Aerosols out: airy against corona

Virus-poor indoor air is a key to keeping the risk of infection with the novel coronavirus low. But how can such clean indoor air be ensured?

A droplet ejected by a person sneezing or coughing races several meters through the air - and falls to the ground within a few seconds. If the novel coronavirus were only transmitted by droplet infection, it would be far less common. But with SARS-CoV-2, so-called aerosols (made up of the ancient Greek word for "air" and the Latin word for "solution") also play an important role. And that's a problem.

Aerosols can float for hours

Aerosols are exhaled when speaking normally. They are significantly smaller than droplets: while the latter are up to half a millimeter in diameter, the diameter of aerosols is no more than two hundredths of a millimeter. This means that aerosols - also known as droplet nuclei - remain in the air significantly longer. It can take several hours for them to sink to the ground or another surface.

It's not a problem outside in the fresh air. "Aerosols with viruses have enough space here and, thanks to air currents, enough movement to disperse in no time. The concentration then drops very quickly to a harmless level," explains Professor Andreas Wille from the Institute for Hygiene and Environment in Hamburg. Because aerosols are diluted so quickly by the air in the open air, the risk of infection is significantly lower and the risk is, so to speak, blown away.

The risk of enclosed spaces

It looks different in closed rooms: If the room air is not constantly renewed or filtered, the aerosols will also stay in the air for a long time. For example, if a person with Covid-19 goes to a restaurant, then theoretically - depending on the size of the room and the ventilation - they can infect all guests there, even those who are sitting at the other end of the room or even only come when they are already back has gone.

SARS-CoV-2 was only able to spread so quickly through transmission via aerosols. A clean indoor air with correspondingly few aerosols is therefore one of the keys to curbing the spread. But how is it possible to keep the concentration of these droplet nuclei in the room air low or to reduce it?

Effective air conditioning systems through special filters

Today air conditioning systems usually also have a filter."Unfortunately, the aerosols are very small, so that many standard filters are of no use," says Privatdozent Dr. Frank-Albert Pitten, specialist in hygiene and environmental medicine and managing director of the Institute for Hospital Hygiene and Infection Control. Only very high quality filters, also known as HEPA filters, reliably remove the aerosols from the air. "They are required by law for air conditioning in operating rooms in hospitals and on airplanes," says Pitten. This can be a bit comforting for air travelers, but it does not mean a complete all-clear: Because people constantly exhale aerosols with their air, the particles always circulate in the room air - albeit in low concentrations.

Apart from airplanes and hospitals, however, one can assume that only a few systems are equipped with the high-quality filters, says Pitten. "They are not only expensive to buy. Because the air has to be pushed through with a lot of force, air-conditioning systems with these filters also consume noticeably more energy," says the hygienist and environmental doctor. Systems with an inadequate filter are also no cause for concern: they may not reduce the risk of infection, but they also do not increase the risk.

UV-C radiation for disinfection at a short distance

UV disinfection is another way of rendering virus-laden aerosols in the room air harmless: the short-wave UV-C radiation destroys the genetic material of the viruses in a short time, often within seconds. The method has been used for several decades, primarily in drinking water purification and in industry. But it has two catches: firstly, the objects to be disinfected - or the air - must ideally be a few centimeters away from the UV light source so that it can develop its disinfecting effect. And secondly, the radiation is also harmful to humans, so it must be used at an appropriate distance, for example, or at times when there are no people in the vicinity.

Of course, these restrictions only allow restricted use in "normal" closed spaces such as shops or restaurants. In Hamburg's Europa-Passage, a large shopping center, the escalator handrails are irradiated at one point with a so-called UV-C disinfection module. And in China there are huge UV sluices through which buses are sluiced for disinfection at night. In some places the concept of open UV disinfection certainly makes sense, but it is certainly not the large, wide-area solution, "says Andreas Wille.

So-called closed UV disinfection systems are somewhat more promising. Here the air is sucked into a closed system, which can be thought of as a kind of box, and there it is disinfected with UV radiation. They are basically similar to an air conditioning system with a HEPA filter. However, only a few such closed UV disinfection systems are still in use.

Nebulized disinfectants

The distribution of disinfectants through nebulizers is a third way of killing viruses. For example, hydrogen peroxide or ozone are possible. "All of these substances can be dangerous for people, which is one of the reasons why they are hardly an option as a widely used disinfection method," says Frank-Albert Pitten.