A kitchen is typically designed to maintain negative air pressure difference in relation to dining
area, with not more than 15 % of the amount of replacement air drawn from adjacent dining
areas10. However, under certain circumstances, the negative air pressure difference
becomes excessive and large amounts of air from dining area will constantly stream into the
kitchen. If there is excessive negative air pressure difference, adjustments to the dampers of
exhaust or supply fans are possible options to improve the situation.
Ensure correct airflow patterns within indoor space; adjust supply air diffuser vanes, air
damper settings and/or use portable air cleaners to supplement existing system
(For operating and new hotels)
Air distribution in any space should allow for a good continuous directional airflow through
the space from supply air grille to return air grille, without any dead zone where air remains
stagnant. To achieve this, (a) there should be appropriate separation distance between
supply and return air grilles to prevent short-circuiting of air flows and (b) appropriate selection
and placement of supply air grilles to ensure conditioned air reaches all areas; the same set
of supply grilles with different vane directions results in a wide variety of air distribution
patterns (see Figure 3). Thus, manufacturer’s recommendations should be closely followed.
Figure 3 Illustration of depth of throw of conditioned air in relation to various grille deflections
For operating hotels with areas that are not well-ventilated with pockets of stagnant air, there
are possible options such as (a) adjustments to supply air diffuser vanes and air dampers
settings, and/or (b) use of portable high-efficiency particulate air (HEPA) air cleaners to
supplement existing system.
Enhance ventilation in elevators (For operating and new hotels)
Elevators are enclosed occupant-dense areas. To improve ventilation, and where lift landing
doors are not required to contribute to the protection of the building against fire, elevators
could be programmed such that the doors are kept open when the elevators are idling.
Ventilation fans should be sized such that the entire air volume of the elevator is replaced
every hour, as per ASME standards11.
Enhance air handling units (AHU) of high-risk (occupant-dense) areas
(For operating and new hotels)
Ballrooms, lobbies, pre-function areas, conference rooms – these occupant-dense areas
have a high-risk of airborne transmissions. Below measures (Figure 4) should be considered:
Figure 4: Enhancements for AHUs of high-risk (occupant- dense) areas
There are some points to take note before implementation of the enhancement measures:
Use of higher-grade air filters results in higher pressure drop and reduced airflow.
Depending on climatic conditions, fully opening outdoor air dampers and closing recirculation dampers may lead to a diminished heating/cooling capacity of the AHU.
The room may not achieve designed temperature/humidity set-points and there may
be acoustic noise issues due to larger amounts of air flowing through the outdoor air
duct and grilles.
If this measure is implemented, the temperature/humidity conditions of the indoor
space needs to be carefully monitored and the occupancy of the space may have to
be reduced accordingly.
Control relative humidity (RH) within a range of 40 – 60%; adjust air-conditioning systems
settings and/or use portable humidifiers to supplement existing system
(For operating and new hotels)
Typically, HVAC systems are designed to control humidity and in turn affect the
transmissibility of pathogens. However, under certain circumstances, indoor environment’s
RH may fall out of range. Studies show that indoor RH below 40% (see Figure 5) is associated
with three factors that increases risk of infections:
Aerosols shrink quickly to suspend in air much longer and gain the ability to travel
further distances
Many viruses have an increased survivability in low-RH conditions
Humans become more susceptible due to dry air causing the protective mucous
membranes of the nasal region to be impaired
This measure is more relevant for very cold or desert climates where the outdoor air has low
moisture content. For these operating hotels, it is important to verify that cooling, heating and
humidification systems are operating at correct design parameters. Portable humidifier is an
option to supplement existing system.
Figure 5: Indirect health effects of relative humidity in indoor environments 12
Ensure minimum 5 meters separation distance between all fresh air intake grilles and
exhaust grilles (For operating and new hotels)
Fresh air intakes and exhaust air grilles are typically located with a minimum 5 meters
separation distance, as per ASHRAE recommendations. This helps prevent carry over of
polluted exhaust air into fresh air intakes. Depending on the wind’s direction and speed, this
distance may need to be extended further. For operating hotels, a site investigation should
be carried out to determine how close/far the exhaust air grilles are to the supply air grilles.
Mitigation measures such as extension of ducting or relocation of grilles may be necessary.
Plan for re-commissioning of the ventilation systems for operating hotels;
Enhance the commissioning of new ventilation systems for new hotels
Commissioning13 is a quality-focused process aimed at verifying that systems are installed,
tested, operated and maintained to meet specifications and standards. A typical
commissioning for ventilation systems include testing and measurements, adjustments and
balancing for all components of including AHU, fans, ducting, dampers and grilles. This
process is typically completed by the general contractor before the premises are occupied.
For operating hotels, a plan should be developed for a space-by-space re-commissioning,
focused on identifying gaps and implementing measures to enhance the ventilation systems
as highlighted previously. For new hotels, the commissioning process should have a renewed
emphasis on indoor air quality.
Proposed Mitigation Measure
Provide minimum ventilation and exhaust for all areas
as per ASHRAE or EN standard
Avoid use of energy recovery ventilation systems that
leak exhaust air into outdoor air supply
Maintain kitchen’s negative air pressure difference
within acceptable limits; adjust fans and air dampers
Ensure correct airflow patterns within indoor spaces;
Applicable areas
All areas
AHU and ventilation systems
Kitchens and restaurants
Adjust supply air diffuser vanes, air damper settings
and/or use portable HEPA air cleaners to supplement
All public areas, conference
rooms and ballrooms
existing system
Enhance ventilation in elevators
All public areas,
conference rooms and
Enhance AHU of high-risk (occupant-dense) areas:
Fully open outdoor air damper, close re-circulating
dampers, where indoor/outdoor conditions permit
Disable demand-controlled ventilation
Operate AHU for minimum 3-hours before and
3-hours after use
Install higher-grade filters of minimum
MERV-13 or F7 grade or higher
Control relative humidity within 40 – 60 %;
Adjust air-conditioning system settings and/or use of
All areas
portable humidifiers to supplement existing system
Ensure minimum 5 meters separation distance
between fresh air intake grilles and exhaust grilles
Plan for re-commissioning of ventilation systems;
AHU and ventilation systems
AHU and ventilation systems
Enhance the commissioning of ventilation systems
Table 1: Summary of mitigation measures and associated costs
Infectious aerosols can be transmitted within buildings through HVAC systems. Several
measures are effective in disrupting transmission, including dilution, filtration, pressurisation,
and having correct airflow patterns.
All hotels should, as a minimum, continue to follow prevailing standards, guidelines and best
practices in engineering design. Existing hotels should continue good maintenance practices
such as filter cleaning/replacement, washing of cooling/heating coils, and ensuring HVAC
equipment are in good working condition. Systems commissioning should have a renewed
focus on indoor air quality.
All above-mentioned HVAC mitigation measures (see Table 1) should be implemented,
where appropriate, as they provide robust and effective level of control in HVAC systems
together with planned preventative maintenance processes. The recommended measures
do not incur capital cost for standards-compliant hotels; some measures will lead to a
marginal increase in operating costs.
Various countries and professional organisations are currently conducting research on how
to enhance building codes and standards. There is also a lot of ongoing research and
development on potentially workable solutions such as ultraviolet germicidal irradiator (UVGI)
and electrostatic air cleaners. Broadly, several new technologies lack a test standard for
verification of removal effectiveness and safety (see Annex). The situation will be closely
monitored to identify proven strategies and solutions suitable for implementation in hotels.
Prepared by:
Joe Ong
Peer reviewed by:
Barry Wormald
Air cleaning
Fibrous filter
How does it work
Collection: Fibrous or porous
material to remove solid particulates
such as dust, pollen, mould and
bacteria. Rely on mechanical means
to capture particles.
Proven and widely used across all
building types
High-efficiency particulate air
(HEPA) or ultra-low particulate air
(ULPA) air filter can remove viral
particles. HEPA/ULPA filters are
used in critical facilities.
Regular replacement is required
HEPA/ULPA filters have high air pressure
drop. Operating hotels fan motors are not
sized to handle such pressure drops
Electrostatictype fibrous
Collection: Air passing through the
fibrous filter is statically charged. As
air passes through subsequent
layers, the charge is released, and
particles are mechanically trapped
in the filter.
Proven and used in critical facilities
Raises capture efficiency of
mechanical filters with a much
lower air pressure drop
Higher capital cost
Collection: Use of high voltage to
charge up particles which collect on
oppositely charged plates
Low pressure drop
Low maintenance
High removal efficiency
May have high ozone and nitrogen oxide
generation rates. Both gases are indoor
air pollutants
Plates require cleaning
May generate crackling noise as dust is
Particle deposition in respiratory tract as
particles become charged
High power requirements
Generates ozone
Low effectiveness due to low airflow
Collection: Similar to ESP,
electrically charge air molecules and
generate ions to attach to airborne
particles. Charged particles are then
collected on oppositely charged
plates in the air cleaner
Destruction: UV-C light
kills/inactivates airborne microbes
Typically, low power draw
Low maintenance
Effective with high dosage and
sufficient exposure duration
Typically used to inactivate
microbes on cooling coils and
surfaces, prevent biofilm formation
Used in hospitals as upper-room
air disinfection
Require minimum exposure duration to be
In-duct airstream irradiation is not proven
Potential for eye injury
Susceptible to debris/dust
Uncoated lamps generate ozone. Need to
be mindful on lamp specifications
Test standards
and rating metrics
ASHRAE Standard 52.2 (MERV)
EN 779
ISO 16890 (for efficiency of
particulate matter filters)
ISO 29463 (HEPA)
Portable air cleaners (HEPA):
Same as above
ANSI/UL Standard 867 for electrical
safety and ozone emissions
Note: standard is only pass/fail; no
rating metric
None specified. Closest is ANSI/UL
Standard 867 for electrical safety and
ozone emissions
Note: standard is only pass/fail; no
rating metric
Air irradiation:
ASHRAE Standard 185.1
Surface irradiation:
ASHRAE Standard 185.2
Air cleaning
carbon filters
How does it work
Collection: Gases are adsorbed into
the media
Plasma bipolar
Conversion: Electric arc to ionise
gas and break down chemical
bonds of gas pollutants
Conversion: Deliberate ozone
High removal efficiency of gaseous
No by-product
Preliminary information shows high
removal efficiency
Ozone is an effective air steriliser
Photo catalytic
Conversion: A high surface area
media is coated with titanium
dioxide; gases are adsorbed into the
media and UV lamps activate the
coating, causing a reaction with the
adsorbed gases to transform them
Can degrade a wide array of
gaseous contaminants
Not effective against viruses
Regular replacement is required
High pressure drops on sorbent media
Standard test methods not widely used
By-products are formed including
particles, ozone, formaldehyde, carbon
monoxide, nitrogen oxide. Needs to be
used in tandem with other filtration such
as activated carbon filters
Ozone is an indoor air pollutant. Needs to
be used in tandem with other less harmful
means such as activated carbon filter to
remove ozone.
High amounts of by-products
Can generate harmful by-product such as
formaldehyde and ozone. Needs to be
used in tandem with other less harmful
means such as activated carbon filter to
reduce harmful by-products.
No standard test methods
Lack of field studies to validate
Note: The above table is closely referenced to:
“Residential Air Cleaners – A Technical Summary” by US Environmental Protection Agency14
ASHRAE Position Document on Filtration and Air Cleaning, 201815
ASHRAE 2019 Handbook – HVAC Applications16
Test standards
and rating metrics
ASHRAE Standard 145.1
In-duct air cleaners:
ASHRAE Standard 145.2
Note: standards do not have any
rating metric or effectiveness
None specified. Closest is ANSI/UL
Standard 867 for electrical safety and
ozone emissions
Note: standard is only pass/fail; no
rating metric
None specified. Closest is ANSI/UL
Standard 867 for electrical safety and
ozone emissions
Note: standard is only pass/fail; no
rating metric
None specified. Closest is ANSI/UL
Standard 867 for electrical safety and
ozone emissions
Note: standard is only pass/fail; no
rating metric
2020. COVID-19 Guidance Document.
2020. ASHRAE Position Document on Infectious Aerosols.
3 US
CDC. 2020. Activities and Initiatives Supporting the COVID-19 Response
4 European
CDC. 2020. Heating, ventilation and air-conditioning systems in the context of COVID-19.
Moser, M.R., et al. 1979. An outbreak of influenza aboard a commercial airliner.
American Journal of Epidemiology Volume 110(1): Page 1-6.
6 L.
Morawska, et al. 2020. How can airborne transmission of COVID-19 indoors be minimised? Environment
7 Baron,
P. n.d. Generation and Behaviour of Airborne Particles (Aerosols). Presentation published at
CDC/NIOSH Topic Page: Aerosols, Center for Disease Control and Prevention.
ASHRAE. 2019a. ANSI/ASHRAE Standard 62.1-2019, Ventilation for Acceptable Indoor Air Quality. Atlanta:
9 EN.
2012. DIN EN 15251 Standard – Indoor Environmental Input Parameters.
10 HVCA.
2017. HVCA DW/172-2017, Specification for Kitchen Ventilation Systems.
London: HVCA.
ASME. 2016. ASME A17.1-2016 – Safety Code for Elevators and Escalators.
Arundel, et al. 1986. Indirect Health Effects of Relative Humidity in Indoor Environments.
Environmental Health Perspectives Volume 65: Page 351-361
CIBSE. 2006. CIBSE Commissioning Code A, Air Distribution Systems
14 US
EPA. 2018. Residential Air Cleaners – A Technical Summary 3rd Edition
2018. ASHRAE Position Document on Filtration and Air Cleaning
2019. ASHRAE Handbook for HVAC Chapter 47: Air cleaners, Chapter 62: Ultraviolet air and
surface treatment