255
PROFESSIONAL ARTICLE
Airborne spread of SARS-CoV-2
Copyright (c) 2022 Slovenian Medical Journal. This work is licensed under a
Creative Commons Attribution-NonCommercial 4.0 International License.
Airborne spread of SARS-CoV-2 – a commentary by the 
Division of Internal Medicine, University Medical Centre 
Ljubljana
Aerogeno širjenje virusa SARS-CoV-2 – komentar Interne klinike Univerzitetnega 
kliničnega centra Ljubljana
Aleš Blinc,1 Jadranka Buturović Ponikvar,2 Zlatko Fras3
Abstract
Slovenia is one of the countries that have been most affected by the autumn/winter 2020/21 wave of the COVID-19 pan-
demic regarding the incidence and excess mortality among the general population as well as regarding the incidence 
among health care workers and nursing personnel. The World Health Organization has underestimated the importance of 
the airborne spread of SARS-CoV-2 and the recommended safety measures have not been entirely sufficient. When people 
breathe, talk, sing, cough, or sneeze, they emit respiratory droplets of various sizes, most of which are always smaller than 
1 μm. Respiratory droplets smaller than 5 μm stay airborne in indoor spaces for a long time and travel over distances much 
longer than 2 m. Thus, an infected person in an indoor environment creates an infectious aerosol that may infect other 
people without close interpersonal contact. This short review presents the mathematical model and internet application 
by authors from the Massachusetts Institute of Technology for calculating the safe time before probable airborne infection 
occurs in indoor spaces. The importance of ventilation, air filtration, air humidity, and air disinfection by ultraviolet light is 
briefly discussed. The principles of preventing the airborne spread of SARS-CoV-2 are summarized.
Izvleček
Slovenija sodi med države, ki jih je jesensko-zimski val 2020/21 pandemije covida-19 najhuje prizadel tako glede obo-
levnosti in presežne umrljivosti med splošnim prebivalstvom kot glede obolevnosti med zdravstvenimi in negovalnimi 
delavci. Svetovna zdravstvena organizacija (SZO) je do nedavnega močno podcenjevala možnost aerogenega prenosa vi-
rusa SARS-CoV-2, zato tudi varnostna priporočila niso bila v celoti ustrezna. Ob dihanju, govoru, petju, kašljanju in kihanju 
1 Department of Vascular Diseases, University Medical Centre Ljubljana, Ljubljana, Slovenia
2 University Medical Centre Ljubljana, Ljubljana, Slovenia
3 Division of Internal Medicine, University Medical Centre Ljubljana, Ljubljana, Slovenia
Correspondence / Korespondenca: Aleš Blinc, e: ales.blinc@kclj.si
Key words: COVID-19; airborne spread; ventilation; air filtration; air humidity; ultraviolet light
Ključne besede: covid-19; aerogeni prenos; zračenje; filtriranje zraka; zračna vlaga; ultravijolična svetloba
Received / Prispelo: 28. 5. 2021 | Accepted / Sprejeto: 5. 11. 2021
Cite as / Citirajte kot: Blinc A, Buturović Ponikvar J, Fras Z. Airborne spread of SARS-CoV-2 – a commentary by the Division of Internal 
Medicine, University Medical Centre Ljubljana. Zdrav Vestn. 2022;91(5–6):255–61. DOI: https://doi.org/10.6016/ZdravVestn.3274
eng slo element
en article-lang
10.6016/ZdravVestn.3274 doi
28.5.2021 date-received
5.11.2021 date-accepted
Public health, epidemiology Javno zdravstvo, epidemiologija discipline
Professional article Strokovni članek article-type
Airborne spread of SARS-CoV-2 – a commen-
tary by the Division of Internal Medicine, 
University Medical Centre Ljubljana
Aerogeno širjenje virusa SARS-CoV-2 – komentar 
Interne klinike Univerzitetnega kliničnega centra 
Ljubljana
article-title
Airborne spread of SARS-CoV-2 Aerogeno širjenje virusa SARS-CoV-2 alt-title
COVID-19, airborne spread, ventilation, air 
filtration, air humidity, ultraviolet light
covid-19, aerogeni prenos, zračenje, filtriranje 
zraka, zračna vlaga, ultravijolična svetloba kwd-group
The authors declare that there are no conflicts 
of interest present.
Avtorji so izjavili, da ne obstajajo nobeni 
konkurenčni interesi. conflict
year volume first month last month first page last page
2022 91 3 4 255 261
name surname aff email
Aleš Blinc 1 ales.blinc@kclj.si
name surname aff
Jadranka Buturović Ponikvar 1
Zlatko Fras 1
eng slo aff-id
Department of Vascular Diseases, University Medical 
Centre Ljubljana, Ljubljana, Slovenia
Klinični oddelek za žilne bolezni, 
Univerzitetni klinični center 
Ljubljana, Ljubljana, Slovenija
1
University Medical Centre Ljubljana, Ljubljana, Slovenia Univerzitetni klinični center Ljubljana, Ljubljana, Slovenija 2
Division of Internal Medicine, University Medical Centre 
Ljubljana, Ljubljana, Slovenia
Interna klinika, Univerzitetni 
klinični center Ljubljana, 
Ljubljana, Slovenija
3
Slovenian Medical Journallovenian Medical Journal
256
PUBLIC HEALTH, EPIDEMIOLOGY
Zdrav Vestn | May – June 2022 | Volume 91 | https://doi.org/10.6016/ZdravVestn.3274
1 Introduction
During the fall and winter season 2020/21 of the 
COVID-19 pandemic, Slovenia was among the hard-
est-hit European countries with a huge increase in ex-
cess mortality (Figure 1) (1). The health care system of 
Slovenia was stressed to its upper limit and an unusu-
ally high proportion of health care workers contracted 
COVID-19. At the Division of Internal Medicine of the 
University Medical Centre Ljubljana, 315 out of 1,347 
employees (23.4%) were infected by SARS-CoV-2 by 23 
December 2020, among them 122 out of 292 (41.8%) 
nurses, 99 out of 471 (21.0%) registered nurses, 47 out 
of 282 (16.7%) physicians, 31 out of 134 (23.1%) ad-
ministrative workers, and 16 out of 168 (9.5%) other 
employees. Since health care workers were well-aware 
of the recommended preventive measures, including 
hand hygiene, wearing surgical masks, and maintaining 
a safe interpersonal distance, it is questionable whether 
the high rate of Covid-19 infection could be attributed 
only to non-compliance or perhaps also to the incom-
pleteness of the recommendations. Until the end of De-
cember 2020, the health care personnel of the University 
Medical Centre Ljubljana dealing with “non-COVID” 
patients, among whom many were actually infected, 
were advised to wear a surgical mask and eye protec-
tion - goggles or a visor. However, surgical masks shield 
only against droplet transmission of COVID-19, but not 
against airborne transmission.
Figure 1: Excess mortality during the autumn/winter 2020/2021 period of the COVID-19 pandemic in Slovenia and Germany, 
according to Our World in Data, 2021 (1).
The percentage difference between the reported number of weekly or monthly deaths in 2020–2021 and the average number 
of deaths in the same period over the years 2015–2019. The reported number might not count all deaths that occurred due to 
incomplete coverage and delays in reporting.
Jan 5, 2020 May 2, 2021Apr 30, 2020 Aug 8, 2020 Nov 16, 2020
0%
20%
40%
60%
80%
100%
Feb 24, 2021
Germany
Slovenia
človek izloča iz dihal v svojo okolico kapljice različnih velikosti, vselej pa je največ kapljic, manjših od 1 μm. Kapljice, ki so 
manjše od 5 μm, se v zaprtih prostorih dolgo zadržijo v zraku in se širijo na razdalje, ki so mnogo daljše od 2 m. Okužena 
oseba v zaprtih prostorih ustvarja kužni aerosol, s katerim se lahko okužijo druge osebe, tudi če z okuženim niso v tesnem 
stiku. Kratek pregledni članek predstavi matematični model in internetno aplikacijo avtorjev z Massachusetts Institute of 
Technology za izračunavanje verjetnosti aerogene okužbe v zaprtih prostorih. Na kratko je predstavljen pomen zračenja 
ter filtriranja, vlaženja in dezinfekcije zraka z ultravijolično svetlobo za preprečevanje aerogenega prenosa. Besedilo pov-
zema načelna priporočila za omejevanje aerogenega prenosa virusa SARS-CoV-2.
257
PROFESSIONAL ARTICLE
Airborne spread of SARS-CoV-2
2 Modes of SARS-CoV-2 transmission
SARS-CoV-2 can be transmitted between individuals 
in three ways (2):
• by touching surfaces that have been contaminated by 
the virus (fomites) and touching their eyes, nose, or 
mouth without cleaning their hands;
• by droplet transmission – through „large“ (> 5 μm) 
infective droplets, exhaled, sneezed or coughed out 
by an infected person, which reach the respiratory 
tract or eyes of the next person in close contact, at a 
distance of up to 1.5 – 2m;
• by airborne (aerosol) transmission - inhalation of 
»small« (< 5 μm) infective droplets that remain sus-
pended in the air for a long time and travel much fur-
ther than 2 m.
Fomite transmission of SARS-CoV-2 seems to be of 
little importance (2,3). Droplet transmission has long 
been regarded as the most important, since the World 
Health Organization (WHO) long acknowledged air-
borne (aerosol) transmission only under special cir-
cumstances of aerosol-generating procedures, such as 
noninvasive high-flow oxygenation, and only recog-
nized transmission by aerosols in poorly ventilated or 
crowded rooms at the end of April 2021 (4). Weighing 
the evidence for and against the possibility of airborne 
transmission has given SARS-CoV-2 a high probability 
score of 8 on a 9 point scale, together with tuberculosis 
and influenza (5). Recently, it has been proposed that the 
dichotomy between the droplet and aerosol transmis-
sion is artificial and that a unique airborne transmission 
mode should be recognized (6). Soon after the begin-
ning of the COVID-19 pandemic, infections were re-
corded in people who had not been in close contact with 
infected individuals (7-9). For example, at the 2.5-hour-
long Skagit Valley Chorale choir practice, 53 out of 61 at-
tendees became infected, most of them without being in 
close contact with the infected individual (7). Airborne 
transmission of SARS-CoV-2 between ferrets has been 
convincingly demonstrated in an experimental setting 
(10).
The importance of airborne spread in poorly venti-
lated indoor spaces is supported by epidemiological data 
that show a much higher incidence of Covid-19 in devel-
oped European countries, including Slovenia, and North 
American countries during the autumn/winter season 
than in African or Asian countries with a milder climate, 
where people spend more time outdoors (Figure 2) (11).
Figure 2: Graphs of daily new confirmed COVID-19 cases per million people show a much larger incidence in Europe, 
including Slovenia, and the United States in contrast to Africa and Asia. Summarized after Our World in Data, 2021 (11).
Mar 1, 2020 May 22, 2021Aug 8, 2020 Nov 16, 2020
0
200
400
600
800
Apr 30, 2020 Feb 24, 2021
Slovenia
Europe
United States
Asia
Africa
258
PUBLIC HEALTH, EPIDEMIOLOGY
Zdrav Vestn | May – June 2022 | Volume 91 | https://doi.org/10.6016/ZdravVestn.3274
3 When does airborne transmission occur?
Aerosol scientists have long known that there is no 
clear boundary between droplet transmission and aero-
sol transmission (12). Since 2009 it has been known that 
people exhale droplets with a continuous distribution 
of sizes at all kinds of respiratory activities and that the 
peak of the distribution is always at sizes smaller than 
1 μm (13). The absolute number of excreted droplets is 
much larger at singing or coughing than at quiet breath-
ing (13). With airflow velocities typical of indoor spac-
es, droplets up to 5 μm in diameter travel several tens 
of meters before falling to the ground (14). These facts 
were emphasized by Lidia Morawska, an aerosol scien-
tist, and by Donald K. Milton, a physician, professor of 
public health, and expert on virus transmission, in an 
invited commentary for Clinical Infectious Diseases in 
July 2020, where they warned of SARS-CoV-2 transmis-
sion in poorly ventilated indoor spaces (15). The com-
mentary was endorsed by 239 scientists from all over 
the world (15). Their message was summarized for the 
lay public by the New York Times (16) and for the sci-
entific community by Nature (17). In October 2020, the 
airborne transmission of SARS-CoV-2 was addressed by 
a letter to Science (18), where the authors claimed that 
droplets not only up to 5 μm in diameter but up to 100 
μm contribute to infective aerosols, and that airborne 
transmission is the major route of spreading COVID-19 
(18). The danger comes from poorly ventilated indoor 
spaces where an infected individual resides for a lon-
ger time (18). The medical community was informed of 
this matter by a commentary in Lancet Respiratory Dis-
eases (19). A review of studies testing air samples from 
hospitals for the presence of SARS-CoV2 RNA found 
the highest concentrations in toilets, bathrooms, inner 
rooms for personnel, and in hallways /waiting rooms 
without windows (20).
The importance of airborne transmission of SARS-
CoV-2 is in good agreement with the results of the re-
search group led by Jure Leskovec from Stanford Uni-
versity, who have published in Nature on patterns of 
mobility and probable sites of contracting COVID-19 
by analyzing the data from 98 million American mobile 
phone users (21). The conclusion was that a minority of 
sites were responsible for the majority of disease trans-
missions (21). Low-income Americans were most likely 
to contract COVID-19 in churches, whereas high-in-
come Americans were most likely to become infected in 
restaurants, cafés, and fitness centres (21).
Authors from the Massachusetts Institute of Tech-
nology (MIT) have developed a mathematical model 
predicting the probable time to airborne infection in 
confined spaces with an infected individual, taking into 
account the number and activity of other people in the 
room, the volume of the confined space, ventilation, air 
filtration, air humidity and some other variables (22). 
An infective dose of a few tens of SARS-CoV-2 virus 
particles was assumed, which is consistent with a re-
cent review and with the fact that the infective dose is 
smaller in transmission through the lower respiratory 
tract than through the upper respiratory tract (23). Ad-
mittedly, the infective dose of SARS-CoV-2 in humans 
is not precisely known, since all assumptions are based 
on animal data and modelling (24). However, according 
to the MIT model, the results can be dire: in a poorly 
ventilated nursing-home room, where an infected indi-
vidual has resided long enough to create stationary con-
ditions of the infective aerosol, another person entering 
the room without protective equipment becomes infect-
ed on average after only 3 minutes (22).
4 Room ventilation
Based on their mathematical model, the authors 
from MIT have developed an application that calculates 
the time to probable infection of another person under 
non-steady-state conditions, after an infected individual 
enters the room (25). When an infected individual en-
ters a (hospital) room with a floor surface of 30 m2 and 
ceiling height of 3.66 m, with an air humidity of 20%, 
ventilated by natural outdoor air exchange rate of 0.3 
air changes per hour (ACH) – corresponding to closed, 
non-sealing windows, the other susceptible person in 
the room will likely (with 10% risk tolerance) become 
infected via airborne transmission, i.e., over distanc-
es larger than 1.8 m, within 13 hours by the wild-type 
(Wuhan) strain or within 9 hours by the alpha strain 
and within 7 hours by the delta strain, assuming that 
both subjects are resting and not wearing face masks 
(25). If room ventilation is improved from 0.3 to 8 ACH, 
the time to likely airborne infection of the other suscep-
tible person increases to 14 days by the wild-type strain, 
to 8 days by the alpha strain, and to 2 days by the delta 
strain, respectively (25). 
5 Air humidity
Relative air humidity is an important factor in air-
borne virus transmission (26). In dry air, water evapo-
rates from exhaled infective droplets, hence the droplets 
decrease in size and float in the air for longer periods 
than larger droplets that fall to the ground sooner (26). 
259
PROFESSIONAL ARTICLE
Airborne spread of SARS-CoV-2
The air’s capacity for water vapour strongly depends on 
its temperature: for instance, air with a temperature of 
30°C can hold more than three times as much water va-
pour as air at 10°C (26). When cold winter air warms 
up to room temperature in indoor spaces, it remains 
dry (26). The optimal air humidity in indoor spaces is 
40- 60%, since dry air not only promotes the airborne 
spread of infective exhaled droplets but also dries the 
respiratory mucosa and reduces its resistance to patho-
genic viruses and bacteria (26). According to the model 
by authors from the MIT, increasing relative air humid-
ity from 20% to 60% prolongs the time to likely airborne 
infection of the next susceptible person in a poorly ven-
tilated hospital room from 13 to 18 hours (25), which 
means that air humidification cannot sufficiently sub-
stitute for good ventilation.
6 Air filtration
High-efficiency particulate air (HEPA) filters are 
increasingly recognized as useful tools for maintaining 
good air quality in indoor environments. According to 
European standards, HEPA filters must remove at least 
99.95% of particles with a diameter of 0.3 μm or larger 
from the air that passes through (27).
HEPA filters are a vital adjunct to ventilation systems, 
especially those that do not directly exchange indoor air 
with outdoor air, but rather air between indoor spaces, 
which is dangerous without proper filtration (28). The 
size of the SARS-CoV-2 virus has been estimated to be 
50 -140 nm (29) and HEPA filters are still efficient in 
retaining particles of this size by the diffusion and in-
terception regime (30,31). It is noteworthy that the New 
York public school system has purchased 30,000 HEPA 
filters as of November 2020, to augment their heating, 
ventilation and air-conditioning systems in classrooms 
(31).
7 Ultraviolet light for pathogen 
inactivation
Ultraviolet (UV) light effectively inactivates patho-
gens in the air and on non-porous surfaces (32). UV 
lights may nowadays only be used in rooms that are not 
simultaneously occupied by people (32,33). However, 
between 1937 and 1943, outbreaks of measles, mumps, 
and chickenpox were highly contained in American 
classrooms that employed UV lights installed at 2.1 m 
height and pointed upwards (34). Promising results for 
contemporary use in inhabited rooms have been report-
ed by far-UVC light with wavelengths of 207-222 nm, 
which should not be harmful to human tissues, since 
such wavelengths penetrate only a few μm into tissues 
but still efficiently inactivate viruses (33).
8 Concluding remarks
The whole world has high expectations especially 
from mRNA vaccines against SARS-CoV-2 that have 
been developed in record time and have been proven 
remarkably effective and safe (35,36). Unfortunately, at 
first due to the limited supply of vaccines, and later due 
to vaccine hesitancy, vaccination has been proceeding 
slowly in many countries, including Slovenia, and herd 
immunity has not yet been reached. Additionally, there 
are other respiratory pathogens besides SARS-CoV-2 
that are transmitted by aerosols, among them influenza 
(37,38). It is therefore prudent to address the prevention 
of the airborne spread of respiratory diseases in addition 
to promoting large-scale vaccination against COVID-19. 
It is still advisable to adhere to proper hand hygiene, 
mask use in indoor spaces and also outdoors under 
crowded conditions, and maintain a reasonable inter-
personal distance. However, for children, it is better to 
wash their hands than to use disinfectants since system-
ic resorption of disinfectants resulting in poisoning has 
been reported in children (39).
From the public health perspective, it is wise to pro-
mote outdoor activities (16-18), which however is not a 
reasonable alternative for hospitals and nursing homes 
that urgently need adequate ventilation and air filtra-
tion systems. Morawska and co-workers have appealed 
in their Science paper that airborne virus transmission 
should be recognized as a serious public health threat 
that should be systematically addressed, just as contam-
inated water and food have been addressed and success-
fully dealt with in the past (40).
From the public health perspective, it is more ra-
tional to focus on preventing super-spreader events in 
poorly ventilated indoor environments than to restrict 
the mobility of people outdoors (21). CO2 meters are 
advisable for public indoor spaces since the concentra-
tion of CO2 approximates the concentration of exhaled 
contagious aerosols (41,42). The recommended safe lev-
el CO2 in the air is less than 750 parts per million, but 
one should keep in mind that CO2 no longer reflects the 
contagious aerosol content of air when people sing or 
shout (41,42).
For nursing and healthcare personnel who are at 
high risk of being in contact with infected individu-
als, personal protective equipment should be provided 
(18). Regarding airway protection, personal protective 
260
PUBLIC HEALTH, EPIDEMIOLOGY
Zdrav Vestn | May – June 2022 | Volume 91 | https://doi.org/10.6016/ZdravVestn.3274
References
1. Our World in Data. Excess mortality: deaths from all causes compared 
to average over previous years. Oxford: University of Oxford; 202 1[cited 
2021 May 24]. Available from: https://ourworldindata.org/grapher/
excess-mortality-p-scores?country=DEU~SVN.
2. European Centre for Disease Prevention and Control. Transmission of 
COVID-19. Solna: European Centre for Disease Prevention and Control; 
2021 [cited 2021 May 24]. Available from: https://www.ecdc.europa.eu/
en/covid-19/latest-evidence/transmission.
3. Goldman E. Exaggerated risk of transmission of COVID-19 by fomites. 
Lancet Infect Dis. 2020;20(8):892,3. DOI: 10.1016/S1473-3099(20)30561-2 
PMID: 32628907
4. World Health Organization. Coronavirus disease (COVID-19): How is it 
transmitted? Geneva: WHO; 2021 [cited 2021 May 23]. Available from: 
https://www.who.int/news-room/q-a-detail/coronavirus-disease-covid-
19-how-is-it-transmitted.
5. Tang S, Mao Y, Jones RM, Tan Q, Ji JS, Li N, et al. Aerosol transmission 
of SARS-CoV-2? Evidence, prevention and control. Environ Int. 
2020;144:106039. DOI: 10.1016/j.envint.2020.106039 PMID: 32822927
6. Drossinos Y, Weber TP, Stilianakis NI. Droplets and aerosols: an 
artificial dichotomy in respiratory virus transmission. Health Sci Rep. 
2021;4(2):e275. DOI: 10.1002/hsr2.275 PMID: 33977157
7. Miler SL, Nazaroff WW, Jimenez JL, Boerstra A, Buonanno G, Dancer SJ, et 
al. Transmission of SARS-CoV-2 by inhalation of respiratory aerosol in the 
Skagit Valley Chorale superspreading event. Indoor Air. 2021;31(2):314-
23. DOI: 10.1111/ina.12751 PMID: 32979298
8. Shen Y, Li C, Dong H, Wang Z, Martinez L, Sun Z, et al. Community 
outbreak investigation of SARS-CoV-2 transmission among bus riders 
in Eastern China. JAMA Intern Med. 2020;180(12):1665-71. DOI: 10.1001/
jamainternmed.2020.5225 PMID: 32870239
9. Quian H, Miao T, Liu L, Zheng X, Luo D, Li Y. Indoor transmission of SARS-
CoV-2. Indoor Air. 2021;31(3):639-45. DOI: 10.1111/ina.12766 PMID: 
33131151
10. Kutter JS, de Meulder D, Bestebroer TM, Lexmond P, Mulders A, Richard 
M, et al. SARS-CoV and SARS-CoV-2 are transmitted through the air 
between ferrets over more than one meter distance. Nat Commun. 
2021;12(1):1653. DOI: 10.1038/s41467-021-21918-6 PMID: 33712573
11. Our World in Data. Daily new confirmed COVID-19 cases per million 
people. Oxford: University of Oxford; 2021 [cited 2021 May 24]. Available 
from: https://ourworldindata.org/grapher/rate-of-daily-new-confirmed-
cases-of-covid-19-positive-rate.
12. Jayaweera M, Perera H, Gunawardana B, Manatunge J. Transmission 
of COVID-19 virus by droplets and aerosols: A critical review on the 
unresolved dichotomy. Environ Res. 2020;188:109819. DOI: 10.1016/j.
envres.2020.109819 PMID: 32569870
13. Morawska L, Johnson GR, Ristovski ZD, Hargreaves M, Mengersen K, 
Dorbett S, et al. Size distribution and sites of origin of droplets expelled 
from the human respiratory tract during expiratory activities. J Aerosol 
Sci. 2009;40(3):256-69. DOI: 10.1016/j.jaerosci.2008.11.002
14. Matthews TG, Thompson CV, Wilson DL, Hawthorne AR, Mage DT. Air 
velocities inside domestic environments: an important parameter in the 
study of indoor air quality and climate. Environ Int. 1989;15(1-6):545-50. 
DOI: 10.1016/0160-4120(89)90074-3
15. Morawska L, Milton DK, Milton DK. It is time to address airborne 
transmission of Coronavirus Disease 2019 (COVID-19). Clin Infect Dis. 
2020;71(9):2311-3. DOI: 10.1093/cid/ciaa939 PMID: 32628269
16. Mandavilli A. 239 Experts With One Big Claim: The Coronavirus Is Airborne. 
New York: The New York Times; 2021 [cited 2021 May 24]. Available from: 
https://www.nytimes.com/2020/07/04/health/239-experts-with-one-
big-claim-the-coronavirus-is-airborne.html.
17. Lewis D. Mounting evidence suggests coronavirus is airborne - but health 
advice has not caught up. Nature. 2020;583(7817):510-3. DOI: 10.1038/
d41586-020-02058-1 PMID: 32647382
18. Prather KA, Marr LC, Schooley RT, McDiarmid MA, Wilson ME, Milton DK. 
Airborne transmission of SARS-CoV-2. Science. 2020;370(6514):303-4. 
DOI: 10.1126/science.abf0521 PMID: 33020250
19. The Lancet Respiratory MedicineCOVID-19 transmission-up in the air. 
Lancet Respir Med. 2020;8(12):1159. DOI: 10.1016/S2213-2600(20)30514-
2 PMID: 33129420
20. Birgand G, Peiffer-Smadja N, Fournier S, Kerneis S, Lescure FX, Lucet 
JC. Assessment of air contamination by SARS-CoV-2 in hospital 
settings. JAMA Netw Open. 2020;3(12):e2033232. DOI: 10.1001/
jamanetworkopen.2020.33232 PMID: 33355679
21. Chang S, Pierson E, Koh PW, Gerardin J, Redbird B, Grusky D, et al. 
Mobility network models of COVID-19 explain inequities and inform 
reopening. Nature. 2020;589(7840):82-7. DOI: 10.1038/s41586-020-2923-
3 PMID: 33171481
22. Bazant MZ, Bush JW. A guideline to limit indoor airborne transmission 
of COVID-19. Proc Natl Acad Sci USA. 2021;118(17):e20218995118. DOI: 
10.1073/pnas.2018995118 PMID: 33858987
23. Karimzadeh S, Bhopal R, Nguyen Tien H. Review of infective dose, 
routes of transmission and outcome of COVID-19 caused by the SARS-
COV-2: comparison with other respiratory viruses. Epidemiol Infect. 
2021;149:e96. DOI: 10.1017/S0950268821000790 PMID: 33849679
24. Usher Network for COVID-19 Evidence Reviews. Review: What is the 
infectious dose of SARS-CoV-2? Edinburgh: Usher institute; 2021 [cited 
2021 May 24]. Available from: https://www.ed.ac.uk/files/atoms/files/
uncover_029-01_review_infectious_dose_of_covid-19.pdf.
equipment according to the Slovenian standards begins 
at the level of FFP2/N95 masks (43-45).
Physical protection against respiratory virus trans-
mission is not cheap, but the costs of a pandemic are by 
far greater.
Conflict of interest
None declared.
Acknowledgement
This mini-review is a follow-up of the lecture pre-
sented at the meeting of the Medical Board of the Uni-
versity Medical Centre Ljubljana on December 21, 2020, 
that is available on-line (46).
A similar text in Slovenian language has been pub-
lished in the July 2021 issue of ISIS, the professional 
journal of the Medical Chamber of Slovenia (47).
261
PROFESSIONAL ARTICLE
Airborne spread of SARS-CoV-2
25. Khan K, Bush JW, Bazant MZ. COVID-19 Indoor Safety Guideline; 
2021 [cited 2021 May 24]. Available from: https://indoor-covid-safety.
herokuapp.com/apps/advanced?units=metric.
26. Ahlawat A, Wiedensohler A, Mishra SK. An overview on the role of relative 
humidity in airborne transmission of SARS-CoV-2 in indoor environments. 
Aerosol Air Qual Res. 2020;20(9):1856-61. DOI: 10.4209/aaqr.2020.06.0302
27. The International Organization for Standardization. ISO 29463-1:2017(en) 
High efficiency filters and filter media for removing particles from air — 
Part 1: Classification, performance, testing and marking. Geneva: ISO; 
2021 [cited 2021 May 24]. Available from: https://www.iso.org/obp/
ui/#iso:std:iso:29463:-1:ed-2:v1:en.
28. Dow A. Ventilation blamed for COVID spread, as design problems are 
detected. Sydney: The Age; 2021 [cited 2020 Dec 21]. Available from: 
https://www.theage.com.au/national/victoria/ventilation-blamed-for-
covid-spread-as-design-problems-are-detected-20201219-p56ox4.html.
29. Cuffari B. The size of SARS-CoV-2 and its implications. Sydney: 
AzoNetwork; 2021 [cited 2021 May 23]. Available from: https://www.
news-medical.net/health/The-Size-of-SARS-CoV-2-Compared-to-Other-
Things.aspx.
30. Wikipedia. HEPA. San Francisco: Wikipedia foundation; 2021 [cited 2021 
May 23]. Available from: https://en.wikipedia.org/wiki/HEPA.
31. Hefernan T. Can HEPA air purifiers capture the coronavirus? New 
York: Wirecutter; 2021 [cited 2020 Nov 18]. Available from: https://
www.nytimes.com/wirecutter/blog/can-hepa-air-purifiers-capture-
coronavirus/.
32. U.S. Food and drug. UV Lights and Lamps: Ultraviolet-C Radiation, 
Disinfection, and Coronavirus. Silver Spring: Food and Drug 
Administration; 2021 [cited 2021 May 23]. Available from: https://
www.fda.gov/medical-devices/coronavirus-covid-19-and-medical-
devices/uv-lights-and-lamps-ultraviolet-c-radiation-disinfection-and-
coronavirus.
33. Buonanno M, Welch D, Shuryak I, Brenner DJ. Far-UVC light (222 nm) 
efficiently and safely inactivates airborne human coronaviruses. Sci Rep. 
2020;10(1):10285. DOI: 10.1038/s41598-020-67211-2 PMID: 32581288
34. Wells WF. Air disinfection in day schools. Am J Public Health Nations 
Health. 1943;33(12):1436-43. DOI: 10.2105/AJPH.33.12.1436 PMID: 
18015919
35. Polack FP, Thomas SJ, Kitchin N, Absalon J, Gurtman A, Lockhart S, et 
al.; C4591001 Clinical Trial Group. Safety and efficacy of the BNT162b2 
mRNA Covid-19 vaccine. N Engl J Med. 2020;383(27):2603-15. DOI: 
10.1056/NEJMoa2034577 PMID: 33301246
36. Baden LR, El Sahly HM, Essink B, Kotloff K, Frey S, Novak R, et al.; COVE 
Study Group. Efficacy and safety of the mRNA-1273 SARS-CoV-2 vaccine. 
N Engl J Med. 2021;384(5):403-16. DOI: 10.1056/NEJMoa2035389 PMID: 
33378609
37. Yan J, Grantham M, Pantelic J, Bueno de Mesquita PJ, Albert B, Liu F, et 
al.; EMIT Consortium. Infectious virus in exhaled breath of symptomatic 
seasonal influenza cases from a college community. Proc Natl Acad Sci 
USA. 2018;115(5):1081-6. DOI: 10.1073/pnas.1716561115 PMID: 29348203
38. Lindsley WG, Noti JD, Blachere FM, Thewlis RE, Martin SB, 
Othumpangat S, et al. Viable influenza A virus in airborne particles 
from human coughs. J Occup Environ Hyg. 2015;12(2):107-13. DOI: 
10.1080/15459624.2014.973113 PMID: 25523206
39. Mahmood A, Eqan M, Pervez S, Alghamdi HA, Tabinda AB, Yasar A, 
et al. COVID-19 and frequent use of hand sanitizers; human health 
and environmental hazards by exposure pathways. Sci Total Environ. 
2020;742:140561. DOI: 10.1016/j.scitotenv.2020.140561 PMID: 32623176
40. Morawska L, Allen J, Bahnfleth W, Bluyssen PM, Boerstra A, Buonanno G, 
et al. A paradigm shift to combat indoor respiratory infection. Science. 
2021;372(6543):689-91. DOI: 10.1126/science.abg2025 PMID: 33986171
41. Bonino S. Carbon dioxide detection and indoor air quality control. 
Dallas: OHS corporate; 2021 [cited 2021 May 23]. Available from: https://
ohsonline.com/articles/2016/04/01/carbon-dioxide-detection-and-
indoor-air-quality-control.aspx.
42. Peng Z, Jimenez JL. Exhaled CO2 as COVID-19 infection risk proxy for 
different indoor environments and activities. Environ Sci Technol Lett. 
2021;8(5):392-7. DOI: 10.1021/acs.estlett.1c00183
43. Šarc L. Izbira osebne varovalne opreme za delo z nevarnimi agensi. In: 
Fras Z, Hugon M. 20. sodobna interna medicina: zbornik predavan. V 
Ljubljani: Katedra za interno medicino, Medicinska fakulteta; 2018.
44. Slovenija. Zakoni. Pravilnik o osebni varovalni opremi, ki jo delavci 
uporabljajo pri delu. Ur l RS. 1999(89); 2005(39);2011(43).
45. Ministrstvo za zdravjeSmernice za delovanje služb NMP ob kemijskih, 
bioloških, radioloških in jedrskih (KBRJ) nesrečah. Ljubljana: MZ; 2019.
46. Blinc A. Pomen aerogenega prenosa pri širjenju COVID-19 - pogled z interne 
klinike. Ljubljana: UKC Ljubljana; 2021 [cited 2021 May 23]. Available from: 
https://www.youtube.com/watch?v=cRUSlmOOWnE&feature=youtu.be.
47. Blinc A, Buturović Ponikvar J, Fras Z. Aerogeno širjenje SARS-CoV-2 – 
pogled z interne klinike UKCL. Isis. 2021(7):51-5.