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How to avoid catching COVID-19 indoors even if you’re around people who are sick

  • German researchers analyzed the risk of coronavirus transmission via respiratory droplets and aerosols and came up with simple means to help people avoid COVID-19 indoors.
  • The use of professional N95 masks combined with social distancing, hand hygiene, and proper and frequent ventilation of indoor spaces and public transportation means can reduce the risk of COVID-19 spread.
  • The use of CO2 monitors, the installation of air purification systems, and the creation of new ventilation systems that pull the air upwards can increase the safety of public places like schools and other indoor settings that can attract crowds.

The novel coronavirus pandemic is at its worst right now, just as vaccines start rolling out in Western countries. The US and Europe have been registering a winter wave of infectious that exceeded the previous ones by far. The US broke all possible records since early November, most of which were previously set by America. The number of daily cases surpassed 220,000 cases a few days ago, with the number of concurrent COVID-19 hospitalizations having crossed 100,000. The number of daily deaths reached a peak of 3,000 this week, another worldwide record.

Several factors are making the virus even more dangerous than in sprint and summer. The colder weather has pushed more people indoors, and that’s where the virus spreads the most, as people keep congregating with others without respecting the health measures they’d adhere to in public places. The holiday season, including Thanksgiving and Christmas, can further fuel the COVID-19 surge, with health experts warning that the worst is yet to come. But hope isn’t entirely lost, and people can take additional precautions indoors that can reduce the risk of infection, including at home. German researchers have come up with new guidelines to mitigate air circulation indoors and reduce the risk of infection.


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The group called the German Working Committee on Particulate Matter released its recommendations online, available at this link, with Fast Company summarizing the proposed measures below:

  • N95 masks. “They should be mandatory in many sectors, instead of the simple hygiene masks,” write the authors.
  • Window ventilation. The more, the better. It’s effective and cheap.
  • Exhaust ventilation systems. The air must be extracted upwards through an overhead exhaust suction, and appropriately filtered if recirculated. The group advises that these ventilation systems be installed immediately in places like classrooms, restaurants, hospitals, buses, and trains.
  • Air purification systems. The use of appropriate purifiers for the size and space are critical. Portable is fine.
  • CO2 monitoring. The results are proxy for how well ventilation is working. If CO2 levels are extremely low in buildings like museums, for example, reopening could be reasonable.
  • No fans. Ceiling fans recirculate air, likely keeping virus particles in the air for longer.

The group focuses on the importance of proper airflow, and that’s not surprising. Germany has already issued guidelines a few months ago, advising people to ventilate their homes and other indoor spaces to reduce the risk of infection. Dr. Anthony Fauci echoed those sentiments a few weeks ago, telling people to wear masks at home if they’re in the presence of people who might be infected and frequently air their homes.

The paper is definitely worth a read, as the German scientists explain why the risk of infection indoors is so significant and why ventilation matters. The scientists offer the following schematic representation of the size of aerosol and droplets that are expelled from the nose and mouth when breathing (A), speaking, singing and shouting (B), and sneezing and coughing (C).

From the paper: “From left to right: (A) Basal respiration, (B): Speaking, singing, and shouting (mouth), (C): Even larger droplets are emitted of the nose and the mouth when sneezing. Suspended viruses are embedded in saliva or dried up lung liquid, cold and damp climate and darkness extend their activity. The particles A can hover longer than one day in unventilated rooms, the particles B several hours. The largest particles (C and mostly bigger) of the sneezing sink to the ground within few seconds. In contrast to cloth face masks N95 and FFP2 masks protect as well against the particles A.”

The A particles are the smallest ones, and they can remain suspended in the air and float with the current, while C droplets are likely to fall on the ground and on surfaces within seconds.

The more people are in a room, the more A, B, and C particles can accumulate, and the risk of infection grows, especially if the place isn’t ventilated. Crowded trains, buses, and airplanes pose similar risks.

Ventilation, especially cross ventilation where doors and windows can be opened simultaneously, can dissipate the particles fasters. Sensors showing low CO2 readings would indicate that the ventilation is successful. If CO2 is removed efficiently, then so are the aerosols.

The researchers make a point of noting that proper ventilation must be used in indoor spaces relying on air conditioning. The air has to move upwards from the ground because that’s the natural movement of aerosols and droplets. When we exhale those particles, they tend to approach a room’s ceiling as warm air climbs. But if the AC blows the air downwards, they would return. Fans could have a similar effect.

The researchers also suggest that recovered infected persons should go through “a medical check-up of the individual particle and virus emission during basal respiration and when speaking.” That’s because some people might be super emitters of particles, and therefore super spreaders. If that’s the case, they could act “extra-responsibly in the future, such as wearing respirator masks as much as possible.”

Aside from having emissions check-ups and installing expensive ventilation systems in public places like schools or public transportation systems, all the German scientists’ measures are simple and affordable. Combining the measures above, including face mask use at home, social distancing, hand hygiene, and proper ventilation, families looking to celebrate Christmas can significantly reduce the risk of infection indoors. However, there’s no way to block the spread of the virus at this time, and infection is still possible even if all recommendations are respected.

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