ASHRAE EPIDEMIC TASK FORCE

FILTRATION & DISINFECTION | Updated 5-4-2020

BACKGROUND/CONTEXT AIR FILTRATION

• Modes of Transmission • Mechanical Air Filters
• ASHRAE Statements on Airborne Transmission • High-Efficiency Particulate Air (HEPA) Filters
• Scientific Data & Literature • Electronic Air Filters
• Gas-Phase Air Cleaners

AIR DISINFECTION
FACILITIES/MAINTENANCE
• Ultraviolet Energy (UV-C)
• Personal Protective Equipment (PPE) Basics
• Photocatalytic Oxidation (PCO)
• HVAC System Maintenance and Filter
• Bipolar Ionization/Corona Discharge
Replacement
• Ozone
• Special Precautions
SURFACE DISINFECTION
• Spray/Wipe Chemical Disinfectants
• Ultraviolet Energy (UV-C)
• Vaporized Hydrogen Peroxide
• Ozone
• Pulsed Xenon Lamps
• 405 nm Visible Light
• Far Ultraviolet
QUESTIONS? COVID-19@ashrae.org www.ashrae.org/covid19
Facilities/Maintenance – PPE Basics

• Refer to CDC Guidance on PPE use, especially banners at bottom of

webpage

• N95 filtering facepiece respirators

−Protects the wearer from respiratory droplets AND aerosols.
−Can be an effective tool for worker protection with proper use.
−Require fit testing and a medical clearance to wear for work.
−Tested for efficiency against 0.3 micrometer airborne particles.
−Certified to filter at least 95% of these particles.
−Generally disposed of after each use, but pandemic has resulted in
limited supplies. CDC issued Strategies to Optimize the Supply of PPE

• Silicone half mask respirators with N95 cartridges (or better) can be used
instead of filtering facepiece respirators.

www.ashrae.org
Facilities/Maintenance – PPE Basics
• Eye Protection
−Safety glasses (side shields preferred)
−Goggles
−Face shields

• Disposable Gloves
−Can be vinyl, rubber or nitrile
−Double gloves reduces likelihood of cuts/punctures
−Can be worn under work gloves if necessary

• Disposable coveralls, gowns and/or shoe covers can be worn to

enhance overall protection.

• After maintenance activities, wash hands with soap and water or use
an alcohol-based hand sanitizer. Change clothes if soiled.
 HVAC System Maintenance
and Filter Replacement during
the COVID-19 Pandemic
• For HVAC systems suspected to be contaminated with SARS-CoV-2,
it is not necessary to suspend HVAC system maintenance, including
filter changes, but additional safety precautions are warranted.

• The risks associated with handling filters contaminated with

coronaviruses in ventilation systems under field-use conditions have
not been evaluated.

• Workers performing maintenance and/or replacing filters on any

ventilation system with the potential for viral contamination should
wear appropriate personal protective equipment (PPE):
−A properly-fitted respirator (N95 or higher)
−Eye protection (safety glasses, goggles or face shield)
−Disposable gloves
HVAC System Maintenance
and Filter Replacement during
the COVID-19 Pandemic
• Consider letting the filter load up further than usual to reduce frequency of
filter changes.
−Don’t let pressure drop increase enough to disrupt room pressure
differentials.
−Confirm filters remain snug in their frames.

• When feasible, filters can be disinfected with a 10% bleach solution or

another appropriate disinfectant, approved for use against SARS-CoV-2,
before removal. Filters (disinfected or not) can be bagged and disposed of in
regular trash.

• When maintenance tasks are completed, maintenance personnel should

immediately wash their hands with soap and water or use an alcohol-based
hand sanitizer.
Modes of Transmission

• SARS-CoV-2, the virus that causes COVID-19, is thought to spread mainly from
person-to-person through respiratory droplets.

• Infectious respiratory droplets are produced when an infected person coughs or

sneezes.
−Droplets can land in the mouths or noses of nearby people.
−Droplets can land on surfaces and be spread through contact with contaminated
surfaces.
−When in close contact with an infected person, droplets can be inhaled into the
lungs.

• Airborne transmission in some circumstances seems probable. See ASHRAE

Statements on Airborne Transmission

• The SARS-CoV-2 virus may be aerosolized by flushing the toilet.

Airborne Transmission
ASHRAE Statement on airborne transmission of SARS-CoV-2:

• Transmission of SARS-CoV-2 through the air is sufficiently likely that

airborne exposure to the virus should be controlled. Changes to building
operations, including the operation of HVAC systems can reduce airborne
exposures.

ASHRAE Statement on operation of heating, ventilating and air-conditioning

systems to reduce SARS-CoV-2 transmission:

• Ventilation and filtration provided by heating, ventilating and air-conditioning

systems can reduce the airborne concentration of SARS-CoV-2 and thus the
risk of transmission through the air. Unconditioned spaces can cause thermal
stress to people that may be directly life threatening and that may also lower
resistance to infection. In general, disabling of heating, ventilating and air-
conditioning systems is not a recommended measure to reduce the
transmission of the virus.
 Transmission Through
Air in Toilet Rooms
Studies have shown that toilets can be a risk of generating airborne droplets
and droplet residues that could contribute to transmission of pathogens.

• Keep toilet room doors closed, even when not in use.

• Put the toilet seat lid down, if there is one, before flushing.

• Vent separately where possible (e.g. turn exhaust fan on if vented directly
outdoors and run fan continuously).

• Keep bathroom windows closed if open windows could lead to re-

entrainment of air into other parts of the building.
Mechanical Air Filters

• Consist of media with porous structures of fibers or stretched membrane material to

remove particles from airstreams.

• Some filters have a static electrical charge applied to the media to increase particle
removal.

• The fraction of particles removed from air passing through a filter is termed “filter
efficiency” and is provided by the Minimum Efficiency Reporting Value (MERV) under
standard conditions.
−MERV ranges from 1 to 16; higher MERV = higher efficiency
−MERV ≥13 (or ISO ePM1)) are efficient at capturing airborne viruses
−MERV 14 (or ISO equivalent) filters are preferred
−High efficiency particulate air (HEPA) filters are more efficient than MERV 16 filters.

• Increased filter efficiency generally results in increased pressure drop through the
filter. Ensure HVAC systems can handle filter upgrades without negative impacts to
pressure differentials and/or air flow rates prior to changing filters.
Mechanical Air Filters

• Generally, particles with an aerodynamic diameter around 0.3 μm are most penetrating;
efficiency increases above and below this particle size.

• Overall effectiveness of reducing particle concentrations depends on several factors:

−Filter efficiency
−Airflow rate through the filter
−Size of the particles
−Location of the filter in the HVAC system or room air cleaner

For more information, see the ASHRAE Position Document on Filtration and Air Cleaning.
ASHRAE Standard 52.2-2017 Minimum Efficiency Reporting Value (MERV)
ASHRAE MERV vs. ISO 16890 Ratings

Approximate Equivalent Ratings for

Filters Tested Under ASHRAE Standard
52.2 (MERV) and ISO 16890
ASHRAE MERV ISO 16890
(Standard 52.2) Rating
1-6 ISO Course
7-8 ISO Course >95%
9-10 ePM10
11-12 ePM2.5
13-16 ePM1
 High Efficiency Particulate Air
(HEPA) Filters
• By definition, true HEPA filters are at least 99.97% efficient at filtering 0.3
μm mass median diameter (MMD) particles in standard tests.

• Most penetrating particle size may be smaller than 0.3 μm, so filtration
efficiency of most penetrating particles can be slightly lower.

Design Point
99.97 0.3 µm

99.96
99.95 Most Penetrating
Particle Size
99.94
Note: Numbers in graph represent MERV values.
High Efficiency Particulate Air
(HEPA) Filters
• HEPA filter efficiency is better than MERV 16.

• Due to high pressure drops, HEPA filters may not be able to be retrofitted
into HVAC systems.

• To function properly, HEPA filters must be sealed properly in filter racks.

• Filters are often delicate and require careful handling to prevent damage
and preserve performance.

• HEPA filters can be located in HVAC systems or in:

−Portable HEPA Air Cleaners
−Pre-Assembled Systems
−Ad Hoc Assemblies
Electronic Air Filters
• Include a wide variety of electrically-connected air-cleaning devices
designed to remove particles from airstreams.

• Removal typically occurs by electrically charging particles using corona wires

or by generating ions (e.g., pin ionizers) and:
−Collecting particles on oppositely charged plates (precipitators),or
−Charged particles’ enhanced removal by a mechanical air filter, or
−Charged particles’ deposition on room surfaces

• The fraction of particles removed from air passing through an electronic filter is termed
“removal efficiency”.
−For portable, self-contained electronic filters, the rate of particle removal from air is
termed the Clean Air Delivery Rate (CADR).
−CADR ≈ airflow rate × removal efficiency

• Overall effectiveness of reducing particle concentrations depends on:

−Removal efficiency
−Airflow rate through the filter
−Size and number of particles
−Location of the filter in the HVAC system or room air cleaner
−Maintenance and cleanliness of electronic filter components
Electronic Air Filters

• It is critical to wipe the wires in electrostatic precipitators as silicone

buildup reduces efficiency.

• Always follow manufacturer’s instructions when using electronic air filters.

For more information, see the ASHRAE Position Document on Filtration

and Air Cleaning.
Gas-Phase Air Cleaners
• Gas-phase air cleaners are those used to remove ozone,
volatile organic compounds and odors from the air.

• Most contain sorbent materials such as carbon (e.g., activated

charcoal).

• Alone, sorbent beds are NOT efficient at removing particles

(including viruses) from airstreams.

• Carbon/sorbent impregnated fiber filters will remove particles;

check for a MERV rating to show efficiency just as you do with
standard particulate filters.
Ultraviolet Energy (UV-C)
• Ultraviolet energy inactivates viral, bacterial and fungal organisms so
they are unable to replicate and potentially cause disease.

• The entire UV spectrum is capable of inactivating microorganisms, but

UV-C energy (wavelengths of 100 – 280 nm) provides the most
germicidal effect with 265 nm being the optimum wavelength.

• The majority of modern UVGI lamps create UV-C energy with an

electrical discharge through a low-pressure gas (including mercury
vapor) enclosed in a quartz tube, similar to fluorescent lamps.

• Roughly 95% of the energy produced by these lamps is radiated at a

near-optimal wavelength of 253.7 nm.

• UV-C light-emitting diodes (LEDs) are emerging for use.

• Types of disinfection systems using UV-C energy:

−In-duct air disinfection
−Upper-air disinfection
−In-duct surface disinfection
−Portable room decontamination
Ultraviolet Energy (UV-C)

• Requires special PPE to prevent damage to eyes and/or skin

from overexposure.

• The Illuminating Engineering Society (IES) Photobiology

Committee published a FAQs on Germicidal Ultraviolet (GUV)
specific to the COVID-19 pandemic.

For more information, see the ASHRAE Position Document on

Filtration and Air Cleaning.
UV-C LEDs
• Have been common in the UV-A spectrum (315 – 400 nm)

• LEDs are starting to be produced in the 265 nm range

• Efficiency is dramatically less than current low-pressure mercury vapor lamps

• Minimal UV output compared to a low-pressure mercury vapor lamp

• For equal output, UV-C LEDs are more expensive than current low-pressure
mercury vapor lamps

• Limited availability; not yet practical for commercial HVAC applications

For more information, see the FAQs on Germicidal Ultraviolet (GUV) published
by the Illuminating Engineering Society (IES) Photobiology Committee:
UV-C In-Duct Air Disinfection
• Banks of UV-Lamps installed inside HVAC
systems or associated ductwork

• Requires high UV doses to inactivate

microorganisms on-the-fly as they pass through
the irradiated zone due to limited exposure time
−Systems typically designed for 500 fpm
moving airstream
−Minimum irradiance zone of two feet
−Minimum UV exposure time of 0.25 second

• Should always be coupled with mechanical

filtration
−MERV 8 filter for dust control
−Highest practical MERV filter recommended
−Enhanced overall air cleaning with increased
filter efficiency
UV-C Upper-Air Disinfection
• UV fixtures mounted in occupied spaces at heights of
7 feet and above.

• Consider when:
−No mechanical ventilation
−Limited mechanical ventilation
−Congregate settings and other high-risk areas
−Economics/other

• Requires low UV-reflectivity of walls and ceilings

• Ventilation should maximize air mixing

• Use supplemental fans where ventilation is insufficient

UV-C In-Duct Surface Disinfection
• Banks of UV-Lamps installed inside HVAC systems, generally
focused on:
−Cooling coils
−Drain pans
−Other wetted surfaces

• UV irradiance can be lower than in-duct air disinfection

systems due to long exposure times.

• Goals are:
−Even distribution of UV energy across the coil face
−Generally, 12 to 36 inches from the coil face
−Operated 24 hours a day, 7 days a week
UV-C Portable Room Decontamination

• For surface decontamination

• Portable, fully automated units; may use UV-C lamps or Pulsed Xenon
technology

• Settings for specific pathogens such as MRSA, C. difficile, both of which are
harder to inactivate than coronaviruses
−>99.9% reduction of vegetative bacteria within 15 minutes
−99.8% for C.difficile spores within 50 minutes
(Rutala et al. 2010)
Photocatalytic Oxidation (PCO)

• Consists of a pure or doped metal oxide semiconductor material

‒ Most Common Photocatalyst is Ti02 (titanium dioxide)

• Activated by a UV light source

‒ UV-A (400-315nm)
‒ UV-C (280-200nm
‒ UV-V (under 200nm) Ozone can be formed at UV-V wavelengths

• Light mediated, redox reaction of gases and biological particles absorbed on the
surface

• Some units claim disinfection from gaseous hydrogen peroxide (sometimes called dry
peroxide)

• Possible by-products formed by incomplete oxidation, including from gaseous

contaminants
Photocatalytic Oxidation (PCO)

• Some air cleaners using PCO remove harmful contaminants to levels below
limits for reducing health risks set by recognized cognizant authorities.

• Some are ineffective in reducing concentrations significantly; manufacturer

data should be considered carefully.

For more information, see the ASHRAE Position Document on Filtration and
Air Cleaning.
Bipolar Ionization/Corona Discharge

• High voltage electrodes create reactive ions in air that react with airborne contaminants,
including viruses.

• The design of the corona discharge system can be modified to create mixtures of
reactive oxygen species (ROS), ozone, hydroxyl radicals and superoxide anions.

• Systems are reported to range from ineffective to very effective in reducing airborne
particulates and acute health symptoms.

• Convincing scientifically-rigorous, peer-reviewed studies do not currently exist on this

emerging technology; manufacturer data should be carefully considered.

• Systems may emit ozone, some at high levels. Manufacturers are likely to have ozone
generation test data.

For more information, see the ASHRAE Position Document on Filtration and Air Cleaning.
Ozone
• Ozone (O3) is a reactive gas that can disinfect air and surfaces by killing
viruses, bacteria and fungi.

• Ozone is harmful for health and exposure to ozone creates risk for
a variety of symptoms and diseases associated with the respiratory
tract.

• ASHRAE’s Environmental Health Committee issued an emerging issue

brief suggesting “safe ozone levels would be lower than 10 ppb” and that
“the introduction of ozone to indoor spaces should be reduced to as low
as reasonably achievable (ALARA) levels.”

• Should only be considered for disinfection on unoccupied spaces; it

should never be used in occupied spaces.
−Available scientific evidence shows that, at concentrations that do not
exceed public health standards, ozone is generally ineffective in
controlling indoor air pollution.
−Reputable cleaning and restoration companies should be used for
effective, safe disinfection of unoccupied spaces.
Chemical Disinfectants
• EPA reviews and registers antimicrobial pesticides, which include disinfectants
for use on pathogens like SARS-CoV-2.

• Carefully read product labels and use as directed.

• Most products have a required contact or dwell time, which is the amount of
time a surface must remain wet to kill a certain pathogen.

• Applying a product in a way that does not align with its intended use may render
the product less effective.

• Products on EPA List N have not been tested specifically against SARS-CoV-2,
however the EPA expects them to kill the virus because they:
− Demonstrate effectiveness against a harder-to-kill virus; or
− Demonstrate efficacy against another type of human coronavirus similar to
SARS-CoV-2.

• All surface disinfectants on EPA List N can be used to kill viruses on surfaces
such as counters and doorknobs.

• Because SARS-CoV-2 is a new virus, this pathogen is not yet readily available
for use in commercial laboratory testing of disinfectant product effectiveness at
killing that specific virus.
Vaporized Hydrogen Peroxide (VHP)

• Liquid hydrogen peroxide (H2O2) is vaporized and the vapor fills the space
to disinfect all exposed surfaces.

• Space MUST be unoccupied during VHP treatment.

• Requires spaces to be sealed including all doorways, plumbing/electrical

penetrations and HVAC supply and return vents to prevent vapor from
escaping.

• After prescribed exposure times, remaining H2O2 vapor is scrubbed from

space and converted back to oxygen and water before space can be
safely reoccupied.

• The effectiveness and safety of VHP when generated inside active HVAC
ducts and occupied spaces.

• VHP is hazardous at high concentrations and lengthy exposure is often

necessary to inactivate bacteria and viruses in sealed spaces.
Pulsed Xenon (Pulsed UV)
• High-powered UV lamps (generally containing xenon gas)
used in rapid pulses of intense energy.

• Emits a broad brand of visible and ultraviolet wavelengths,

with a significant fraction in the UV-C band.
−Uses significantly higher power outputs that usual UV-C
techniques
−Inactivates viruses, bacteria and fungi using the same
mechanisms as standard UV-C systems

• Typically used for healthcare surface disinfection but can be

used in HVAC systems for air and surface disinfection.

For more information, see the FAQs on Germicidal Ultraviolet

(GUV) published by the Illuminating Engineering Society (IES)
Photobiology Committee.
405 nm Visible Light

• Sometimes referred to a “Near UV,” although not in the UV spectrum.

• Generally integrated into standard room lighting systems.

• Kills bacteria and fungi via different mechanism than UV-C.

−Targets and excites naturally-occurring porphyrin molecules inside organisms creating
reactive oxygen species
−Reactive oxygen species kill by a mechanism similar to bleach

• Effectiveness at killing viruses, including SARS-CoV-2, is not as well documented.

• Provides continuous disinfection of air and exposed surfaces in occupied spaces.

• In the FAQs on Germicidal Ultraviolet (GUV), the Illuminating Engineering Society (IES)
Photobiology Committee notes that effectiveness is approximately 1000 times less than
UV-C and the effective doses are not practical in an occupied environment.
Far Ultraviolet
• Far UV spectrum is 205 to 230 nm

• Some deactivation of bacteria and viruses at the 207 nm and 222 nm range

• 222 nm said to effectively penetrate microorganisms 1µm in size and

smaller

• Unable to fully penetrate larger microorganisms

• UV Dose required to inactivate microorganisms is significantly higher at

these wavelengths than in the UV-C range

• While safety concerns are reduced, Far UV can still cause damage to eyes
and skin

For more information, see the FAQs on Germicidal Ultraviolet (GUV) published
by the Illuminating Engineering Society (IES) Photobiology Committee.
Special Precautions
• Exposure to UV-C energy can cause eye and skin damage.
‒ Photokeratitis (inflammation of the cornea)
‒ Keratoconjunctivitis (inflammation of the ocular lining of the eye)

• Symptoms may not be evident until several hours after exposure and may
include an abrupt sensation of sand in the eyes, tearing and eye pain,
possibly severe.
‒ Symptoms usually appear 6 to 12 hours after UV exposure
‒ Symptoms are fully reversible and resolve within 24 to 48 hours

• Maintenance workers should receive special training before working on

UV-C systems.

• If exposures are likely to exceed safe levels, special personal protective

equipment (PPE) is required for exposed eyes and skin.
‒ Eyewear that blocks UV-C energy
‒ Clothing, suits or gowns known to be nontransparent to UV-C
Summary

• It is likely, but not yet shown, that COVID19 could be spread through the air.

• Air cleaning can help mitigate disease transmission.

• Options for air cleaning include:

‒ HVAC systems
‒ In-Room devices

• Technologies that can be effective include:

‒ Mechanical Air Filters
‒ Electronic Air Filters/Air Cleaners
‒ UV-C Systems
‒ Other Emerging Technologies

• Care and professional judgement should be taken to understand choices for

filtration and air disinfection, pros and cons of each and impact(s) on existing
buildings systems.