The use of air purifiers in health care setting to prevent transmission of COVID-19

air purifiers

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Compiled by

Dr Nadisha Badanasinghe
Senior Lecturer, Department of Microbiology, Faculty of Medicine, University of Kelaniya MBBS, Dip (Med-Micro), Med (MedMicro)

Dr Veranja Liyanapathirana
Senior lecturer, Department of Microbiology, Faculty of Medicine, University of Peradeniya MBBS, MPhil, PhD

COVID-19, caused by SARS-CoV-2, is spread in the community predominantly by droplets expelled from an infected patient when they cough, sneeze or exhale. Droplets can deposit on the nose, mouth or eyes of those who are nearby and on surfaces. Other people then catch COVID-19 by touching these objects or surfaces, then touching their eyes, nose and mouth 1.  Therefore, most cost effective method to break this chain of transmission is to practice standard precautions and droplet based precautions in general healthcare settings.

A recent publication in the New England Journal of Medicine has shown that when aerosols are GENERATED using a high powered machine in an experimental setting, aerosols containing the SARS-CoV-2, may remain for up to 3 hours in air 2. However, this finding may not reflect a clinical setting in which aerosol-generating procedures are performed. Aerosol generation occurs in health care environments during certain procedures. These include, intubation/extubation, non-invasive ventilation, suctioning of body fluids etc. In this context, whether SARS-CoV-2 can be transmitted by aerosols remains controversial, and the exposure risk for close contacts has not been systematically evaluated3. However, WHO has clearly stated that the evidence for possible airborne transmission is still lacking1.

Ideal setting requires patients with COVID-19 likely to undergo aerosol generating procedures to be cared in Airborne Infection Isolation Room (AIIR) with controlled air-circulation and negative pressure (CDC) 4. Even when a patient is cared for in such a room, routine cleaning and disinfection need to be performed while healthcare professionals entering such rooms are required to wear appropriate personal protective equipment (PPE), depending on whether they are performing an aerosol generating procedure or not (CDC) 4.

In laboratories, when samples from patients with suspected or confirmed COVID-19 are handled, laboratory professionals are expected to work in Biosafety level 2 containment level. This include appropriate PPE and working in a class 2 safety cabinet (WHO) 5.

Air-purifiers/ filters

The air purifiers and filters are used to improve indoor air quality in domestic and health care settings where indoor air pollutants are commonly found. They have also been in use for disinfection and sterilization of air in health care facilities such as in patient isolation rooms and in some industries where a sterile air circulation is needed.

Filtration method is used to filter airborne infectious particles containing bacteria and viruses (bio-aerosols) while there are various controlling technologies for inactivating and/or attracting aerosolized microorganisms, including air ionizers, use of antimicrobial material-embedded filters, ultraviolet (UV) light, and photocatalysis 6 (refer annex 1 for types of air purifiers).

If they are used to sterilize bio-aerosols, an air purifier has to be capable of consistently drawing in enough air to reduce the amount of infectious particles in the air. The faster an air purifier can cycle air through the filter, the better its chances of catching infectious particles. This is indicated in air purifiers as its CADR (clean air delivery rate) number on the packaging. CADR reflects, in cubic feet per minute, the volume of clean air (after removal of pollutants) that an air purifier produces at its highest speed setting.

There is a need to analyze the dynamics of air flows in the presence of these air filters as the mixing of air may cause multiple desirable and undesirable effects. These can include inhomogeneous flows that concentrate deposition of particles in certain parts of patient care rooms and disturbances to airflow due to surfaces in the room such as furniture, clothing etc5. These may also result in turbulences in the airflow in a closed environment which might cause dispersion of droplets to a longer distance. It has been reported that even a strong airflow from the air conditioner could propagate droplets for a long distance which had resulted in spread of COVID in a restaurant in China 7.

It is not known whether general air purifiers are sufficiently effective at protecting people all over a room from bio-aerosols because the amount of air passing through them may be smaller than the volume of ventilated air in a room. Thus, when using air purifiers, it is necessary to ensure that people are in close proximity to them.

Air purifiers and COVID-19

There is still no direct evidence to state if the use of air-purifiers are effective to prevent spread of COVID-19. Various organizations and individuals, including industry are providing opinion on the use of air purifiers. The WHO and CDC have not recommended the use of air-purifiers in their infection prevention and control recommendations. 

The published guideline by the Federation of European Heating, Ventilation and Air Conditioning Associations suggest that the air purifiers use is dependent on the dimension of the room used and that regular ventilation is more effective in most instances 8. Further, the Society of Heating, Air-Conditioning and Sanitary Engineers of Japan state that air purifiers should not be used alone in the absence of other measures9.

There is one published scientific paper on dispersion of droplets due to use of air purifiers had resulted in transmission of COVID-19 in an enclosed environment in S. Korea10.

In the context of COVID-19 pandemic, SLCM wishes to express its views on air purifiers. When making the recommendations, the following concerns were taken in to account;

  1. The lack of recommendations for use of air purifiers to reduce transmission of COVID-19 in WHO/CDC infection prevention and control guidelines,
  2. The lack of evidence or recommendations on benefits of using air-purifiers as a standalone method to prevent transmission of COVID-19,
  3. the lack of evidence for use of such purifiers as part of airborne precautions in global context,
  4. Evidence for causing harmful effects on human health due to the byproducts generated by some air purifiers
  5. The associated procurement and maintenance  cost which can be  used worthily for more pressing needs such as for personal protective equipment (PPE) instead,
  6. The lack of evaluation mechanism of the effectiveness of these machines and a mechanism for commissioning  in local setting,
  7. Evidence in scientific literature that some air purifiers actually resulted in propagation of COVID-19 due to turbulence in airflow

Considering the above, the Sri Lanka College of Microbiologists currently does not recommend the use of air-purifiers alone, irrespective of their mechanism, in health-care settings for prevention of transmission of COVID-19.The SLCM would review available evidence regularly and change this view if sufficient evidence becomes available. 

Conflict of interest

The SLCM, being a professional college not aimed at profit making, declares no conflict of interest.

Annex 1

An account on different types of air purifiers

  1. HEPA Air Purifiers/ filters

HEPA filters are very effective, certified to capture 99.97 % of particles that are 0.3 micron in diameter. The SARS-CoV2 is 0.125 microns (unattached to any fluid droplet), but the droplets when people cough, talk, or breath, initially are larger, around 1 micron. HEPA filters reliably capture particles of this size, assuming the particle reaches the filter. ULPA (Ultra-Low Penetration Air) filters are even better, catching 99.99% of particles of 0.12 microns.

HEPA filters have three mechanical mechanisms of action to prevent particulate matter from passing through. The smallest particles diffuse, which is when particulate matter becomes trapped by gas particles and the fibers (diffusion). Secondly, slightly larger particles will stick to the fibers upon contact (interception) and larger particles will go through impaction, where they collide with the fibers and become stuck within them 11.

CDC recommends that any AGPs be conducted inside of an AIIR 4.  However, availability of such facilities is scarce. Therefore, alternative, lower cost methods which might be effective in filtering infectious air particles during AGP is desirable. Their use may reduce the risks that are present due to gaps in Personal Protective Equipment (PPE) 11.  For this to be effective, it need to be combined with an adequate number of air changes per-hour 12,13. In the U.S., the CDC also recommended the use of HEPA purifiers to help reduce viral concentrations of the SARS virus in the air when properly ventilated hospital rooms weren’t available 14.

  1. Ionizer purifiers / Electrostatic precipitation/ Charged media filters

Ionizer purifiers use charged electrical surfaces or needles to generate both positive and negative ions. These ions attach to airborne particles which are then electrostatically attracted to a charged collector plate. This mechanism produces trace amounts of ozone and other oxidants as by-products.

Electrostatic precipitation use the same mechanism of action, but it makes the airborne particles to be charged by using an electric field formed between the plates by a high voltage. The charged airborne particles are then attracted to the plate. Charged media filters use a process similar to electrostatic precipitators. The key difference is that charged media filters make use of filters in place of plates.

The efficacy of these methods depends on the ion concentration, ion polarity, and ion exposure time15.

While there have been reports on inactivation of airborne bacteria with ion emission, there is limited data for virus inactivation. Huang et al. (2008) performed a removal test of the influenza virus A strain (H11N9) with unipolar air ions. They reported the removal efficiency achieved not by inactivation but by filtration 16. However, Junho Hyun et al 15 showed the antiviral efficiency for MS2 bacteriophage virus with bipolar ions was higher than that with unipolar ions, but the electric field was not effective for inactivation of viruses. However, there are no reports on antiviral activity of bipolar ions against SARS CoV-2 in the literature.

The airborne particles floating in the air which normally have a neutral charge end up with a charge when they come in contact with charged ions. That causes them to stick to surfaces around the room such as walls and to stick to each other making them too heavy to remain airborne and settle on floor.  When these particles stick to walls and surrounding solid items in the room, instead of completely eliminating them.  Further there is evidence to show that negatively charged ions could significantly reduce the level of serotonin in blood or brain thus causing profound neurovascular, endocrinal, and metabolic effects in humans17. In addition, if these ionizer purifiers produce ozone and other oxidants as a byproduct, that can also have serious side effects to humans 15.

  1. Ultraviolet germicidal irradiation and ozone generators

UV air purifiers use UVC light to sterilize air that passes through it via forced air. The mechanism of action of UVC germicidal air purifiers with their limitations are given in the document published in SLCM web site (

The ozone generators have similar limitations to UVC air purifiers since they cause severe adverse effects on human health. It must be present at a concentration above levels considered safe for humans. When inhaled ozone can damage lungs and can worsen chronic respiratory diseases 18.

  1. Sorbent Air Cleaners

These air cleaners are mainly used to adsorb gaseous contaminants in the air by physical adsorption (physisorption) and chemisorption.  However, gases once adsorbed can later desorb back into the airstream since weak forces are used to attract them. The most common adsorbent used is activated carbon and others include activated aluminas. The adsorption process when using activated carbon must reach equilibrium thus it may be difficult to completely remove contaminants. They are not used as a standalone method for air purification, but they may be used in combination with HEPA filters to sterilize air 19.

  1. Air Cleaners Using Photo-catalytic Oxidation

Term “photo-catalysis” applies to a chemical change enabled by photon activated catalysis. The chemical change is usually oxidation. The catalyst that is used is a metal oxide semiconductor such as   titanium dioxide and it’s energized by ultraviolet (UV) light to initiate the chemical change 20.

Photo-catalytic air cleaners claim to efficiently decompose and eliminated organic chemicals, acetaldehyde, and bio-aerosols containing viruses, bacteria and fungi21. However, in a paper published in the journal of Building and Environment, authors have found that some of the gases to come through the system are dangerous to humans including formaldehyde which is a known carcinogen 22.


  1. WHO Scientific Brief – Modes of transmission of virus causing COVID-19: implications for IPC precaution recommendations dated 29/03/2020.
  2. van Doremalen N, Bushmaker T, Morris DH, Holbrook MG, Gamble A, Williamson BN, Tamin A, Harcourt JL, Thornburg NJ, Gerber SI, Lloyd-Smith JO, de Wit E, Munster VJ. Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1. N Engl J Med. 2020 Apr 16;382(16):1564-1567. doi: 10.1056/NEJMc2004973. Epub 2020 Mar 17. PubMed PMID: 32182409; PubMed Central PMCID: PMC7121658.
  3. Guo Z, Wang Z and  Zhang S et al. Aerosol and Surface Distribution of Severe Acute Respiratory Syndrome Coronavirus 2 in Hospital Wards, Wuhan, China, 2020; July 2020; Journal of  Emerging Infectious Diseases
  4. CDC – Interim Infection Prevention and Control Recommendations for Patients with Suspected or Confirmed Coronavirus Disease 2019 (COVID-19) in Healthcare Settings. Dated 13th April 2020. Available at Accessed on 13/05/2020
  5. World Health Organization. (‎2020)‎. Laboratory biosafety guidance related to coronavirus disease 2019 (‎‎COVID-19)‎‎: interim guidance, 12 February 2020. World Health Organization.
  6. Kim J & Jang J. Inactivation of airborne viruses using vacuum ultraviolet photocatalysis for a flow-through indoor air purifier with short irradiation time. Aerosol Science and Technology, 52:5, 557-566, DOI: 10.1080/02786826.2018.1431386
  7. Lu J, Gu J, Li1 K, Xu C, Su W, Lai Z, Zhou D, Yu C, Xu B , Yang V. COVID-19 Outbreak Associated with Air Conditioning in Restaurant, Guangzhou, China, 2020; Journal of Emerging Infectious Diseases.
  8. REHVA COVID-19 guidance document, April 3, 2020. Accessed on 14/05/2020
  9. The Society of Heating, Air-Conditioning and Sanitary Engineers of Japan (SHASE). Available from . Accessed on 14/05/2020
  10. Ham S. Prevention of exposure to and spreadofCOVID-19using air purifiers: challenges and concerns. Epidemiol Health. 2020 Apr 17:e2020027. doi: 10.4178/epih.e2020027.
  11. Elias B and  Bar-Yam Y. Could Air Filtration Reduce COVID-19 Severity and Spread? March 9, 2020.
  12. CDC, Guidelines for Environmental Infection Control in Health-Care Facilities (2003), Appendix B. Available from Accessed on 14/05/2020
  13. WHO. Severe Acute Respiratory Infections Treatment Centre Practical manual to set up and manage a SARI treatment centre and a SARI screening facility in health care facilities. March 2020. Available from Accessed on 14/05/2020
  14. CDC. Supplement I: Infection Control in Healthcare, Home, and Community Settings Public Health Guidance for Community-Level Preparedness and Response to Severe Acute Respiratory Syndrome (SARS) Version 2/3. Available from . Accessed on 14/05/2020  
  15. Hyuna J , Leea S , Hwanga  J. Application of corona discharge-generated air ions for filtration of aerosolized virus and inactivation of filtered virus Received 9 August 2016; doi:10.1111/j.1600-0668.2007.00512.x
  16. Wu Y et al. MS2 virus inactivation by atmospheric-pressure cold plasma using different gas carriers and power levels(Article)(Open Access) 2015, American Society for Microbiology.
  17. Jiang S, Ma A and Ramachandran S. Negative Air Ions and Their Effects on Human Health and Air Quality Improvement. Int. J. Mol. Sci. 2018, 19, 2966; doi:10.3390/ijms19102966.
  18. Indoor Air Quality. United States Environmental Protection Agency. Available at
  19. ASHRAE Position Document on Filtration and Air Cleaning; Approved by ASHRAE Board of Directors January 29, 2015 , Reaffirmed by Technology Council January 13, 2018,  Expires January 23, 2021. Available at
  20. Hay S, Obee T , Luo Z,  Jiang T, Meng Y , He J , Murphy  S and Suib S.The Viability of Photocatalysis for Air Purification. Molecules 2015, 20, 1319-1356; doi:10.3390/molecules20011319. Available at
  21. Kim J and Jang J. Inactivation of airborne viruses using vacuum ultraviolet photocatalysis for a flow-through indoor air purifier with short irradiation time. Journal Aerosol Science and Technology. Volume 52, 2018 – Issue 5.
  22. Zhong L, Haghigha F. Photocatalytic air cleaners and materials technologies – Abilities and limitations. Building and Environment ; Volume 91, September 2015, Pages 191-203

Author: SLCM