Current evidence shows that SARS-CoV-2, the virus that causes Covid-19, can be transmitted through droplets of different sizes. This happens when a person is in close contact (within 1m) with someone who has respiratory symptoms, such as sneezing or coughing. While the World Health Organization does not address that Covid-19 can be transmitted airborne, more than 230 scientists have already written to the agency, urging the health officials to update their guidelines. They said that the virus can be spread through microscopic respiratory droplets not only within 6 feet but up to several meters in an enclosed indoor space.
Is Covid-19 airborne?
Now, a team of researchers from the Leibniz Institute for Tropospheric Research in Germany has added to the already growing evidence that the coronavirus can be spread as aerosols – suspension of fine solid particles. The team pointed out that coronavirus SARS-CoV-2 spreads more indoors at low humidity. So, they recommend at least 40 to 60% relative humidity to reduce the spread of SARS-CoV-2 and their absorption through the nasal mucous membrane. If it’s below 40% humidity, the particles emitted by infected people remain lighter, fly further through the room, absorb less water, and are more likely to be inhaled by healthy people.
Ajit Ahlawat from TROPOS and colleagues wrote that the transmission routes of Covid-19 are still debated but recent evidence strongly suggests that it can be transmitted via air in a poorly ventilated place. Previous studies pointed out to the presence of many people in one room, such as in slaughterhouses or choir rehearsals. The safety distance of 1.5m is not sufficient when infected and healthy individuals are in one room for a long time.
Indoor humidity and aerosol research
For instance, Dutch researchers highlight that a small drop of 5 micrometers in diameter of virus produced when speaking can float in the air for up to 9 minutes. As for Ahlawat and the team’s study, they focused on indoor humidity. This area of research relating to Covid-19 has received little attention so far but the Indo-German research team believes that it could be particularly important in the next flu season.
Physicists at TROPOS, in collaboration with the CSIR National Physical Laboratory in New Delhi, have been studying the aerosol particles for years to better estimate their effects on cloud formation or air quality. Dr. Ahlawat said via Science Daily that when it comes to aerosol research, it has been established that air humidity plays a major role. “The more humid the air is, the more water adheres to the particles and so they can grow faster.,” the author said.
So, the team evaluated 10 most relevant studies from 2007 to 2020 by other researchers who investigated the effect of humidity on the spread, survival, and infection with the pathogens of influenza as well as the coronaviruses MERS, SARS-CoV-1, and SARS-CoV-2.
Three different ways air humidity influences the spread of the virus
They said there are three different ways the air humidity influences the spread of the virus. The first is that through the behavior of microorganisms within the virus droplets, second is the inactivation or survival of the virus on the surfaces, and third is the role of dry indoor air in the airborne transmission of viruses.
The Indo-German researchers said that indeed, droplets containing viruses dry out more quickly in low humidity. However, the survivability of the viruses remains high. Thus, they recommend a 40 to 60% relative humidity in indoor air. The team likewise found that dry air makes the mucous membranes in people’s noses dry and more permeable to viruses.
Significance of the study
Aside from lowering the risk of Covid-19 transmission, the new findings are important for the upcoming winter season in the northern hemisphere, according to the researchers. Millions of people in the northern hemisphere countries, including the United Kingdom, Norway, Poland, and Sweden, will be staying in heated rooms. Prof. Alfred Wiedensohler warned that in temperate and cold climate zones, the indoor climate is often very dry during the heating season (coldest months of the year). However, this could encourage the spread of Covid-19 indoor. It is the air humidity that determines how much water a particle can bind, he added.
The surface of the particles changes significantly (kind of water bubble forms) when air humidity is high, though. The indoor space serves as a miniature ecosystem with chemical reactions. At higher humidity, the droplets also grow faster, fall to the ground faster, and can be inhaled less by healthy people. Therefore, a humidity level of at least 40% in local transport and public buildings would not only reduce the effects of Covid-19 but of other viral diseases, like seasonal flu.
Dr. Sumit Kumar Mishra of CSIR demands that authorities should consider the humidity factor in their future indoor guidelines. The researchers encouraged those living in cool climates to maintain a minimum indoor humidity. On the other hand, those living in hot and tropical climates should make sure that indoor rooms are not extremely undercooled by air conditioning systems. Although it makes people in tropical countries feel comfortable indoor when air is cooled, the dry particles will stay in the air for a longer duration.
Their study, which appeared in Science Daily, pointed out that more attention should be given to indoor air in preventing the outbreaks of viral disease. The moisture content of the air is an important aspect and fresh air from the outside can reduce the risk of transmission, the authors added. Other safety measures are also mentioned, having few people per room volume, keep social distancing, and wearing masks as possible. So far, the lowest risk of infection is when there are no viruses in the air.
South Korea’s relative humidity
In South Korea, the average temperature last year was 13.5 degrees Celsius, according to Statista. The country has a distinct four-season climate. In 2011, the average temperature was 12.4 °C, which has risen steadily over the following years: 2014 (13.1 °C), 2015 (13.4 °C), 2018 (13 °C), and 2019 (13.5 °C).
On average, July is the most humid and April is the least humid month. The average annual percentage of humidity is 68.0%, according to Weather and Climate.
It has now become more important to set a minimum relative humidity standard for indoor environments, such as public transports, offices, and hospitals, to minimize the spread of Covid-19.