REMARK: This Q&A was part of the AIVC special COVID-19 newsletter published in November 2020. To subscribe to the newsletter please click here.

There are various measures to reduce the risk of exposure to the virus that causes COVID-19 from spreading indoors and generally fall into three main categories: source control, ventilation control, and removal control. Source control means reducing the number of people in the enclosed space (1). This enables social distancing, which reduces the number of infected people (if there is an infected person in the enclosed space) and provides more outdoor air per person. Increased ventilation dilutes the infectious aerosols produced by infectious human pathogens. However, the capacity to increase ventilation rates may be somewhat limited by the original design specifications and implementation (2). 

The third measure is removal by air cleaning, which can reduce the risk of exposure to the virus. There are several portable air cleaners on the market; for instance, portable HEPA (high efficiency particulate air) filters and germicidal ultraviolet light (GUV). HEPA filters have the ability to remove 99.97% of particulate matter, smog and microorganisms that have a size at 0.3 μm. The filtration efficiency increases for particle diameters both less than and greater than 0.3 μm. For instance, a HEPA H13 filter is capable of removing up to 99.95% of Maximum Penetration Size Particles (MPPS). According to EN-1822, the filters must be tested with the particle of maximum penetration size (MPPS - Most Penetrating Particle Size). The MPPS for each filter ranges from 0.12 µm to 0.25 μm (3). The size of the virus that causes COVID-19 is estimated to be between 0.12 µm and 0.16 μm and the minimum size of a respiratory particle that can contain SARS-CoV-2 is calculated to be approximately 4.7 μm. In addition, the minimum size of the particles can decrease due to the evaporation of water on the particle surfaces (4). Therefore, portable air cleaners equipped with HEPA filters can reduce the aerosol transmission risk for COVID-19. It should be recognized that such devices must have a clean air delivery rate (CADR) that is sufficient for the characteristics of the desired room.

GUV (also known as UVGI) uses ultraviolet light in the UV-C wavelength range (200 nm to 280 nm) to inactivate microorganisms. Most systems use low-pressure mercury lamps that produce a peak emission of approximately 254 nm. The virus that causes COVID-19 is susceptible to GUV, so if it is irradiated for a sufficiently long period of time, it becomes inactivated. There are three air disinfection applications on the market. One application is upper-room germicidal systems and the other application is UVGI cleaners used in HVAC systems and portable air cleaners. The upper-room systems can reduce the amount of active virus in the air by an equivalent of 10 air changes per hour or more of outdoor air at a much lower energy cost (5). The other application is UVGI cleaners in HVAC systems that are designed to destroy/inactivate viruses in the flowing air stream as they pass through the device. Portable air cleaners can incorporate UV-C lamps in their design in order to destroy and remove viruses trapped on air filter medium surfaces. According to SAGE_EMG, there is good evidence that GUV, using UV-C light, is likely to be a viable decontamination approach against COVID-19 for unoccupied rooms (6). There are several portable devices on the market that show good single-pass efficiency. However, their effectiveness in a room is dependent on their flow rate relative to the room size; many devices have an insufficient airflow rate to be very effective in practice. Finally, it is important to mention that air cleaners cannot fully replace a ventilation system. Please visit the IEA EBC Annex 78 “Supplementing Ventilation with Gas-phase Air Cleaning, Implementation and Energy Implications website to know more.


Alireza Afshari, Aalborg University


  1. Melikov A. K., Ai Z.T , Markov D.G., Intermittent occupancy combined with ventilation: An efficient strategy for the reduction of airborne transmission indoors, 2020, Science of the Total Environment 744 (2020) 140908.
  2. Morawska L. et al., How can airborne transmission of COVID-19 indoors be minimised?, 2020, Environment International 142 (2020) 1058322.
  3. EN 1822: High efficiency particulate air filters (HEPA and ULPA), parts 1-5, Beuth Verlag GmbH, Berlin 1998/2001.
  4. Byung Uk Lee, Minimum Sizes of Respiratory Particles Carrying SARS-CoV-2 and the Possibility of Aerosol Generation, Int. J. Environ. Res. Public Health 2020, 17, 6960; doi:10.3390/ijerph17196960.
  5. Riley R. L., Knight M., Middlebrook G.,  Ultraviolet susceptibility of BCG and virulent tubercle bacilli, Am Rev Respir Dis . 1976 Apr;113(4):413-8. doi: 10.1164/arrd.1976.113.4.413.
  6. SAGE – Environmental and Modelling Group, Application of UV disinfection, visible light, local air filtration and fumigation technologies to microbial control, 2020.
  7. AIVC. AIVC Newsletter Special Issue on COVID-19. November 2020