Improved ventilation now and for the future
The German Engineers’ Association issued its VDI 6022 guidelines 20 years ago, aiming to guarantee that the air inside buildings is non-detrimental to health. Likewise, workplace regulations exist, designed to ensure “a sufficient quantity of healthy air” in interior spaces. Industrial safety law calls for risk assessments to be carried out at every workplace, including checks on the ventilation systems. But now, with the threat of infection by the SARS-Co V-2 virus, ensuring good indoor air quality has become more important than ever.
Classic ventilation systems quickly and conveniently replace stale indoor air with fresh air from outside. They are designed to reduce ‘normal’ – that is, non-virally contaminated – aerosol concentration inside buildings. But viral loads are another matter. To deal effectively with them, the capacity of existing, regular-use systems would need to be extended to impractical levels. Assuming it takes exposure to an increased viral load for approx. 10 to 15 minutes to lead to infection in healthy people, we can roughly estimate the minimum air replacement flow needed to dilute a single viral load indoors by means of ventilation technology.
Ideally, a mixed flow system works by an exponential function, that is, with high displacement at first that gradually diminishes. Applying the equation that describes this function (e.g. in SIA 382/1: 2014-07 – ventilation and air conditioning systems) to find out the rate of indoor air replacement required to lower the initial viral load by 50 percent in 15 minutes, the result is: fresh air flow equivalent to three times the usual indoor air replacement per hour. The outdoor air flow cannot force the indoor air out like a plunger, only dilute it, which is why only a 50 percent reduction can be achieved in 15 minutes. If we factor in additional effects, e.g. a non-ideal blending of the exterior air with contaminated internal air, or the presence of an ongoing viral source despite airing, the required air exchange rates become much higher.
Basically, that means to reliably reduce the concentration of any viral sources that may be present, interior spaces should always be additionally aired by opening windows, whether there are ventilation systems in use or not. And to minimize any potential viral sources indoors, visitors to any buildings should wear mouth/nose coverings as a preventive measure.
New ideas and new solutions needed
A steadily increasing number of mobile air cleaning devices has appeared on the market in recent years. But they do not replace stale indoor air with fresh air from outside. Most of them just filter the indoor air with conventional particle filters. To eliminate micro-organisms and viruses, particle filters need the finest of meshes, which soon become blocked, causing noise levels to rise when the ventilators are working.
Increasingly, devices work with additional UV-C-light treatment or a kind of air ionization. This enables them to improve the quality of indoor air over a longer period. The effectiveness can usually only be sporadically spot-checked, so professional technical societies in Germany, Austria and Switzerland have devised hygiene requirements for ventilation systems that also encompass hygiene checks.
The complete set of rules is in VDI 6022. Page 5 concerning mobile devices lists the requirements for testing and evaluating technologies for avoiding allergen exposure. However, methods of testing and determining the rate of microorganism reduction (bacteria, fungus spores, virus particles) have not yet been standardized.
Below we will look at two different ionization procedures. Air ionization is an electro-chemical procedure for reducing the microorganisms in the incoming air as well as in the room air. As it does not involve high pressure loss filters, this method consumes less energy and is usually fairly quiet.
The older and better-known ionization procedure is based on so-called corona discharge. For this, an actuator and a collector are placed in the airflow (also known as electro-filtration). It has been reliably used in industry and kitchen air extractor technology for some time. The drawback is that it generates ozone. So, with residual ozone entering after cleaning via the incoming air, organic atmospheric pollution (e.g. smells, bacteria) can be reduced in the room but at the cost of an increased ozone level and often with unclear oxidation by-products.
An ingenious procedure has been used for some time in industrial air improvement and occasionally in recreational air conditioning. It involves the air inside a building being made electrically conductive by means of a special ionization technique (exterior air is, incidentally, electrically conductive). For this, the positive and negative air ion content indoors is raised with the help of actuators (mostly arranged in the supply air outlet). These air ions then look for opposite-poled carriers in the air (analogous to magnetic poles, positive and negative attract each other). In this way, fine dust particles are collected into coarse dust clusters, making the room air not only cleaner but also purer, since microorganisms never move through the air alone, but always attached to a particle (i.e., as aerosol).
In comparison to the previously described procedure and UV light technology, this method allows cleaning to take place in the room air, i.e. at the source of the contamination. There is no need for redirecting the air through a ventilation system for it to be cleaned. And, crucially, no ozone is generated by this technology. While it is based on raising air ion content, the ion concentration can be regulated.
New methods of reducing indoor viral loads using ventilation technology are already in the pipeline, or even in production, for regular, widespread usage. In the current circumstances, we would like to plead the case for promoting air cleaning procedures that have hitherto been specialized, niche technologies. They need to be purposefully developed and tested, and testing procedures and construction standards established in parallel. And the whole process needs to be conducted from the start in cooperation with virologists, microbiologists and engineers. Because the current pandemic has shown us the supreme importance of healthy room air.