Miele AirControl: Reducing the viral load in enclosed spaces with air purifiers

Filtration capacity

Air purifiers filter room air and, in doing so, retain particles which carry viruses. The filtered air should on no account be enriched with noxious chemicals. Against this backdrop, Miele Professional has opted for a purely physical filtration process involving multiple filter layers to guarantee high-level efficiency.

1. Coarse air inlet filter

The coarse filter serves to hold back and filter out coarse particles >10 μm – such as pollen, lint and small paper clippings – as air enters the unit. This protects the filter cartridge inside the unit and extends replacement intervals, thereby reducing the need for maintenance.

2. Fine filter

The fine filter retains particles which are 1 to 10 μm in size. These include smoke, germs and bacteria. Together with the HEPA H14 filter⁽³⁾, this filter forms a single unit housed in a cartridge and is fitted in an air-tight manner to prevent bleed air bypassing the unit. In other words, the entire air stream passes through the filter cartridge without any bleed air. A so-called filter leakage test can be performed according to the DIN EN ISO 14644-3 standard.

3. HEPA H14 high-performance particulate filter

The HEPA H14 filter is tested and certified according to the EN 1822-1 standard. It holds back 99.995% of particles with dimensions of 0.1 - 0.3 μm, including viruses and bacteria. Certified HEPA H14 filters are specifically designed to remove air-borne particulates in this order of magnitude, hence in principle ideal for the purpose.⁽³⁾ Alongside these filters, HEPA H13 filters are also considered effective in current scientific publications. These filters have a retention rate of 99.95%.

4. Active charcoal filter

The active charcoal filter is located downstream from the HEPA filter and additionally removes odours from the air.

5. Exhaust filter

The exhaust filter optimises air discharge, ensures the uniform distribution of air and standardises air-flow velocities. In addition to this, the interior of the unit is protected against soiling and coarser particles which might enter the unit from above.

Air volume flow rate

The filtration capacity of an air purifier isn't everything: the volume of filtered air is also a key criterion. This is a measure of how many times the air within an enclosed space is displaced per hour, expressed in terms of m3/h. The current thinking among experts is that a six-fold air displacement rate per hour is technically achievable to ensure a sufficient margin of safety⁽²⁾. In a typical classroom or meeting room with a cubic volume of, for example, 231 m3 (5 m x 14 m x 3.30 m), this would mean displacing approx. 1,400 m3 of air. Ideally, the air throughput rate of an air purifier should be scalable to suit a variety of room sizes.

Air distribution

A further measure of the efficiency of an air purifier is its ability to distribute air. Air intake and discharge points should be at a distance to each other. This ensures that the filtered and unfiltered air flows do not blend and allows filtered, laminar air discharge directed deep into the room in a manner which prevents any mixing with the incoming air stream. Under optimum conditions, a unit should take in air at floor level and return it to the room at the top.

Sound emissions

Nothing is more disturbing than loud background noises. As it is to be presumed that air purifiers will be operated while people are in the room, sound emissions should not be excessively high and should be perceived by those in the vicinity as being quiet, pleasant and of a normal level.

Deactivation of contaminants inside the filter

The highly effective HEPA H14 filter retains the minutest of particles, such as viruses. In order to prevent exposure of service personnel and the unit's environs when routinely replacing the HEPA filter, the deactivation of viruses has an important role to play. This process takes place inside the unit and penetrates deep into the filter, i.e. deactivates not only the viruses which adsorb onto the outer surfaces of the filter. Furthermore, it was important that the process should not involve noxious substances such as ozone. The technology seen to best serve this purpose is thermal deactivation. Aerosol-borne SARS-CoV-2 viruses are not particularly temperature-resistant and hence they can be deactivated at temperatures of 60°C to 80°C. ^{(4, 5)}

Draught-free air distribution

When using an air purifier, it is not simply a matter of distributing air uniformly throughout the entire room, it must also be done in an unobtrusive manner without any inconvenience to those in the room. Draughts should be avoided at all costs as they can result in stiff limbs and dry mucous membranes.

Automatic power control with CO2 sensor

Monitoring CO2 concentrations in the air represents a reliable indicator as to the efficiency of room ventilation and hence indirectly of the risk of infection. For a good room micro-climate, the mean concentration should be kept below 1,000 ppm (parts per million) (3). Air purifiers with an integrated CO2 sensor may not be able to reduce carbon dioxide concentrations directly but they can indicate elevated levels, sensitising those present and prompting to action. In the best of cases, the air purifier enhances and accelerates the distribution of fresh air to remedy the situation. This results in shorter ventilation times and an increased sense of wellbeing among persons in the room.

Dimensions and operation

Machines must be tilt-protected whilst at the same time being mobile – for example installed on wheels. As the pre-filter which removes the coarsest soil requires more frequent changing, replacement should be simple, accessible and avoid any contact with live components.