Technical Advisory on Use of Air-Cleaning Technologies to Mitigate COVID-19 Aerosol Transmission Risk

1.            Introduction

COVID-19 is mainly transmitted by close contact and respiratory droplets which are released when an infected person coughs, sneezes, talks, or sings. It can also be spread through virus aerosols in the air under certain settings, such as enclosed environments which are poorly ventilated. Hence, it is critical to mitigate aerosol transmission risk by improving ventilation and air quality in indoor environments.

This Guidance Note provides current knowledge on air-cleaning technologies that can be used to reduce airborne contaminants, including particles containing viruses, in enclosed environments. Improving ventilation is the primary measure to reduce aerosol transmission risk (please refer to recommendations in the Joint BCA-NEA-MOH Guidance on Improving Ventilation and Indoor Air Quality in Buildings Amid the COVID-19 Situation). If deployed correctly, air-cleaning technologies can be a useful supplementary strategy, especially in spaces where ventilation cannot be easily improved (e.g. no doors or windows), and/or in spaces with high-risk activities^*. Air-cleaning technologies are not a substitute for ventilation as fresh air is not introduced into the treated areas. Spaces that are already highly ventilated may benefit less from air-cleaning technologies.

Among the air-cleaning technologies, mechanical filtration and ultraviolet germicidal irradiation (UVGI) have been demonstrated to be effective in removing or inactivating SARS-CoV-2 in the air.^[1-2] A combination of multiple complementary air-cleaning technologies may be used as part of a comprehensive plan to manage indoor air quality. For example, installation of efficient filters (at least MERV 14, F8 or ISO ePM1 70-80% is recommended) in air-conditioning and mechanical ventilation (ACMV) systems to treat recirculated air may be used in combination with upper-room UVGI to clean the air in occupied spaces.

Despite recent growing interest in electronic air-cleaning technologies and chemical sprays for air cleaning, there is a lack of robust scientific evidence to support their safety and effectiveness against virus aerosols.

Each of these technologies is further discussed in the subsequent sections.

^* Such spaces include those where bioaerosol-generating procedures are performed on people (e.g. nasopharyngeal swab taking and dental procedures), where COVID-19 patients may be present, or where masks must be removed.

2.            Mechanical filtration

Mechanical air filters remove airborne particulates by capturing them on filter materials (e.g. fiberglass, polyester, cotton). This class of filters does not include electronic filters which remove the particles from the air electronically (e.g. using static electricity or ions).

Efficient filters (at least MERV 14, F8 or ISO ePM1 70-80% is recommended; refer to Table 1 for efficiency ratings) should be installed in ACMV systems to treat recirculated air. In spaces where ventilation cannot be easily improved and/or high-risk activities are ongoing, portable air-filtering devices for localised air cleaning may be considered as an interim measure.

Table 1. Efficiency rating of mechanical air filters

Portable air-filtering devices

Filter efficiency

Portable air-filtering devices are often equipped with a high-efficiency particulate air (HEPA) filter. HEPA filters are at least 99.97% efficient at capturing particles 0.3 µm in size, and can capture particles both larger and smaller than 0.3 µm with even higher efficiency, due to the use of multiple particle collection mechanisms (Figure 1). A particle size of 0.3 µm approximates the filters’ most penetrating particle size (MPPS), i.e. the worst case. As the SARS-CoV-2 virus is about 0.1 µm in size and is likely exhaled in larger respiratory droplets, HEPA filters are therefore at least 99.97% efficient at capturing viral particles associated with SARS-CoV-2.

Products with HEPA filters are recommended for use in environments with higher risk of COVID-19 transmission,^[1] while products with non-HEPA filters may be used in lower-risk environments.

Figure 1. Filtration efficiency of HEPA filters at different particle sizes. HEPA filters are at least 99.97% efficient at capturing particles 0.3 µm in size, and can capture particles both larger and smaller than 0.3 µm with even higher efficiency. This is due to the use of multiple particle collection mechanisms such as interception, impaction, and diffusion, which act on a range of particle sizes. A particle size of 0.3 µm approximates the filters’ most penetrating particle size (MPPS), at which the filtration efficiency is the lowest. Figure taken from ^[3].

Sizing of portable air-filtering devices

The effectiveness of a portable air-filtering device at removing particulates depends on both the airflow rate through the device and the filter efficiency. One standard measure of effectiveness is the Clean Air Delivery Rate (CADR), which is assigned to the device upon testing. CADR may be reported in cubic feet per minute (cfm) or cubic meter per hour (cmh) for removal of smoke (0.1-1.0 µm), dust (0.5-3.0 µm) and pollen (5-11 µm) from the air. The smoke CADR best represents the effectiveness at filtering virus particles.

When choosing a portable air-filtering device, select a unit that is appropriately sized for the space. The minimum smoke CADR that a unit should provide for a particular room size can be estimated as follows, according to AHAM AC-1 standard:^[4]

smoke CADR (cmh) ≥ room size (cubic meter) × 5

smoke CADR (cfm) ≥ room size (cubic feet) ÷ 12

For example, a room with a floor area of 24 m² and a ceiling height of 2.6 m (room volume of 62.4 m³) will need a portable air-filtering device with a minimum smoke CADR of 62.4 x 5 = 312 cmh (i.e. 184 cfm).

Some portable air-filtering devices may contain additional electronic air-cleaning technologies such as ionisers.  As the effectiveness of electronic air-cleaning technologies against virus particles is not well established, it is recommended that sizing of portable air-filtering devices be based on smoke CADR of the filters alone. Portable air-filtering devices with a high ratio of smoke CADR (based on filters alone) over airflow rate are more efficient at cleaning the air (i.e. higher output of clean air per pass through the units), and are recommended for use in environments with higher risk of COVID-19 transmission. For example, portable air-filtering device A with smoke CADR (based on filters alone) of 312 cmh and an airflow rate of 400 cmh will be more efficient at cleaning the air than portable air-filtering device B with the same smoke CADR (based on filters alone) but an airflow rate of 800 cmh.

3.            UVGI

UVGI uses short-wave ultraviolet (UV-C) energy to inactivate microorganisms. UV-C irradiation has been shown to be effective at inactivating SARS-CoV-2,^[5] and has been applied in several air-cleaning solutions, discussed in the subsequent sections. The most effective UV wavelength range for inactivation of microorganisms is 220 – 300 nm, with peak effectiveness near 265 nm.^[6] The typical source of UV-C is low-pressure mercury vapor lamps, which emit mainly near-optimal 253.7 nm. ASHRAE has recommended a minimum UV-C (254 nm) dose of 1,500 µJ/cm² for 99% inactivation of SARS-CoV-2 in the air.^[7] While other UV-C sources such as UV-C light-emitting diodes (LEDs, emission in the 265 – 280 nm range) and excimer lamps (emission in the far UV-C region, 205 – 230 nm) are becoming more commonly available, more research on safety and effective dose against aerosolised SARS-CoV-2 is required for such emerging UV-C sources to support their use amid the COVID-19 situation.^[7-10] Although there is some evidence that far UV-C lamps emitting at 222 nm are less harmful to skin and eyes than the mercury vapor lamps emitting at 254 nm, they may still cause damage^[7-10] and should be used in accordance with NEA’s Safety Guidelines for Use of UVC Devices in Commercial/Industrial Settings, which is applicable for all UV-C systems.

The following should be considered during implementation of all UV-C systems:

1. UVGI lamp fixtures must be carefully installed to minimise exposure of UV-C to occupants in the room. The exposure to UV incident radiation on unprotected skin or eyes should be within the limits (8 hr exposure: 3 mJ/cm² for 270 nm, 6 mJ/cm² for 254 nm, 25 mJ/cm² for 220 nm, etc) specified under the International Commission on Non-ionizing Radiation Protection (ICNIRP) guidelines on limits of exposure to UV radiation of wavelengths between 180 nm and 400 nm (incoherent optical radiation) and the International Electrotechnical Commission (IEC) 62471 Photobiological safety of lamps and lamp systems.
2. UV-C systems should not generate ozone gas in occupied spaces beyond the acceptable limit (0.05 ppm, 8 hr) stipulated by Singapore Standard SS554: Code of Practice for Indoor Air Quality for Air-Conditioned Buildings.
3. If mercury lamps are used as the UV-C source in cold moving air, higher UV irradiance may be necessary to achieve the desired UV dose for virus inactivation. Mercury lamps may not function optimally at low temperatures.^[6]
4. UV-C systems should be designed for the output at the end of effective life when the UV-C intensity levels are 50-85% of that measured at initial operation. Depreciation over useful life should be verified by lamp manufacturers’ specification data.^[6]
5. All organic material components within 1.5 m of the UV lamp should be shielded, as UV-C energy can be detrimental to organic materials (e.g. plants, certain filter media and insulation or gasket foams in ACMV systems, wood surfaces, and wallpapers).^[6]
6. If UV-C systems are intended to be operated intermittently, this must be factored into the initial system design. Cycling UV lamps on and off may negatively affect the lamp/ballast performance, and life.^[11]
7. Regular verification of the performance of the UV-C systems should be carried out after installation to ensure continued safety and efficacy.
8. Companies and households with used UV-C lamps are encouraged to recycle them via the voluntary lamp recycling programmes in Singapore.

Upper-room UVGI

Upper-room UVGI systems^[11-12] use UV lamps to create an irradiation zone in the upper portion of the room for inactivation of microorganisms in the air (Figure 2). As the irradiation zone is located above room occupants, such systems avoid direct exposure to UV-C.

Upper-room UVGI systems should be customized for the spaces in which they will be used. Parameters such as room configuration, UV fixture placement, air mixing and relative humidity will affect UV-C effectiveness. Spaces that are already highly ventilated may benefit less from upper-room UVGI systems.  The following are some considerations when implementing upper-room UVGI.

1. UV irradiation should be distributed uniformly and parallel to the plane of the ceiling. Minimal UV dose of 1500 µJ/cm² (if 254 nm wavelength is used) should be maintained in the upper irradiation zone.^[7]
2. The minimum ceiling height in a room should be 2.6 m, with upper room UVGI installed at heights of 2.1 m and above.^[12]
3. ACMV systems should be designed and operated to provide optimal airflow patterns and prevent air stagnation. Fans can be used if air is not well mixed vertically between the lower and upper portions of the space.^[11]
4. Relative humidity in the room should be less than 60% if attainable, as levels over 80% may reduce the effectiveness of the UV-C systems.^[11]
5. Installation of upper-room UVGI systems should be carried out by professionals or reputable UV-C system manufacturers to ensure that the UV-C irradiation is directed above the occupied space, and that the irradiance levels in the lower occupied room do not exceed the acceptable limits (e.g. 0.2 µW/cm² for 254 nm based on 8 hr exposure).^[13]

Figure 2. Example of placement of an upper-room UVGI system. Figure taken from ^[11].

UV-C airstream disinfection systems

UV-C systems (Figure 3) intended for airstream disinfection are typically installed inside air-handling units (AHUs) or AHU rooms where the lowest maximum air speed in an ACMV system usually occurs, though UV-C systems may also be located in air ducts.^{[7], [11]} The following should be considered for installation of UV-C airstream disinfection systems:

1. UV energy should be distributed uniformly in all directions throughout the AHUs, AHU rooms or the length of the air ducts to achieve required UV dose (minimum 1,500 μJ/cm², if 254 nm wavelength is used).^[7]
2. Highly reflective materials (e.g. aluminium) can be used inside AHUs or AHU rooms to increase effective UV dose by reflecting UV-C energy back into the irradiated zone.^[11]
3. UV-C airstream disinfection systems should be designed to provide the required single-pass disinfection level under worst-case conditions of air speed and temperature in the irradiated zone, and installed in locations that can provide a minimum UV exposure time of 0.25 s (i.e., minimum irradiance zone of 63.5 cm in length if air speed is 2.54 m/s).^{[7], [11]}
4. UV-C systems should be coupled with mechanical filtration to enhance the overall air cleaning efficiency and to protect UV lamps from dust and debris accumulation which may reduce UV output over time.^[11]
5. Warning signs to indicate the presence of a UV-C hazard should be placed near the lamps and on AHU access panels where the internal UVGI lamps are installed. Activation switches should be clearly labelled and protected with switch guards to prevent accidental activation by unauthorised persons.^{[8], [11]}

Figure 3. Example of a UV-C airstream disinfection system. Figure taken from ^[7].

Some UV-C devices installed inside the ACMV systems are intended for surface decontamination of cooling coils, condensate pans and other wetted surfaces, and may not be effective at disinfecting moving air due to lower UV irradiance.^{[7], [11]} Users should request the correct product specifications for the intended use.

4.            Electronic air cleaners

Electronic air cleaners use technologies such as bipolar ionisation, negative-ion generators, photocatalytic oxidation and non-thermal plasmas to generate reactive ions or reactive oxygen species which can react with airborne contaminants, including microorganisms.^{[2], [7]} Despite recent growing interest in electronic air-cleaning technologies, the efficacy of most products in inactivating virus aerosols is not supported by consistently robust scientific evidence. Moreover, the disinfection efficacy of such products in real-world settings, as well as potential unintended side effects and their avoidance, are not adequately documented. Electronic air cleaners may generate ozone and other undesirable secondary by-products which could have potential health effects (e.g. respiratory or skin irritation). Consumers are encouraged to exercise caution and to ensure that product claims and the intended use of the product are supported by efficacy and safety data.

5.            Chemical sprays for air cleaning

Some devices generate and release sprays/mists/fogs/vapors of chemicals such as ozone, chlorine dioxide, glycols, alcohols and bleach into indoor spaces for air and surface cleaning.^{[2], [14]} The efficacy of most such products in inactivating virus aerosols is not supported by consistently robust scientific evidence, and is dependent on the chemicals used and their concentrations in the air. Moreover, the disinfection efficacy of such products in real-world settings, as well as potential unintended side effects and their avoidance, are not adequately documented. Prolonged exposure to such chemicals may lead to health consequences, particularly for vulnerable populations such as children and the elderly. In addition, the aerosolised chemicals could land on surfaces, resulting in secondary exposure via touch.  Consumers are encouraged to exercise caution and to ensure that product claims and the intended use of the product are supported by efficacy and safety data.

6.            Maintenance of air-cleaning devices

Most air-cleaning devices require maintenance to function optimally. Emission of visible light by UV lamps does not necessarily imply that the lamps are effective. Air filters and UV lamps should be replaced according to manufacturers’ recommendations to ensure continued effectiveness of the devices.

7.            References

^[1] Ventilation in Buildings, Centers for Disease Control and Prevention, Updated on 2 Jun 2021, URL: https://www.cdc.gov/coronavirus/2019-ncov/community/ventilation.html.

^[2] Potential Application of Air Cleaning Devices and Personal Decontamination to Manage Transmission of COVID-19, Scientific Advisory Group for Emergencies – Environmental and Modelling group (SAGE-EMG), UK, 4^th November 2020.

^[3] Ana Maria Todea, Frank Schmidt, Tobias Schuldt, Christof Asbach, Development of a Method to Determine the Fractional Deposition Efficiency of Full-Scale HVAC and HEPA Filter Cassettes for Nanoparticles ≥3.5 nm, Atmosphere, 11, 1191 (2020).

^[4] Guide to Air Cleaners in the Home, 2^nd Edition, EPA-402-F-08-004, United States Environmental Protection Agency, July 2018.

^[5] Biasin M. et al, UV-C Irradiation is Highly Effectively in Inactivating SARS-CoV-2 Replication, Sci Rep, 11, 6260 (2021).

^[6] 2020 ASHRAE Handbook – HVAC Systems and Equipment, Chapter 17: Ultraviolet Lamp Systems, 2020.

^[7] Filtration and Air Cleaning Summary, ASHRAE, Last Access on 11 Jun 2021, URL: https://www.ashrae.org/technical-resources/filtration-disinfection#cdcposition

^[8] IES Committee Report: Germicidal Ultraviolet (GUV) – Frequently Asked Questions, IES CR-2-20-V1, Illuminating Engineering Society, 2020.

^[9] UVC Lamps and SARS-CoV-2, International Commission on Non-Ionizing Radiation Protection, May 2020, URL: https://www.icnirp.org/en/activities/news/news-article/sars-cov-2-and-uvc-lamps.html

^[10] CIE Position Statement on the Use of Ultraviolet (UV) Radiation to Manage the Risk of COVID-19 Transmission, International Commission on Illumination, Last Access on 20 Aug 2021, URL: https://cie.co.at/publications/position-statement-use-ultraviolet-uv-radiation-manage-risk-covid-19-transmission

^[11] 2019 ASHRAE Handbook – HVAC Applications, Chapter 62: Ultraviolet Air and Surface Treatment, 2019.

^[12] Upper-Room Ultraviolet Germicidal Irradiation (UVGI), Centers for Disease Control and Prevention, Updated on 9 Apr 2021, URL: https://www.cdc.gov/coronavirus/2019-ncov/community/ventilation/uvgi.html#print

^[13] Environmental Control for Tuberculosis: Basic Upper-Room Ultraviolet Germicidal Irradiation Guidelines for Healthcare Settings, Centers for Disease Control and Prevention, March 2009.

^[14] Safety Precautions: Cleaning and Disinfecting for COVID-19, Updated on 3 Jun 2024, URL: https://www.cdc.gov/covid/php/public-health-strategy/?CDC_AAref_Val=https://www.cdc.gov/coronavirus/2019-ncov/php/eh-practitioners/sprayers.html.