The Effect of Mobile Indoor Air Cleaners on the Risk of Infection with SARS-CoV-2 in Surgical Examination and Treatment Rooms with Limited Ventilation Options

Research Article

Austin J Public Health Epidemiol. 2021; 8(1): 1094.

The Effect of Mobile Indoor Air Cleaners on the Risk of Infection with SARS-CoV-2 in Surgical Examination and Treatment Rooms with Limited Ventilation Options

Oberst M¹*, Klar T² and Heinrich A²

1Clinic for Orthopedics, Trauma and Spinal Surgery, Germany

2Aalen University, Center for Optical Technologies, Germany

*Corresponding author: Michael Oberst, Clinic for Orthopedics, Trauma and Spinal Surgery, Ostalb Klinikum Aalen, Im Kälblesrain 1, 73430 Aalen, Germany

Received: March 10, 2021; Accepted: April 20, 2021; Published: April 27, 2021


Objective: Due to the airborne transmission of the Coronavirus Disease (Covid-19) via aerosols, we investigated the effect of a mobile air filter system in a surgical examination room.

Methods: A mobile indoor air cleaner (AP 90, DEMA-airtech, Germany) was run during regular surgical consulting hour in our outpatient’s clinic. Aerosol concentration was measured by Fidas Frog fine dust monitoring system (Palas, Germany) by constantly recording PM1.0, PM2.5, PM4, PM10 and the total particle load PMtot.

Results: The use of the air filter system led to a significant reduction of aerosols in the room despite the fact that there were various numbers of persons in the room constantly.

Conclusion: The use of a high efficiency air filtration device, especially in examination rooms with poor ventilation, e.g., lack of windows or local exhaust is recommendable.

Keywords: Covid-19; Aerosol; PMtot; Ventilation


Even though the ways in which SARS-CoV-2 is transmitted are not completely understood, droplet infection from airborne particles contaminated with the virus obviously play a decisive role in humanto- human transmission [1-3]. Different forms of emission (speaking, breathing, singing, coughing, sneezing, etc.) produce a wide range of particle sizes and transmit them to the surrounding air. Breathing and speaking produce particle cells of between 0.75μm and 1.1μm, while coughing and sneezing produce much larger particles (larger than 5μm) [4]. Particle distribution within a space also varies by particle size. Droplets (>50μm) fall to the floor relatively quickly, but aerosols (<5μm) can be detected in the air after several hours, and convection and other air movements can transport them several meters [3,5-7]. If they are inhaled, these particles can (depending on their size) penetrate deep into the respiratory tract, even reaching the alveoli [2,5,8].

These particulate matter emissions are defined by the PM10 unit based on the National Ambient Air Quality Standards introduced by the U.S’s Environmental Protection Agency in 1987. The 10 here does not stand for a sharp distinction at 10μm of aerodynamic diameter, but reflects an attempt to recreate the separation behavior in the upper respiratory tract: All particles with an aerodynamic diameter of less than 1μm are considered, while a certain percentage of larger particles are included. That percentage falls as particle size increases until 0% is reached at 15μm. This is ultimately, where the PM10 designation comes from: 10μm is the exact halfway point in the size of particles considered. In 1997, PM2.5 was added to the guidelines. It refers to respirable (alveolar) particulate matter. The definition is analogous to PM10, but the weighting function is significantly steeper (100% weighting <0.5μm; 0% weighting >3.5μm; 50% weighting at about 2.5μm) [9]. The regulations were further expanded to include PM1, which is analogous to PM2.5, but for 1μm.

Given the transmission methods mentioned above, precautionary measures have been taken (or mandated) throughout Germany with the goal of reducing the risk of transmitting SARS-CoV-2 by reducing aerosol and droplet exposure. However, at this point German officials (Federal Environment Agency) doubt the effect of indoor air cleaners [10]. The German Society of “Hospital Hygiene sees” a high need for further studies” on the topic [11].

Therefore, by now it is especially important to increase air circulation in closed rooms and buildings by airing them regularly (Germany’s “AHA+L” rule) [12]. This measure for potentially reducing viral load tied to airborne particles can scarcely be implemented in rooms with no windows (or at least not without major structural and technical interventions in ventilation systems).

In a previous trial, we had already noticed the positive effect of an indoor air cleaner on the concentration of airborne particles/aerosols in the ambient air of a consultation room without ventilation options [13], so we set up a repeat experiment under the same conditions but with improved, high-quality measurement technology and a mobile air cleaner with higher filtration capacity (see Material and Methods). The aim of the presented study was to find out which level of air replacement a device would need to achieve a relevant aerosol reduction and how the decline in air particles (half-life) is related to the device power level and the “personnel load” (public traffic during consultation hours).

Materials and Methods

In Examination Room 2.148 (floor space of 21m², room volume of 52m³) in our Clinic for Orthopedics, Trauma and Spinal Surgery (Figure 1), an ambient air filter device (DEMA-airtech, Stuttgart, Germany, Type AP-90) was deployed during routine surgical outpatients- clinic hours on 11/30/2020. According to manufacturer information, the device’s maximum filter capacity is 720m³/h. In addition to an activated carbon filter, a Class H13 HEPA (highefficiency particulate air) filter was installed (European Standard 1822, minimum filter efficiency 0.3μg/m³/h, efficiency 99.95%). The filtered air is also treated with plasma and UV light, which the manufacturer says will kill 99% of viruses and bacteria (Guangdong Detection Centre of Microbiology, Report No. 2020SP8365R03E) after the filter, has eliminated airborne particles.