National Institute ofEnvironmental Health Sciences
Worker Training Program

Selection and Use of Portable Air Cleaners
to Protect Workers from Exposure to SARS-CoV-2
This fact sheet provides guidance to help employers, building operators, and union officials selectand use portable air cleaners to remove virus-contaminated air in indoor spaces.

Overview
Ventilation and filtration are important to prevent transmissionof COVID-19. SARS-CoV-2, the virus that causes COVID-19,
is mainly spread through inhalation of virus-contaminated air
when an infected person speaks, laughs, coughs, sings, orsneezes. Physical distancing alone will not prevent the buildup of viral particles in a room or workspace (Figure 1).

(1) Source(2) Pathway

The Centers for Disease Control and Prevention
defines close contact as within 6 feet of a person for
15 minutes or more during a 24-hour period. Wearing
a cloth or surgical mask, while helpful, cannot be
relied upon to prevent the spread of COVID-19.

(3) Recipient

AIRPersonwith
COVID-19

Personwithout
COVID-19

Figure 1. For transmission to occur via tiny airborne particles,
three things are necessary: source, pathway, and recipient.1

Approximately 40% of people who are infectious areasymptomatic (no symptoms) or presymptomatic (beforesymptoms begin) and may contaminate air unknowingly.
Work settings with inadequate ventilation and/or those that
require people to be close together for extended periodsof time, increase the risk of COVID-19 transmission. The
illustration at right outlines the recommended steps toimprove ventilation in buildings (Figure 2).

Figure 2. Illustration of the order in which to consider
improvements to ventilation in buildings.2
2 Source, Jones et al, 2020. Schools for Health, Risk

1 Based on an image from the Center for Infectious Disease

Reduction Strategies for Reopening Schools. Harvard
Healthy Buildings Program.

Research and Policy. https://www.cidrap.umn.edu/covid-19/
preparedness-and-response/protecting-essential-workers

1Consider using portable air cleaners (Figure 3) tosupplement heating, ventilation, and air conditioning (HVAC)
system ventilation and filtration, especially in buildings orspaces where adequate ventilation is difficult to achieve.
Directing the airflow so that it does not blow directly fromone person to another reduces the potential for inhalation ofvirus-contaminated air.

Caution: The use of air cleaners is a ventilationintervention that provides one layer ofprotection against COVID-19 transmission. By
itself, air cleaning or filtration is not enough toprotect people from exposure to the virus thatcauses COVID-19.

How Can I Tell If an Air Cleaner Will BeEffective?
For an air cleaner to be effective in removing viruses
from the air, it must be able to remove small, microscopic
airborne particles (in the size range of 0.3 -1 µm4).
Manufacturers of air cleaners report this capability inseveral ways such as:• Documenting particle removal efficiency for specificparticle sizes (e.g., removes 99.9% of particles as small
as 0.3 µm).
• Use of High Efficiency Particulate Air (HEPA) filters.
• Use of the Clean Air Delivery Rate (CADR) rating system
to rate air cleaner performance.

Key Steps to Select Portable
Air Cleaners

Figure 3. Example of a portable air cleaner.3

1. Determine what type of ventilation system is used in
the building.

What Are Air Cleaners and HVAC Filters?
Air cleaners and HVAC filters are designed to capture
pollutants that pass through them out of the air. When usedproperly, air cleaners and HVAC filters can help reduce thespread of airborne viruses in a building or enclosed space.
Portable air cleaners may be particularly helpful whenadditional ventilation with outdoor air is not possible, or
when it compromises indoor temperature or humidity, or
when outdoor air pollution is high.

2. Calculate cubic footage of the space(s) where the aircleaners will be used.
3. Determine the capability and number of air cleaners
needed for the workspace.
4. Identify price and value based on ongoing filter
replacement, maintenance, and upkeep.

When used along with other protective measures, filtration
can be part of a workplace COVID-19 exposure control plan
to reduce the potential for inhalation of the SARS-CoV-2
virus indoors.

4 µm is the symbol for micrometer. A micrometer is one millionth of ameter, invisible to the human eye.

3 Source: www.rabbitair.com

2Determine Type of Ventilation System
Used in the Building

The Bottom Line: If feasible, increase outside
air and filtration in the building’s mechanicalventilation system. If that is not feasible,
consider using portable air cleaning units.

The first step is to determine if the building hasmechanical and/or natural ventilation systems. With amechanical ventilation system, outdoor air will be drawn
into the building with a fan. That outdoor air will be heatedor cooled, filtered, and then distributed to the occupied
spaces through supply vents called diffusers (Figure 4). In
most ventilation systems, air is recycled through exhaustor return vents where a portion of the air is exhausted tothe outside and a portion is combined with outside air andrecycled. Some systems only supply enough air to meet
outdoor air requirements and do not recirculate. Somemechanical systems have economizers that recycle 100%of the air to save on energy costs by reducing heatingor cooling of outside air. Recycled air that is not properly
filtered is more likely to be contaminated. Avoid recycling
unfiltered air.

Do not use air cleaners that generate ozone inoccupied spaces.
• Some products sold as air cleaners generateozone. These products are not safe to use
when people are present because ozone canirritate breathing.
• Ozone generated by commercially available
air cleaners in indoor spaces does noteffectively remove viruses, bacteria, mold, orother biological pollutants.

Unit ventilators provide outside air, filtration, andexhaust for a single room only. In general, they are notcapable of increasing ventilation and filtration to therecommended levels.

How Do I Select A Portable Air Cleaner?
Consider the following characteristics when selecting aportable air cleaner:

Natural ventilation is when outside air comes in throughopen windows and doors and there is no specific equipment
to move and filter air in and out of the building.

1. Select a unit that DOES NOT include ultraviolet
irradiation, ozone generators, or other disinfection
features. These extra features may pose a health hazard
to building occupants and provide little added benefit to
air cleaning.

See the checklist at the end of this document(Appendix) to learn more about your building’s
ventilation systems.

2. Consider the noise rating and whether it will disrupt
activities in the room.
3. Select a unit that uses a HEPA filter. A HEPA filter willremove at least 99.97% of sub-micron particles.
4. Select the correct size of portable air cleaner based on
the room size by calculating the necessary cubic feet per
minute required to achieve the desired air exchange rate
for the individual room. The portable air cleaner will haveinformation on the label (Figure 5) or in the user manual
describing its capabilities. Unfortunately, there is not astandard approach to labeling these devices and often
the labeling is inaccurate or misleading.

Figure 4. Typical supply air vent (diffuser).

3

Rate (CADR), expressed in CFM (Table 1). For
manufacturers that use this standard, three CADR
numbers are given on the label, one for smoke, dust,and pollen. The smoke particles are the smallest,
so that CADR number applies best to viral particles
related to COVID-19. A portable air cleaner witha smoke CADR of 30 CFM can be interpreted as
equivalent to an additional 30 CFM of fresh outdoor
air ventilation. The larger the CADR the greater
amount of air the unit will clean. The CADR isdetermined at the highest fan speed (if the fan speed
is adjustable). Keep in mind that the CADR assumes
good room air mixing, which is not typical.
Warning: operating the air cleaner at lower
fan speeds will greatly reduce their ability toclean air.

a. Calculate the cubic feet of the room by multiplying
the width, length, and height of the room in feet.
Calculation: Width x Length x Height = Cubic Feetb. Calculate the necessary cubic feet per minute (CFM)
for the space where the portable air cleaner willbe used. For most buildings, a minimum of 5-6 airchanges per hour (ACH) of additional air cleaning isdesired. One ACH means that in one hour, a volume of
air has been introduced to the space that is equivalent
to the volume of the space. This does not mean all
the air in the room has actually been replaced due
to inefficient air mixing. It takes many air changes tocompletely replace all the air in a room.
Providing 5-6 ACH is recommended as it
will replace about 99% of the volume of
air in an indoor space with fresh filtered oroutdoor air every 45 – 60 minutes.

i. Cubic Feet per Minute (CFM) Calculation:
Multiply 5 ACH with the cubic feet of the room,
then divide this number by 60.
Calculate CFM for a room
CFM = 5 ACH x Width x Length x Height60

ii. Consider increasing the required CFM by 10%
to account for variability between specified andactual performance.
iii. Select an air cleaner based on your calculated
CFM requirement.
Figure 5. Example of a label from a portable air cleaner.5Note: this label uses square feet, assuming an 8 foot ceiling. Inrooms with ceiling greater than 8 feet, the calculation needs tobe adjusted downward or multiple air cleaners should be used.

c. Many air cleaners only indicate their capabilities
based on square feet. Their ratings are based on
8-foot ceilings. In areas with higher ceilings, therating for these units should be adjusted downward.
If you have an area that is larger than any available
product will serve, use multiple air cleaners.

5 Source: HVAC filtration for controlling infectious airbornedisease transmission in indoor environments: Predicting
risk reductions and operational costs Parham Azimi, Brent
Stephens, Build Environ . 2013 Dec;70:150-160. doi:
10.1016/j.buildenv.2013.08.025. Epub 2013 Sep 4.

d. A recommended standard for rating the effectivenessof air cleaners is the use of a Clean Air Delivery
4Other Important Considerations
Regarding Portable Air Cleaners

Portable Air Cleaner Sizing for Particle Removal
Room area(square feet)

100

200

300

400

500

600

Minimum
CADR (cfm)

65

130

195

260

325

390

Use and Room Configuration• Consider the location of furniture and people in the roomand the placement of units relative to room layout. The
goal is to get as much of the room air as possible to flow
through the HEPA filter as this is what is cleaning theair which is then returned to the room. Obstructions anddistance can interfere with drawing the air into the unit.
• DO NOT place air cleaners next to corners, doorways,curtains, walls, or furniture. Preferably they should beabout 3 feet away from obstructions.
• While open windows are a ventilation improvement option
during the COVID-19 pandemic, air cleaners should beplaced away from open windows to provide the maximum
protective benefit (i.e., the units should filter air frominside the room, not air directly from outdoors).
• In a large room, use more than one portable air cleaner
with a HEPA filter to promote better mixing and overall aircleaning.
• If feasible, air cleaners should remain on and maximizedin the space 2 hours before and after occupancy.

Table 1. This chart is for estimation purposes. The CADRs arecalculated based on an 8-foot ceiling. If you havehigher ceilings, you may want to select a portable air cleaner
with a higher CADR. This chart only applies to aircleaners with HEPA filters.6

If the outdoor air ventilation rate is not reliably known, a
simple rule of thumb for sizing a portable unit for a roomof 500 square feet and 8-foot ceiling height is to select aportable unit with CADR of 325 CFM. This portable unit can
provide a supplemental 5 ACH in the room.
The Clean Air Delivery Rate (CADR) ratings for
portable units are specific to three pollutants(tobacco smoke, dust, or pollen) and an air
cleaner will receive a rating for each of thesepollutants. The efficiency of the device isbased on the difference between pollutant
concentrations in a test chamber with andwithout air cleaner use. These efficiencies arethen translated to CADR ratings which describe
efficiencies at various room sizes.

Maintenance• Schedule preventive maintenance for the units.
• Ensure a sufficient supply of the correct filters needed forchange-out of both the pre-filter and HEPA filter.
• Follow the manufacturer maintenance and filter changerequirements.
• Ensure staff are aware of these requirements.
• Ensure good fit of filters in the frame and no leakage of
air around the filter unit.

Cost• Portable air cleaners range in cost from about $150 to$2,000 depending on their size and features.
• One handy feature is a shut off timer that allows the unit
to shut down and turn on at set times.
• The average annual electricity costs for running portable
HEPA air cleaners 100% of the time are just under $200
per year, with individual units ranging from just over$100 to nearly $250 per year. The cost of replacement
filters should also be considered in estimating overall
annual cost.

6 Source EPA Guide to Air Cleaners in the Home, 2nd Edition.

5Appendix
How Do I Learn More About My Building’s
HVAC System?
To learn about your building’s HVAC system, meet with building
managers, maintenance personnel, landlords, or an HVAC contractor.
Ask them about the specific design, operation, and maintenance ofyour building’s systems. The best approach includes an inspection sothat you can see how things work. A simple test is holding a tissue
near air supply and exhaust vents to see if air is moving or not. Also
look to see if vents have been covered or blocked. This may occur
when buildings are renovated.
Be sure to ask the following questions:• Does the building have a mechanical or natural ventilation system?
If natural, then skip the rest of the questions below and consider
using portable air cleaners. If mechanical find out the following:• How do the systems work?
• How many air changes per hour of outside air is brought into thebuilding or produced by filters in the HVAC equipment?
• Where does the outside air enter the building?
• What is the MERV7 rating of the filter used in this building? MERV= minimum efficiency reporting value. Experts recommend a
MERV rating of 13 or higher for control of COVID-19 if the building
systems can function with that level of filtration. Filters with aMERV of 13 and above are required to demonstrate at least 50%
removal efficiency for the smallest particles tested.
• How often are the filters changed?
• How is the filtered air distributed to the occupied spaces?
• Does the system have economizers?
• Are supply and exhaust returns all functioning properly?
• When was the system last tested and balanced to ensure it isworking as designed?
• Have renovations changed conditions in some of the rooms?
• How many zones and thermostats are used by the HVAC system?
Warning: often setting a thermostat to “automatic”, causes thefan bringing in outside air to only function when the system
requires heating or cooling. Setting the fan to the “on” position will
avoid this problem
For more information: https://www.cdc.gov/coronavirus/2019ncov/community/ventilation.html
7 The MERV is an output of the American Society of Heating, Refrigerating andAir-Conditioning Engineers (ASHRAE) Standard Test 52.2. This test method assigns
the filter with a number (from 1 to 16) based on a performance test comparing
concentrations of particles sized between 0.3 and 10 μm, upstream and downstreamof the filter. The MERV for each filter corresponds to the particle removal efficiency ofthe filter, based on the specific size category of particles tested.

6Referencesand Resources
1. EPA Air Cleaners, HVAC Filters, andCoronavirus (COVID-19). https://www.epa.gov/coronavirus/air-cleaners-hvac-filters-andcoronavirus-covid-19
2. EPA Guide to Air Cleaners in the Home, 2ndEdition, EPA 402-F-08-004, July 2018, EPA
Indoor Environments Division. https://www.
epa.gov/sites/production/files/2018-07/
documents/guide_to_air_cleaners_in_the_home_2nd_edition.pdf
3. Residential Air Cleaners, A Technical
Summary, 3rd Edition, Portable Air Cleaners,
Furnace and HVAC Filters, EPA 402-F-09-002,
July 2018, Indoor Environments Division.
www.epa.gov/iaq
4. Coronavirus (COVID-19) Response Resources
From ASHRAE and Others. https://www.
ashrae.org/technical-resources/resources
5. Curtius et al. (Oct 5, 2020). Testing Mobile Air
Purifiers in a School Classroom Reducing theAirborne Transmission Risk for SARS-CoV-2.
Pre-print downloaded Oct 6 from https://doi.
org/10.1101/2020.10.02.20205633
6. Portable Air Cleaners: Selection andApplication Considerations for COVID-19 Risk
Reduction, Healthy Buildings Program at theHarvard T.H. Chan School of Public Health,2020
7. ASHRAE, In-Room Air Cleaner Guidance For
Reducing COVID-19 in Air in Your Space/Room, January 21, 2021. https://www.ashrae.
org/file%20library/technical%20resources/
covid-19/in-room-air-cleaner-guidance-forreducing-covid-19-in-air-in-your-space-orroom.pdf
8. Centers for Disease Control and Prevention,
Ventilation in Buildings, March 23, 2021.
https://www.cdc.gov/coronavirus/2019-ncov/
community/ventilation.html