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One of the most important measures to reduce the viral load in indoor areas and to reduce or eliminate infectiousness is the regular ventilation of indoors (if there are several people in the room, ventilate at least every 30 minutes. The closer the frequency the better). These measures are paramount where aerosols (tiny droplet particles) can stay suspended in the air for up to a few hours, and even a mouth and nose protection does not protect. The regular mouth and nose coverings do not prevent aerosol transmissions; this requires the virus filter masks (FFP2 / KN95 masks).

A good flow of air through the room is important so that the room air is exchanged quickly. With no wind and small temperature differences between the indoor and outdoor areas, the ventilation can be insufficient and should be performed by ventilators (fans).

In rooms where windows cannot be opened or cannot be opened adequately, ventilation or air exchange must take place differently. Air filters such as those used in aircraft are recommended. Where are a higher risk of infection and greater spread, it would be even better to use filters like in hospitals (e.g. surgery room) and laboratories.

It will also be difficult to carry out the ventilation measures in winter so that additional air filters are also recommended in rooms with otherwise adequate windows.

Filter Systems

There are EPA (Efficiency Particulate Air filter) HEPA (High Efficiency Particulate Air filter) and ULPA (Ultra Low Penetration Air filter) filter systems (the European filter standard is based on EN 1822-1: 2009).

The EPA filter class is less efficient. The HEPA, and even better the ULPA filter classes are necessary in the case of viruses; they are used in hospitals, research laboratories - HEPA also in airplanes. In addition to the filter class (correspondingly increasing percentage of filtering out - e.g. HEPA 13 to 14 or ULPA 15-17), the exchange capacity (the filter volume per unit of time) should be adapted to the size of the room.

Some filters can decontaminate the separation of airborne germs, viruses, and bacteria, as well as spores or microfibers themselves via a thermal regeneration system (heating device).

Recommendation

Since it will take even longer before herd immunity can be achieved (through automatic spread or potential future vaccination), such filter systems are recommended now also in rooms such as offices, restaurants, school classes, daycare centers, trains, buses, and all facilities where several people automatically come closer together.

Annotation

The reason why some people with close contact to COVID19 infected people (positive people) were not infected (e.g. within a family) can be explained by the fact that, in addition to a good primary immune defense of the not yet infected, a certain viral load (higher virus numbers and high virus excretion) of the infectious person was not current anymore - so the person was COVID positive but no longer really (significant) infectious.

In cases where this also occurred without a filter, the time and probably the partial immunity of the people spreading the virus played a role (we do not yet sufficiently know the factors that make people less infectious).

An adequate filter (room or mask) can accelerate such a condition since the aerosols containing the virus are then intercepted, and the viral load is no longer infectious.

In cases where someone e.g. has nevertheless been infected in an aircraft, the infectious person must have been in the immediate vicinity (next to the seat) and have transmitted the virus to the neighbor over a longer time window – and both wore none virus filter mask (FFP2 / KN95 mask).