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Special Issue on COVID-19 Aerosol Drivers, Impacts and Mitigation (V)
Aerosol and Air Quality Research, 20: 1856–1861, 2020
ISSN: 1680-8584 print / 2071-1409 online
Publisher: Taiwan Association for Aerosol Research
https://doi.org/10.4209/aaqr.2020.06.0302
Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.
An Overview on the Role of Relative Humidity in Airborne Transmission of
SARS-CoV-2 in Indoor Environments
Ajit Ahlawat1*, Alfred Wiedensohler1, Sumit Kumar Mishra2
1 Leibniz Institute for Tropospheric Research (TROPOS), Permoserstraße, 15 Leipzig, Germany
2 CSIR-National Physical Laboratory, New Delhi, India
ABSTRACT
COVID-19 disease is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which originated in
Wuhan, China and spread with an astonishing rate across the world. The transmission routes of SARS-CoV-2 are still
debated, but recent evidence strongly suggests that COVID-19 could be transmitted via air in poorly ventilated places. Some
studies also suggest the higher surface stability of SARS-CoV-2 as compared to SARS-CoV-1. It is also possible that small
viral particles may enter into indoor environments from the various emission sources aided by environmental factors such
as relative humidity, wind speed, temperature, thus representing a type of an aerosol transmission. Here, we explore the role
of relative humidity in airborne transmission of SARS-CoV-2 virus in indoor environments based on recent studies around
the world. Humidity affects both the evaporation kinematics and particle growth. In dry indoor places i.e., less humidity
(< 40% RH), the chances of airborne transmission of SARS-CoV-2 are higher than that of humid places (i.e., > 90% RH).
Based on earlier studies, a relative humidity of 40–60% was found to be optimal for human health in indoor places. Thus, it
is extremely important to set a minimum relative humidity standard for indoor environments such as hospitals, offices and
public transports for minimization of airborne spread of SARS-CoV-2.
Keywords: Aerosol; COVID-19; SARS-CoV-2; Indoor; Humidity.
INTRODUCTION
The World Health Organization (WHO) declared a global
pandemic for the outbreak of novel coronavirus disease
(nCOVID-19), which is a highly transmittable and pathogenic
viral infection caused by severe acute respiratory syndrome
coronavirus 2 (SARS-CoV-2) (Sanders et al., 2020; WHO,
2020b). There are more than 7 million confirmed COVID-19
cases worldwide through June 10, 2020 (Worldometer, 2020)
since its first reported case in Wuhan, China in December,
2019 (WHO, 2020a). The overall geographic range of
COVID-19 spread is much larger as compared with the
epidemic of the severe acute respiratory syndrome (SARS)
in 2003 (WHO, 2004). SARS-CoV-2 is identified as an
enveloped, non-segmented, positive ribonucleic acid (RNA)
virus with a diameter of 65–125 nm, containing single
strands of RNA with spikes in a crown shape on the outer
surfaces (Astuti and Ysrafil, 2020). SARS-CoV-2 is majorly
transmitted from human-to-human via direct or indirect
contact between people and with contaminated surfaces
* Corresponding author.
Tel.: +49-163-3019-921
E-mail address: ahlawat@tropos.de
(Prather et al., 2020). It is clear that airborne transmission of
SARS-CoV-2 is likely although many countries have not
acknowledged this possibility (Tellier et al., 2019; Asadi et
al., 2020; Hadei et al., 2020; Hsiao et al., 2020; Morawska
and Cao, 2020; National Academies of Sciences, Engineering,
and Medicine, 2020; Prather et al., 2020; Setti et al., 2020;
van Doremalen et al., 2020). Similarly, Paules et al. (2020)
recently pointed out that the airborne transmission of SARS-
COV-2 may also occur besides close distance contacts.
Recently, SARS-CoV-2 was found in aerosols in the
hospitals in Wuhan, China farther than 6 ft. distance (Liu et
al., 2020). Generally, respiratory infections occur through
the transmission of aerosols (< 5 µm) and viral droplets
(> 5–10 µm) exhaled from the infected persons. The larger
respiratory viral droplets will fall due to gravitational
settling, which leads to contact transmission, whereas smaller
droplets will lose mass through evaporation and remain in
the air for a longer time (Prather et al., 2020). It has been
shown in a recent study that small droplets of radius approx.
5 µm will take 9 minutes to reach the ground when produced
at height of 160 cm i.e., average speaking height. These small
droplets are of specific interest because of their association
with aerosol transmission of SARS-CoV-2 (Somsen et al.,
2020). These exhaled droplets are basically dilute saline
solutions with salts, water and some organics material along
with the attached virus (Kumar and Morawska, 2020). The
Ahlawat et al., Aerosol and Air Quality Research, 20: 1856–1861, 2020
1857
large population of these fine droplets originating from
coughing, sneezing and speech remain airborne for many
hours and can infect healthy persons (Prather et al., 2020).
The coronavirus transmission can also be affected by various
factors such as climate conditions (majorly temperature,
humidity and wind speed), population density, available
medical facilities (Dalziel et al., 2018). It is shown in previous
studies that wintertime climate and host behavior can favor the
influenza transmission (Shaman et al., 2011; Chattopadhyay et
al., 2018) and other human coronaviruses (Killerby et al.,
2018; Neher et al., 2020). Current studies indicate that
temperature and humidity have a significant influence on the
number of confirmed cases for a certain location (Bukhari
and Jameel, 2020). Therefore, precise understanding of the
influence of humidity on the transmissibility of COVID-19
in indoor environments is important for general public
awareness. For indoor areas with poor ventilation facilities,
people inhale the recirculating air. In cold and temperate
climates, within an indoor environment, the RH values are
typically low. Due to low RH, the droplets will evaporate at
a more rapid rate forming particles with smaller sizes (Feng
et al., 2020). The smaller size could lead to more airborne
suspension time of viral droplets and ultimately, they could
be transported to farther distances depending on ventilation
conditions (Bourouiba, 2020). But, in humid places, as the
humidity increases, the viral droplet size increases and falls
from the air faster providing less chances for other people to
breathe in the infectious viral droplets. The role of humidity
seems to be extremely important to the airborne spread of
COVID-19 in indoor environments.
ROLE OF RELATIVE HUMIDITY IN AIRBORNE
TRANSMISSION OF SARS-COV-2 IN INDOOR
CONDITIONS
As the exhaled droplets comes out from an infected person,
they will start either evaporating or there will be some
droplet growth. Both these scenarios are relative humidity
dependent (Feng et al., 2020). In dry indoor conditions,
when the aerosol droplets containing viruses and other fluids
are expelled in the air, they evaporate so that the water vapor
pressure at aerosol surface equilibrates with the ambient
conditions. The resulting water loss causes change in solute
concentration like proteins, salts or changes in other properties
such as pH (Marr et al., 2019). After evaporation of water,
the microdroplets will become quite small and suspended in
the air for longer durations. After some time, the suspended
viral particles concentration will increase depending on the
stagnant air and poor ventilation facilities, thus increasing
the infection risk in public places such as hospitals, restaurants
(Kumar and Morawska, 2020).
Based on literature, we have found that there are three
different scenarios where RH affects virus transmission in
the indoor surroundings (a) fate of microorganisms inside the
viral droplets (b) survival or inactivation of virus on surfaces
(c) role of dry indoor air in airborne transmission of viruses.
Fate of Microorganisms inside the Viral Droplets
Highly infectious diseases transmission such as COVID-19
requires pathogens to remain active outside of the host body.
RH affects the survival of some of these microorganisms in
the environment. A recent study explained that the viruses
survived well at RHs below 33% and at 100%, whereas, at
the intermediate RHs the viability was considerably reduced
(Lin and Marr, 2020). Lin and Marr (2020) investigated the
effect of RH on the viability of viruses both in suspended
aerosols and in droplets using culture-based approaches.
Based on the Lin and Marr (2020) findings, the viability is
typically much lower at a RH around 60% (~55%). This is
because evaporation kinetics plays an important role in
modulating the survival of the microorganism within the
droplets or aerosols. RH controls the evaporation kinetics of
the droplets. The enrichment factor (i.e., the calculated
concentration of the solute given a certain amount of water
loss or the fold increase in concentration of the solutes, as
the droplets evaporate) is an important parameter while
explaining the evaporation kinetics process. At lower RH,
i.e., at 43% and below, Lin and Marr (2020) found the
enrichment factor could increase rapidly with droplet
evaporation and dry out completely, while at higher RH,
evaporation occurred more slowly leading to a gradual
increase in the enrichment factor and the droplet never drying
out. Lin and Marr (2020) characterized the impact of this
evaporation with concentrations of solutes harmful to virus
viability (e.g., salts) by calculating their cumulative dose, or
sum of the products of the solute concentration and time. At
lower RH, due to rapid evaporation, solute concentrations
increased but then became irrelevant after the droplets dried
out, allowing virus viability to remain high. At the highest
RH levels, the cumulative dose increased slowly and did not
greatly impact virus viability, while at intermediate RH,
cumulative dose was a crucial factor to reduce virus viability
as the solute concentrations significantly increased while the
droplet never completely evaporated. Thus, virus inactivation
within droplets or aerosols is linked to the cumulative dose
of a harmful substance in solution, which itself has a
nonlinear response to RH.
Survival or Inactivation of Virus on Surfaces
A report based on humidity’s role on virus survival and
inactivation on surfaces showed that high temperature at
high relative humidity has a collegial impact on inactivation
of SARS-CoV-1 viability (Chan et al., 2011). Whereas,
lower temperatures and low humidity support prolonged
survival of virus on contaminated surfaces. Another important
point to mention here is that, the virus transmission has often
occurred in well air-conditioned environments such as
hospitals or hotels in some countries which has intensive use
of air-conditioning (Chan et al., 2011).
The Role of Dry Indoor Air in Airborne Transmission of
Viruses
There is a significant contribution of dry indoor air in both
disease transmission and poor resident health. During cold
winters, an outdoor air is drawn indoors and then heated to a
comfortable temperature level. This process will significantly
lower the indoor RH, which creates an extremely dangerous
situation for indoor residents, particularly during the
Ahlawat et al., Aerosol and Air Quality Research, 20: 1856–1861, 2020
1858
COVID-19 pandemic. When the indoor RH is less than
40 percent, humans becomes more vulnerable to viral
respiratory infections making SARS-CoV-2 virus more
infectious in the inhaled air. Earlier studies have shown that
for human health, a relative humidity between 40 to 60% is
optimum (Condair Ltd., 2007). When the indoor RH is lower
than 40 percent, the resulting moisture-free air yields optimal
route for long distance transmission of small infectious
aerosols. These viral airborne particles will further travel,
become inhaled by other residents, or finally settle on surfaces
where they can survive for many days. The infectivity of
many viruses, including SARS-CoV-2 are actually enhanced
due to low RH levels. Dry air also causes a significant
impact on our respiratory immunity. During the inhalation
of low RH air, the mucus in our nose and throat becomes dry
and more viscous, which diminish cilia’s capability to expel
viral aerosols. The low RH compromises the immune
system’s ability to effectively respond to microorganisms
(Taylor, 2020). Human ear, nose and throat areas are more
effective as virus fighter at high RH values rather than when
room air is very dry (Hohmann-Jeddi, 2019). While social
distancing reduces the risk of getting COVID-19 from other
inhabitants through short range contamination by large
droplets, it does less to prevent the transmission of tiny
infectious aerosols in the air.
COMPARISON OF INDOOR AND OUTDOOR
(AMBIENT) RH RELATIONSHIP WITH COVID-19
A quick comparison based on recent literature between
indoor and outdoor (ambient) RH relationship with COVID-19
will provide more insights into this topic. An indoor
environment is a microenvironment in which most people
spend the major portion of their daily life. As a result, indoor
air contributes to population exposures more than those
outdoors, although of course being influenced by factors
present at indoors as well as outdoors. In dry indoor places i.e.,
less humidity (< 40%), the chances of airborne transmission
of COVID-19 are high. Based on an indoor experiment from
Chinese cities during Jan-March 2020, the airborne spread
of SARS-CoV-2 was reduced by increasing RH from
23.33% to 82.67% (Yao et al., 2020). Feng et al. (2020)
recently investigated the influence of RH using numerical
modeling. In the study, they considered 40% RH as lower
bound and 95% RH as upper bound. They found that 40%
RH activates the evaporation of water in the cough droplets,
leading to droplet shrinkage and prolonged suspension in air
whereas high RH at 95% will increase the droplet size due
to hygroscopic growth with higher deposition fractions both
on humans and on ground. Biktasheva (2020) emphasized
on the air humidity control for indoor environment as a
feasible way to mitigate patients’ SARS-CoV-2 exposure.
When considering ambient air humidity, an important role
of humidity was found in rapid transmission of COVID-19
within the New York city (Bashir et al., 2020). Pani et al.
(2020) found the positive correlation of absolute humidity (AH)
with COVID-19 spread based on the daily data provided by
Ministry of Health, Singapore. Similarly, a positive correlation
was found between COVID-19 and RH (r = 0.106, p = 0.001)
in Kuala Lumpur, Malaysia (Suhaimi et al., 2020). Because,
in more humid outdoor environments, the population is more
likely to use drier indoor air and thus promote more COVID
viability. Considering outdoor absolute humidity factor
during cold winters, it was found that 73% of confirmed
cases in region of study with AH in range 3–10 g m–3 (Huang
et al., 2020). When the outdoor temperature is low and the
indoor environment is heated, indoor RH is closely correlated
with outdoor AH, resulting in more COVID-19 cases.
Another study pointed out that COVID-19 spread was found
to be significant in US with AH in range 4–6 g m–3 (Gupta
et al., 2020).
For better understanding, Table 1 depicts the influence of
RH on the survival, transmission and infection of H1N1,
SARS-CoV-1, MERS and SARS-CoV-2 viruses.
POLICIES FOR CONTROLLING THE OUTBREAK
OF SARS-COV-2 INCLUDING THE RH FACTOR
Nowadays, there is an immense need for rapid development
of effective vaccination and anti-viral medications which will
save humanity from a brutal pandemic. But, apart from that,
the building supervisors and government officials to play an
extremely important role in reducing the viral transmission
of these deadly diseases, such as SARS-CoV-2, by issuing
guidelines and standards. Governments around the world have
already set some indoor air quality standards for temperature
and indoor pollutants, but to the best of our knowledge there
are no such regulations and policies worldwide that require
a minimum RH standard in public buildings and indoor
environments. Based on research findings, for future scenarios,
setting a minimum RH standard of 40% for public buildings
will not only reduce the impact of COVID-19, but it will also
reduce the impact of further viral outbreaks, both seasonal
and novel. Though it is not an easy task to predict the
outbreaks of viral infections, gathering enough knowledge on
how these viral infections spread and developing counter plans
accordingly will certainly prevent us from such large-scale
pandemic like SARS-CoV-2. For countries in colder climates,
minimum RH standard for the indoor environments should be
kept into consideration. While, for tropical and typical hot
countries, humidity control measures are recommended while
avoiding extreme cooling of indoor places. Hygroscopic
growth at high RH will play an important role in reducing
the airborne spread of virus. Although virus viability will not
be minimized at high RH, the large droplet size will ensure
the of being airborne is minimized. Overall, air humidifying
is advised.
In order to implement the abovementioned guidelines, we
need a concrete plan along with the relationship between
different communities such as medical professionals, policy
makers, planners and government officials. In order to curb
the disease outbreaks, we must focus on the role of indoor air
on disease transmission and resident health. Other precautions
apart from RH optimization is to increase natural ventilations
like opening of windows during indoor stay, using proper face
masks (face shields along with face mask could provide better
results), avoid staying in direct periphery of the infected or
other persons, and maintaining social distancing.
Ahlawat et al., Aerosol and Air Quality Research, 20: 1856–1861, 2020
1859
Table 1. The influence of RH on the survival, transmission and infection of H1N1, SARS-CoV-1, MERS and SARS-CoV-2 viruses.
Reference Viruses Type of Study Typical Conditions Remarks
Lowen et al.,
2007
H1N1, Influenza Experimental, Indoor, Chamber
study
Transmission in cold and dry environment
(i.e., low RH of 20%–35%) conditions.
Transmission found to be completely stopped
at high RH of 80%.
Range which was tested (RH 20%–80%).
Low RH due to indoor heating in winter
supports virus spread in humans.
Tamerius et al.,
2013
H1N1, Influenza Modeling, Globally Temperate regions show a seasonal cycle with
low humidity in the winter and occurs in
some tropical locations during the rainy
season.
Low specific humidity (SH) conditions
favors the airborne survival and
transmission in temperate regions
during the cold-dry season.
Yuan et al., 2006 SARS-CoV-1 Meteorological data and
statistical analysis, Outdoor
(Ambient), Beijing, China
The peak transmission was found at mean RH
52.2%.
RH was found to be an important
meteorological parameter affecting the
transmission.
Cai et al., 2007 SARS-CoV-1 Meteorological data and
statistical analysis, China
Association of daily RH was found up to
certain extent.
Contribution of heaters and air
conditioning to the long-lasting
outbreak.
Chan et al., 2011 SARS-CoV-1 Experimental, Individual plastic
plate representing non-porous
surfaces
Prolonged survival of viruses was found at low
humidity on contaminated surfaces.
There were no major community
outbreaks found in Asian countries in
tropical area with high RH
environment
van Doremalen
et al., 2013
MERS Experimental Stability for a long time (as droplets on solid
surface and as aerosol) in low-humidity
Potential to be transmitted via contact or
aerosol transmission due to long
environmental presence.
van Doremalen
et al., 2020
SARS-CoV-2 Experimental, Stability on
aerosols and surfaces (plastic,
stainless steel, copper, and
cardboard)
Stable on plastic and stainless steel (65% RH),
Poor stable on copper and cardboard
(65% RH).
Viable virus was detected up to 72 h after
application in all surfaces.
Bukhari and
Jameel, 2020
SARS-CoV-2 Meteorological data and
statistical analysis
Lower number of cases in tropical countries
due to warm-humid conditions.
High absolute humidity (> 10 g m
–3
) was
a factor for slowdown in transmissions.
Yao et al., 2020 SARS-CoV-2 Experimental, Indoor, China The spread of SARS-CoV-2 was reduced by
increasing RH from 23.33% to 82.67%
RH being an important factor in reducing
the airborne transmission
Ma et al., 2020 SARS-CoV-2 Meteorological data and
statistical analysis, Wuhan,
China
Absolute humidity is negatively associated
with daily death counts.
Patients during therapy felt quite stable
and a comfortable environment
Ahlawat et al., Aerosol and Air Quality Research, 20: 1856–1861, 2020
1860
ACKNOWLEDGEMENTS
The authors confirm that no funding was received for this
work. The authors declare that there are no competing
interests.
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Received for review, June 16, 2020
Revised, July 17, 2020
Accepted, July 21, 2020
