Coronavirus SARS-CoV-2 spreads more indoors at low humidity
Indian-German research team recommends at least 40 percent humidity in
public buildings

Date:
August 20, 2020
Source:
Leibniz Institute for Tropospheric Research (TROPOS)
Summary:
The airborne transmission of the coronavirus SARS-CoV-2 via aerosol
particles in indoor environment seems to be strongly influenced
by relative humidity, concludes the analysis of 10 most relevant
international studies on the subject. Therefore, they recommend
controlling the indoor air in addition to the usual measures such
as social distancing and masks. A relative humidity of 40 to 60
percent could reduce the spread of the viruses and their absorption
through the nasal mucous membrane.



FULL STORY ==========================================================================
The airborne transmission of the coronavirus SARS-CoV-2 via aerosol
particles in indoor environment seems to be strongly influenced by
relative humidity.

This is the conclusion drawn by researchers from the Leibniz Institute
for Tropospheric Research (TROPOS) in Leipzig and the CSIR National
Physical Laboratory in New Delhi from the analysis of 10 most relevant international studies on the subject. Therefore, they recommend
controlling the indoor air in addition to the usual measures such as
social distancing and masks. A relative humidity of 40 to 60 percent
could reduce the spread of the viruses and their absorption through the
nasal mucous membrane. To contain the COVID-19 pandemic, it is therefore extremely important to implement standards for indoor air humidity in
rooms with many people, such as hospitals, open-plan offices or public transport, writes the research team in the scientific journal Aerosol
and Air Quality Research.


========================================================================== According to the WHO, the coronavirus SARS-CoV-2 has led to at least
21 million infected persons and over 750,000 deaths worldwide in over
half a year. The health and economic effects of the pandemic pose major
social challenges for practically all countries. Worldwide, therefore,
ways are being sought to stem the spread of the virus in order to avoid
drastic measures such as lockdowns and contact restrictions. For a long
time, the main transmission route of viral droplets was considered to
be direct human-to-human contact, because of infected people sneezing
or coughing and secreting the virus. Because these drops are relatively
large and heavy, they fall very quickly to the ground and can only cover
very short distances in the air. The recommendation to keep a minimum
distance of 1.5m to 2m (social distancing) is based on this assumption.

Recently, however, COVID-19 outbreaks have also been recorded, which
seem to be due to the simultaneous presence of many people in one room
(choir rehearsals, slaughterhouses, etc.). A safety distance of 1.5m is apparently not sufficient when infected and healthy people are together
in one room for a long time. For example, Dutch researchers have
now been able to prove that tiny drops of 5 micrometres in diameter,
such as those produced when speaking, can float in the air for up to
9 minutes. In July, 239 scientists from 32 countries -- including the
chemist Prof. Hartmut Herrmann from TROPOS -- therefore appealed to the
World Health Organization (WHO) to focus more closely on the long-lived infectious particles suspended in the air. In order to contain the
spread via the aerosol particles floating in the air, the researchers
recommend not only continuing to wear masks but also, and above all,
good indoor ventilation.

An Indo-German research team is now pointing out another aspect that
has received little attention so far and could become particularly
important in the next flu season: Indoor humidity. Physicists at the
Leibniz Institute for Tropospheric Research (TROPOS) in Leipzig and the
CSIR National Physical Laboratory in New Delhi have been studying the
physical properties of aerosol particles for years in order to better
estimate their effects on air quality or cloud formation. "In aerosol
research, it has long been known 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. So, we were curious: what studies have already
been conducted on this," explains Dr. Ajit Ahlawat from TROPOS.

Therefore, they evaluated a total of 10 most relevant international
studies between 2007 and 2020 by other researchers who investigated
the influence of humidity on survival, spread and infection with the
pathogens of influenza and the corona viruses SARS-CoV-1, MERS and
SARS-CoV-2. Result: Air humidity influences the spread of corona viruses indoors in three different ways: (a) the behaviour of microorganisms
within the virus droplets, (b) the survival or inactivation of the virus
on the surfaces, and (c) the role of dry indoor air in the airborne transmission of viruses. Although, low humidity causes the droplets
containing viruses to dry out more quickly, the survivability of the
viruses still seems to remain high. The team concludes that other
processes are more important for infection: "If the relative humidity
of indoor air is below 40 percent, the particles emitted by infected
people absorb less water, remain lighter, fly further through the room
and are more likely to be inhaled by healthy people. In addition, dry
air also makes the mucous membranes in our noses dry and more permeable
to viruses," summarizes Dr. Ajit Ahlawat.

The new findings are particularly important for the upcoming winter season
in the northern hemisphere, when millions of people will be staying in
heated rooms. "Heating the fresh air also ensures that it dries. In cold
and temperate climate zones, therefore, the indoor climate is usually
very dry during the heating season. This could encourage the spread of
corona viruses," warns Prof.

Alfred Wiedensohler of TROPOS. The air humidity determines how much
water a particle can bind. At higher air humidity, the surface of
the particles changes considerably: a kind of water bubble forms --
a miniature ecosystem with chemical reactions. The liquid water content
of aerosols plays an important role in many processes in the atmosphere,
as it influences the optical properties, leading for example to haze or
altered effects of aerosols on the climate.

At a higher humidity, the droplets grow faster, fall to the ground earlier
and can be inhaled less by healthy people. "A humidity level of at least
40 percent in public buildings and local transport would therefore not
only reduce the effects of COVID-19, but also of other viral diseases
such as seasonal flu.

Authorities should include the humidity factor in future indoor
guidelines," demands Dr. Sumit Kumar Mishra of CSIR -- National
Physical Laboratory in New Delhi. For countries in cool climates, the researchers recommend a minimum indoor humidity. Countries in tropical
and hot climates, on the other hand, should take care that indoor rooms
are not extremely undercooled by air conditioning systems. When air is extremely cooled, it dries out the air and the particles in it, making
people inside the room feel comfortable. But the dry particles will
remain in the air for longer duration.

From a researchers' point of view, more attention should be paid to
indoor air to prevent future outbreaks of viral disease. The moisture
content of indoor air is an important aspect but not the only one. Fresh
air from outside can also reduce the risk of transmission. And of course,
the measures already known and practised: Keep social distancing, having
as few people per room volume as possible, and wearing masks. The lowest
risk of infection still where there are no viruses in the air.


========================================================================== Story Source: Materials provided by Leibniz_Institute_for_Tropospheric_Research_(TROPOS).

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Ajit Ahlawat, Alfred Wiedensohler, Sumit Kumar Mishra. An Overview
on the
Role of Relative Humidity in Airborne Transmission of SARS-CoV-2
in Indoor Environments. Aerosol and Air Quality Research, 2020;
20 DOI: 10.4209/aaqr.2020.06.0302 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200820102503.htm

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