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\begin{document}

\title{Drying-resistant nature and droplet nuclei transmission of
SARS-CoV-2~~~~~~~~}



\author[1]{Daisuke Miyazawa}%
\affil[1]{Affiliation not available}%


\vspace{-1em}



  
  \date{June 14, 2024}


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Droplet nuclei are sometimes defined only by size. The Centers for
Disease Control and Prevention (CDC) defines droplet nuclei as ``dried
residues of less than 5 microns in size'' {[}1{]}. Additionally, it is
sometimes referred to as ``small drops of moisture carrying infectious
pathogens'' {[}2{]}.~The World Health Organization (WHO) defines droplet
nuclei as follows: ``when the droplet particles are \textgreater{}5--10
\selectlanguage{greek}μ\selectlanguage{english}m in diameter, then they are referred to as respiratory droplets, and
when then are \textless{}5 \selectlanguage{greek}μ\selectlanguage{english}m in diameter, they are referred to as
droplet nuclei'' {[}3{]}, and they do not require dessication.

The term ``droplet nuclei'' must be clearly defined in terms of
desiccation. For the purpose of this study, droplet nuclei will be
discussed using CDC's definition, which includes a requirement for
desiccation. The WHO and CDC's current position is based on the
conventional medical definition that in human exhalation origin, only
droplet nuclei can cause airborne transmission {[}1{]}. Presently, the
WHO and CDC suspect but do not officially recognize airborne severe
acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission
{[}4{]}. According to the CDC, airborne transmission is currently
limited to measles, chickenpox, disseminated shingles, and smallpox
{[}5{]}.~Airborne transmission is suspected in certain other viruses but
has not been confirmed yet. Suspected cases of SARS-CoV transmission
through apartment building air {[}6{]}, outbreak of influenza in an
airplane delayed on the ground with an inoperative ventilation system
{[}7{]}, and reports of simultaneous coronavirus disease (COVID-19)
outbreaks in large enclosed spaces {[}8{]} suggest that these enveloped
viruses are transmitted through dry droplet nuclei because it is
unlikely that the fine droplets will stay moist for long distances and
time. Wells found that droplets smaller than 100 \selectlanguage{greek}μ\selectlanguage{english}m would completely dry
out before falling approximately 2 m to the ground {[}9{]}.~

Morawska reported that droplets with sizes of the order of 1 \selectlanguage{greek}μ\selectlanguage{english}m
evaporate within a few milliseconds even under conditions of high
relative humidity, droplets with size of the order of 10 \selectlanguage{greek}μ\selectlanguage{english}m exist for up
to a few tens of a second, while very large droplets that are 100 \selectlanguage{greek}μ\selectlanguage{english}m in
diameter survive for up to a minute {[}10{]}. SARS-CoV-2 and SARS-CoV
exhibited similar half-lives in aerosols, with median estimates of
approximately 2.7 hours {[}11{]}. In this study, all aerosols are
considered to be dried out during the course of the process and turned
into droplet nuclei. A couple of hours of half-life is sufficient for
the dry droplet nuclei to be dispersed and become clinically infectious,
especially in an enclosed environment. The presence of infectious,
replicating virions in \textless{}1 \selectlanguage{greek}µ\selectlanguage{english}m of aerosol samples has been
evident, and there were increases in viral RNA during cell culture of
the virus from aerosol samples {[}12{]}. It has been shown that droplets
up to 1\selectlanguage{greek}μ\selectlanguage{english}m in size evaporate within a few milliseconds {[}10{]}; this
indicates that SARS-CoV-2 can be transmitted through droplet nuclei.
SARS-CoV-2 is not inactivated on plastic and stainless steel surfaces
for a couple of days {[}11{]}. Generally, enveloped viruses with a lipid
envelope tend to be more persistent at lower relative humidity, while
non-enveloped viruses are more stable at higher relative humidity
{[}13{]}.

Airborne transmission is meant by most authors to be synonymous with
aerosol transmission {[}14{]}. The WHO states the following: ``Airborne
transmission is defined as the spread of an infectious agent caused by
the dissemination of droplet nuclei (aerosols) that remain infectious
when suspended in air over long distances and time'' {[}7{]}. This
statement includes two definitions. First, the WHO defines that the term
``airborne transmission'' is limited to transmission by droplet nuclei.
Second, the WHO also mentions that airborne, droplet nuclei, and aerosol
transmissions are all synonymous. The CDC states, ``The definition of an
aerosol, as used here, is a suspension of tiny particles or droplets in
the air, such as dusts, mists, or fumes.'' {[}15{]}. CDC's definition
requires droplet nuclei to be dry, and airborne transmission is further
limited to transmission by droplet nuclei.

Therefore, aerosol transmission is not synonymous with airborne or
droplet nuclei transmission in CDC's definition. Additionally, most
reports stating that SARS-CoV-2 spreads through air and/or aerosol do
not clearly specify whether the definitions of the terms ``airborne''
and ``aerosol'' refer only to dry droplet nuclei or fine, moist droplets
floating in the air {[}14{]}.

The fact that the definitions of aerosols and droplet nuclei are not
uniform worldwide, even between the WHO and the CDC, is a huge
detriment.

~

Equilibrium moisture content~(EMC) is the~moisture content~at which the
material is neither gaining nor losing~moisture in the air.~The droplet
nuclei is considered dry when the EMC state has been reached.. Because
the value of the EMC depends on the relative~humidity~and temperature of
the air, the moisture content of droplet nuclei varies with relative
humidity and temperature and their infectivity is also likely to vary.
This is an issue to consider when discussing whether or not droplet
nuclei are infectious.

~

Indeed, it is difficult to determine how small and how dry a particle
should be to be referred as a droplet or droplet nuclei. To prevent this
ambiguity, a suitable alternative is to abolish the current
classification of droplets and droplet nuclei and reclassify them as
droplets and aerosols based, solely, on how long they are suspended in
the air, regardless of their size and dryness.

Along with the global unification of the definitions of aerosols and
droplet nuclei, it is essential to recognize the above theoretical
evidence showing that SARS-CoV-2 has a drying-resistant nature and is
likely to have airborne transmission through dry droplet nuclei.

~

\textbf{References}

1.~~~~~~~~ U.S. Department of Health \& Human Services. (2012, May 18).
Principles of Epidemiology. Retrieved July 28, 2020, from
https://www.cdc.gov/csels/dsepd/ss1978/lesson1/section10.html

2.~~~~~~~~ Medical Advisory Secretariat. Air cleaning technologies: an
evidence-based analysis.~\emph{Ont Health Technol Assess Ser}.
2005;5(17):1-52.

3.~~~~~~~~ Modes of transmission of virus causing COVID-19: implications
for IPC precaution recommendations. World Health Organization.
https://www.who.int/news-room/commentaries/detail/modes-of-transmission-of-virus-causing-covid-19-implications-for-ipc-precaution-recommendations.
Published July 9, 2020. Accessed July 29, 2020.

4.~~~~~~~~ Morawska L, Milton DK. It is Time to Address Airborne
Transmission of COVID-19 {[}published online ahead of print, 2020 Jul
6{]}.~\emph{Clin Infect Dis}. 2020;ciaa939. doi:10.1093/cid/ciaa939

5.~~~~~~~~ Isolation Precautions.
https://www.cdc.gov/infectioncontrol/guidelines/isolation/index.html.
Published July 22, 2019. Accessed July 28, 2020.

6.~~~~~~~~ Yu IT, Li Y, Wong TW, et al. Evidence of airborne
transmission of the severe acute respiratory syndrome virus.~\emph{N
Engl J Med}. 2004;350(17):1731-1739. doi:10.1056/NEJMoa032867

7.~~~~~~~~ Moser MR, Bender TR, Margolis HS, Noble GR, Kendal AP, Ritter
DG. An outbreak of influenza aboard a commercial airliner.~\emph{Am J
Epidemiol}. 1979;110(1):1-6. doi:10.1093/oxfordjournals.aje.a112781

8.~~~~~~~~ Transmission of SARS-CoV-2: implications for infection
prevention precautions. World Health Organization.
https://www.who.int/news-room/commentaries/detail/transmission-of-sars-cov-2-implications-for-infection-prevention-precautions.
Published 2020. Accessed July 28, 2020.

9.~~~~~~~~ Atkinson J, Chartier Y, Pessoa-Silva CL, et al., editors.
Natural Ventilation for Infection Control in Health-Care Settings.
Geneva: World Health Organization; 2009. Annex C, Respiratory
droplets.~Available from:
\url{https://www.ncbi.nlm.nih.gov/books/NBK143281/}

10.~~~~ Morawska L. Droplet fate in indoor environments, or can we
prevent the spread of infection?.~\emph{Indoor Air}. 2006;16(5):335-347.
doi:10.1111/j.1600-0668.2006.00432.x

11.~~~~ van Doremalen N, Bushmaker T, Morris DH, et al. Aerosol and
Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1.~\emph{N
Engl J Med}. 2020;382(16):1564-1567. doi:10.1056/NEJMc2004973

12.~~~~ Santarpia JL,~Herrera VL,~Rivera DN, et al. The Infectious
Nature of Patient-Generated SARS-CoV-2 Aerosol~

medRxiv~2020.07.13.20041632;~doi:~https://doi.org/10.1101/2020.07.13.20041632

13.~~~~ Sobsey, M.D. and Meschke, J.S., 2003. Virus survival in the
environment with special attention to survival in sewage droplets and
other environmental media of fecal or respiratory origin. Report for the
World Health Organization, Geneva, Switzerland

14.~~~~ Tellier R, Li Y, Cowling BJ, Tang JW. Recognition of aerosol
transmission of infectious agents: a commentary.~\emph{BMC Infect Dis}.
2019;19(1):101. Published 2019 Jan 31. doi:10.1186/s12879-019-3707-y

15.~~~~ Aerosols. Centers for Disease Control and Prevention.
https://www.cdc.gov/niosh/topics/aerosols/default.html. Published June
29, 2010. Accessed July 29, 2020.

~

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