Which places are particularly dangerous?

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Which places are particularly dangerous?


Restaurants open again, parks and baths fill up, supermarkets anyway. How to assess the risk of infection: whether on the go or in air-conditioned rooms.



© left: Gaurav Baya / unsplash right: imago images
Enclosed spaces such as elevators are more dangerous than places in the fresh air.


On some days, life in Germany seems almost normal. Many people go back to work, since last Friday beer gardens and restaurants have reopened in some federal states. Have a coffee with a friend on the balcony? Now visit grandpa or aunt? People feel safer, but the virus still infects people – even if not as many new infections per day to be added as a few weeks ago.

In order for this number to decrease further, it depends on how we behave and implement what we have learned from the past few weeks. This also includes knowing what is important in which location. Where is it particularly likely to be infected? And what is the best way to behave to avoid exactly that? A guide

How the virus behaves in the air

How the virus is transmitted has been known since the pandemic began. Namely above all about the droplets that we throw into the environment when we cough, sneeze and speak. The larger particles fall to the ground within one and a half meters. The most important rule to avoid infecting yourself and others is therefore: keep your distance. But there are also smaller, microscopic droplets – the so-called aerosols. These can continue to spin through the air and infect someone. The virologist Christian Drosten from the Berlin Charité now believes that almost half of the infections from aerosol transmission happens, almost the other half from larger droplets, and only about ten percent from smear infections, when someone touches an area where the virus sticks and then touches the nose, mouth or eyes with the same hand.

A researcher from the Massachusetts Institute of Technology came to the conclusion in a study that the smallest particles that arise when coughing can travel almost five meters – those that arise when sneezing can even move almost eight meters (Jama: Bourouiba, 2020). But aerosols are also created when speaking and even breathing. Masks interrupt the trajectory, but many let small particles through or past the sides. How they are distributed and how high their concentration is depends on many things: How high is the virus concentration of the infected person? How long does she speak? How big is the room? How much air is there?

Infected people do not always give off the same amount of virus: A study from Singapore found that 30 corona patients examined only exhaled so much virus in the first week that they had symptoms of the disease that a significant amount collected on the ground. In the second week of infection, the wipe samples were no longer positive (medrxiv: Chia et al., 2020).

Now let’s take a look at the different locations:

Don’t worry about trips to the park

Theoretically, the virus can survive in the smallest floating particles for up to three hours (NEJM: van Doremalen et al., 2020). Would it be conceivable for an infected person to walk in the park, exhale the viruses on the way, and the walker, who follows the same route a few minutes or even hours later, inhale the tiny droplets again? Theoretically, yes, but it is unlikely. So far, studies have only been carried out for closed rooms. In principle, the exhaled air quickly dilutes outside, the wind disperses the particles. If the minimum distance of 1.5 meters is maintained there, you do not have to worry.

If you want to be safe, you can keep a little more distance. In the countryside there is more space on meadows and in the forest anyway, but there are also enough green spaces in cities. If you spread out all the residents of Munich on the green spaces of the city, four people share a hundred square meters, in Berlin it would be – at least in theory – only one and a half people. In order to really have so much space, it helps to use off-peak times or to go to a remote location on weekends.

One thing helps in closed rooms: ventilation

Eat, drink, chat and have a good time together. This is now possible again, for example in the beer garden, but also in the interior of cafes and restaurants. However, caution is required especially in closed rooms: the particles that we exhale move in the rooms and follow a pattern very similar to that of smells that spread in the air. Even larger particles can remain in the air for around ten minutes, so that they could theoretically float from one end to the other end of a room, as two physicists have shown in a study (Anchordoqui / Chudnovsky, 2020, PDF). The movement depends on several factors, such as the position of the air conditioning systems, the radiators in the room that let the air rise, the windows, the objects and, last but not least, people who create small air vortices through their movements. In meeting and office rooms, but also in supermarkets and department stores, it is therefore not sufficient from a purely physical point of view to keep a minimum distance of 1.5 to two meters, the researchers write.

In January in Guangzhou, someone carrying the virus but not yet feeling sick infected nine other people in a restaurant. A study (Emerging Infectious Diseases: Jianyun Lu et al .: 2020) describes how the air conditioning system moved the particles through the five by six meter restaurant room. From the seating arrangement of the guests, the research team concluded that the transmission in the room could not have taken place solely through falling droplets, since at least two of the people later infected were sitting several meters away from the first infected person – and even against the wind direction in which the Air conditioner blew the particles. Although this is a single case, it shows that it depends on how a room ventilation system works, whether it only sets the air in motion or actually exchanges it.

It is safer to simply ventilate in the restaurant by opening windows, says also Christian Drosten. Because that ensures that the air is diluted and exchanged. Fans in the window or on the ceiling can also help. This is a good option, especially in early summer, when the room temperature can use cooling anyway.

Shopping can be managed quite well

In the supermarket, there is no avoiding meeting other people. Since the shopping tour does not usually take as long as a visit to a restaurant and many supermarkets exchange their indoor air, one does not have to have the fearful idea that the air there is full of viruses, says Drosten in his Corona podcast.

How about the handle of the shopping cart? Infection is theoretically possible if a person touches a contaminated area first and then his face. However, a study by the group around the epidemiological modeler Christophe Fraser assumes that in no more than ten percent of cases is there a smear infection behind a transmission (Science: Fraser et al., 2020). However, if you want to be sure, you should always wash your hands thoroughly when you touch potentially contaminated surfaces. Many supermarkets offer their customers to disinfect their hands before shopping. This protects the subsequent buyers, but not yourself. That is why it is important to clean your hands even after you have finished shopping.

We are crammed together in the office

Experts point out that the likelihood of infection is particularly high where you spend a long time with other people in a closed room – for example in an office or in the same household. Based on data analyzes, two Chinese studies estimate the likelihood of catching an infected person in the household at between 14 (medrxiv: Jing et al., 2020) and 17 percent (Clinical Infectious Diseases: Li et al., 2020). The risk is particularly high among spouses, but lower among children. What is striking is that the likelihood of getting infected with Sars-CoV-2 is significantly lower than for other diseases that spread via the air, such as measles, where it is 75 to 90 percent.

What does this mean for work in the office? Fewer people in the open-plan office, fixed walkways and places that are far apart – all this helps to reduce the likelihood that someone will be infected. Past infection chains show that transmission in the office is likely if certain behaviors and safety rules are not established. When we knew little about the virus Already in January one person could infect several others. The virus was passed on in meetings, small meeting rooms, but also in the canteen. The first case in Germany also shows that people who do not even know that they are sick can be infectious (Lancet: Böhner et al., 2020). The outbreak in a South Korean call center in March also shows that the likelihood of infection increases when you are in a room. A single infected employee infected 92 of the 94 people who worked on the same floor as him over a period of one week (Emerging Infectious Diseases: Park et al., 2020). One thing is certain: Open-plan offices in which many people spend a long time are places where an infection is quite likely. It can be reduced with hygiene rules and keeping a distance, but not excluded.

Viruses drive the elevator and train, and are also flight attendants

So it still means: keep your distance. If you run more, ride a bike or use your own car, you lower the risk of getting infected. But what if that doesn’t work? Is it because you have to take a taxi or rely on public transport? Bus, train and taxi are small capsules in which people meet or in which they are in quick succession. There is a possibility that the taxi driver or the other person on the train is infected.

How is it in small rooms with the germs that someone leaves behind? Richard Corsi, an engineer and computer scientist at Portland State University and a specialist in indoor air quality, did this using a Modeled elevator ride. Assume that an infected person goes up ten floors in an empty elevator for thirty seconds and speaks on their phone during that trip. At the top, the doors open for ten seconds, the person leaves the elevator, which then goes back down without opening the doors again. Below is the next person who concerns the elevator. Corsi assumes that at this point in time there is still a quarter of the virus volume in the elevator that the first person left behind. So for the moment it is still advisable to take the stairs whenever possible.

The problem with the capsule and the many people also arises when traveling by air. Every aircraft does have a filter system that constantly cleans the air in the aircraft and enriches it with new oxygen. But these filters only held back bacteria, not viruses, said the epidemiologist Timo Ulrichs told ZEIT ONLINE. The decisive factor for air travel is also the duration: the longer people have to sit in a closed room, the more likely an infection is. All an individual can do is observe the general hygiene rules and try to keep their distance – even if the latter is hardly feasible on an airplane. For this purpose, airlines would have to free certain seats and that at least does not seem to be planned at the moment.

It is difficult to judge whether it is more dangerous to sit in a taxi, take the elevator or fly to the Philippines. In general, however, the following rule of thumb applies: the smaller the room, the more people in one place, the more difficult it is to maintain the minimum distance and the poorer the air exchange in a room, the higher the risk of infection.

The same distance rules apply in the swimming pool

Weekends at the bathing lake? Sounds good at first. But what if other people have the same idea – can I catch the virus when an infected person is in the water? This is also theoretically possible, but is unlikely in practice. It is conceivable that someone carries the virus into the water or that contaminated waste water flows into lakes. So far, the World Health Organization (WHO) no evidence of thatthat people got infected by water. The fact that there is an infection in the water that Sars-CoV-2 is an enveloped virusthat cannot survive in the water for very long.

According to current knowledge, it is still unclear whether people can also become infected with Sars-CoV-2 via the wastewater (International Journal of Infectious Diseases: Amirian 2020). However, the wastewater in Germany goes through a complex cleaning process before it is discharged into nature. And even if viruses get into the bathing water and survive there for some time, the viral load in the water dilutes so quickly that an infection in the water is unlikely.

Swimming pools and outdoor pools are still closed almost everywhere – but that could change if the federal states further relaxed their corona measures in May and June. The advantage of swimming pools and outdoor pools compared to open water is that the water is filtered regularly and disinfected with chlorine – an effective measure to inactivate viruses. In bathing establishments there is a risk of infection if too many people crowd in a small space, whether in the water or on the bathing lawn. That is why the same rule applies everywhere at the lake, in the open-air swimming pool or swimming pool: Keep your distance, at least 1.5 meters.

Hospitals are corona hotspots

Hospitals are hotspots for coronavirus transmissions, the Robert Koch Institute points this out in its management reports. It is understandable that people in hospitals contract Sars-CoV-2 – after all, all those who have a serious condition come to the hospital, and this includes a severe course of Covid-19. Individuals who tested positive for the corona virus or who have symptoms typical of corona are treated in separate wards. But Hajo Grundmann, head of the Institute for Infection Prevention and Hospital Hygiene at the University Medical Center in Freiburg, says that especially in the early days of the corona pandemic, it was more common for patients to be infected with the corona virus but not to be recognized as such. They were then in normal wards and came into contact with many clinic staff, but also with non-infected patients.

In the meantime, however, the number of new infections in Germany has decreased to such an extent that it is very unlikely to run into an infected person in the hospital, says Grundmann. So if you have to go to the hospital, you shouldn’t put it off longer than necessary. In order to avoid possible infections, the hospital hygienist recommends a number of measures: The hospital staff should wear masks with FFP2 protection level when treating Covid-19 patients directly, and mouth-nose protection for all other patients. Patients themselves are given mouth and nose protection and should, if possible, keep the minimum distance from other patients, for example by placing the beds sufficiently far apart. Grundmann also points out that it is very important to regularly ventilate the rooms. Because only a few rooms are equipped with a ventilation system that can filter pathogens from the air. Therefore, conventional ventilation methods would have to be used, but they would also have proven themselves in tuberculosis patients. Patients should not be afraid of hospital meals: at the University Hospital Freiburg, the food is hygienically arranged and distributed by staff with disinfected hands.

Some sports are more Covid 19 compatible than others

Football training with the entire team, yoga in the park or playing tennis with your colleague – is it still possible where physical contact and closeness are actually taboo? The federal states have now allowed sporting activities again – at least if the sport takes place outdoors and a whole range of infection protection rules can be observed. Specifically, this means: Body contact is prohibited, athletes must keep a minimum distance of 1.5 meters, changing rooms and shower cubicles remain closed, exercise equipment should be disinfected after use.

This shows that some sports are more Covid 19 compatible than others. In tennis, canoeing or athletics, the chances of getting too close to a teammate are lower than in rugby or football. But soccer players can also go back to the field in some places – as long as they train in small groups. The players could therefore do a circuit training with different stations instead of standing on the field as a collective team.

The federal states, on the other hand, disagree about sports in closed rooms: during indoor sports, for example remains prohibited in Saxony-Anhalt until May 25th, Saarland has already allowed the operation of dance studios. It is questionable whether this makes sense from an epidemiological point of view: A research team from South Korea found that eight fitness trainers infected a total of 217 people who had taken a course with the infected coaches in various fitness studios (Emerging Infectious Diseases: Jang et al. 2020). The authors suspect that small classrooms, many participants and intensive workouts could have led to the athletes becoming infected.

The role of schools and daycare centers remains unclear

So far, the federal states have only very carefully opened schools and daycare centers: in some places, only individual grades were allowed to go back to school, in other places Students are taught in small groups, Tables moved two meters apart, breaks staggered in time or, for example in Berlin schools, Entrance controls for toilets carried out, so that there are not too many students in the toilet at once.

But only recently four medical associations in one letter to municipalities and the federal states and demanded that day-care and school operations be resumed as normal – in day-care centers, kindergartens and primary schools, even without major restrictions, such as the obligation to wear a face mask or lessons in small groups. In the appeal, the doctors point out that children “do not play an outstanding role in the dynamics of expansion” of Sars-CoV-2. They also refer to research results that are intended to show that children in infection chains are rarely the starting point (read an analysis of these results here).

However, research on this topic is very ambiguous. Christian Drosten recently published a research report in which he and his colleagues analyzed the viral load in Covid 19 sufferers from different age groups (Jones et al., 2020, not yet peer-reviewed). Drosten comes to the conclusion that children carry as much viral load as adults and could therefore be similarly contagious. Epidemiological data on school outbreaks also show that the virus can spread quickly within a school and then spread to the families of the students (MedRxiv: Fontanet et al., 2020, not yet peer-reviewed). The role that children and young people really play in the spread of the pandemic cannot therefore be conclusively stated.

Not every major event is equally dangerous

We will have to do without festivals and village festivals for a while, because the federal and state governments have decided that major events will be held nationwide remain prohibited until August 31, 2020. Depending on the federal state, smaller celebrations such as weddings or funerals may take place – but only under certain conditions, which in turn vary from country to country. Demonstrations are also allowed under certain conditions.

If you want to take part in an event with several people, can answer a few questions in advance and become aware of the risk of infection. Because not every event is equally risky, and the risk of infection is not the same everywhere: Does the event take place outdoors or in a closed room? How many people will there be? How close will the contact between those present be? Can it be estimated whether a particularly large number of old people or people with previous illnesses will take part?

An event at which people dance and have close physical contact harbors a greater risk of infection than events where the minimum distance can be maintained. And the grandmother’s funeral, at which many acquaintances from the senior meeting will be present, could have worse consequences than a family reunion among the younger generations.

With all these considerations in mind, however, one should keep in mind: Events with many participants have made a significant contribution to the virus being able to spread so quickly – be it the Carnival in Gangelt or the après ski party in Ischgl, Austria. Anyone who can avoid crowds of people should continue to do so.

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