Air filters in closed rooms can help in the fight against the coronavirus. Nevertheless, distance and mask must not be dispensed with. The performance of small systems is often not sufficient.
Regular ventilation is one of the effective measures against the spread of the coronavirus. However, the further outside temperatures drop, the less willing people are to let cold air into rooms on a regular basis. An alternative can be devices that filter the room air.
Filters help only against aerosols
However, the Robert Koch Institute (German FDA warns against foregoing other protective measures when using filter devices. For example, the distance rule of 1.50 meters must be observed or a mouth-nose covering worn. The problem: Even highly effective filters can only remove aerosols, i.e. the finest droplets, from the air. They do not reduce the risk of infection at close range, such as during a conversation. Indirect transmission through viruses on surfaces is also not reduced.
Small devices do not help much
Filter units must be matched to the size of a room, i.e. the air volume. Small, portable units often have too low a throughput to clean the air in a reasonable amount of time. For example, a room with a floor area of 20 square meters and a room height of 2.5 meters contains about 50 cubic meters (m³) of air. The capacity of the filtering devices is expressed in m³ per hour (h).
Multiple air changes per hour required
Prof. Christian Kaehler, an expert on air flow at the University of the Federal Armed Forces in Munich, advises filtering the air six times per hour. This means that a unit for the 20-square-meter room mentioned would have to have a capacity of at least 300 m³/h. A single air change does not mean that all viruses are filtered out of the air.
The filter performance refers to ideal, theoretical conditions. In practice, the decisive factor is how the air is distributed and moves in the room. There are therefore well-ventilated and poorly ventilated areas. The devices must be set up in such a way that they draw in the polluted air as completely as possible and the purified air is mixed as little as possible with polluted air.
The Federal Environment Agency points out that mobile air cleaners with integrated HEPA filters in classrooms, for example, are not sufficient “to effectively remove suspended particles (e.g. viruses) from the room air over the entire duration of the lesson. This would require precise recording of the airflow and air conduction in the room, as well as targeted placement of the mobile devices.”
Tested HEPA filters required
In addition to air performance, numerous other factors are important. The most important is the type of filter. Only so-called HEPA filters (High Efficiency Particulate Air Filter) of classes H13 and H14 can effectively stop particles of the size found in aerosols with corona vapors. The filters must comply with the European standard EN 1822-1:2009. Another criterion is the noise level of the filter devices.
Fresh air is important
The first measure against aerosols in indoor air should always be to let fresh air into the room, says Prof. Martin Kriegel of the Technical University of Berlin. He also confirms that technical devices can filter viruses from the air. However, good air quality includes other criteria such as a low CO2 concentration. Like the amount of aerosols, it increases the more people are in an enclosed space and cannot be improved with air filters.
The consumer centers warn against uncritically trusting manufacturers’ advertising claims. Filter devices could not replace airing. Devices that work with ozone are not recommended because they can additionally pollute the room air with substances that are harmful to health.
Exhaust systems exchange the air
In the discussion about coronaviruses in indoor areas, ventilation or extraction systems are also being discussed in addition to air filters. They work on a different principle and exchange the polluted air for fresh air from outside.
(c) Article was translated from ndr.de
