*Corr. Author’s Address: Department of Vascular Diseases, University Medical Centre Ljubljana, Zaloška 7, 1525 Ljubljana, Slovenia, ales.blinc@kclj.si 265
Strojniški vestnik - Journal of Mechanical Engineering 68(2022)4, 265-271 Received for review: 2022-02-12
© 2022 The Authors. CC BY 4.0 Int. Licensee: SV-JME Received revised form: 2022-03-17
DOI:10.5545/sv-jme.2022.64 Original Scientific Paper, Special Issue: SARS-CoV-2 Accepted for publication: 2022-03-17
The Prevalence of COVID-19 Among Health Care Personnel in a 
University Hospital by the End of 2020, and Ambient Air CO
2
 in 
Hospital Rooms Ventilated by Window-Opening in 2021/22
Blinc, A. – Buturović Ponikvar, J. – Fras, Z.
Aleš Blinc
1,4
 – Jadranka Buturović Ponikvar
2,4
 – Zlatko Fras
3,4
1
University Medical Centre Ljubljana, Department of Vascular Diseases, Division of Internal Medicine, Slovenia 
2
University Medical Centre Ljubljana, Slovenia 
3
University Medical Centre Ljubljana, Division of Internal Medicine, Slovenia 
4
University of Ljubljana, Faculty of Medicine, Slovenia
In autumn and winter 2020/21 and again in 2021/22 Slovenia has ranked among countries with the highest incidence of COVID-19 per 
million inhabitants and high excess mortality over the average of previous years. Many patients on non-COVID-19 hospital wards were in fact 
infected by SARS-CoV-2. Health care personnel at the University Medical Centre Ljubljana (UMCL) were falling ill by Covid-19 in large numbers 
despite wearing surgical masks and eye protection when dealing with patients.  We compared the prevalence of COVID-19 among health care 
personnel of the Division of Internal Medicine, UMCL by the end of December 2020 with the national average of Slovenia.  After instructions 
had been issued to increase room ventilation by opening windows every hour for at least 10 minutes, ambient air CO
2
 was measured in 
an intensive care room and in an outpatient clinic room during a 10-month period, from April 2021 to February 2022. The prevalence of 
COVID-19 by the end of December 2020 was 42 % among nurses, 21 % among registered nurses and 17 % among medical doctors, whereas 
the national of average of the population was significantly lower at 5.5 %. Between April 2021 and February 2022, the average CO
2
 (ppm) 
in the intensive care was 633 (standard deviation 198, range 376 to 1540), while in the outpatient clinic the average was 552 (standard 
deviation 199, range 380 to 1910). During 2020, before the instructions for the use of personal protective equipment were up-graded and 
before regular window-opening was advised, the prevalence of Covid-19 among health care personnel at the Division of Internal medicine, 
UMCL exceeded the national average by 3- to 8-fold. After regular window-opening was advised, the peak CO
2
 levels still often exceeded the 
recommended “safe” level of 750 ppm.
Key words: COVID-19, prevalence, airborne spread, room ventilation, CO
2
 level
Highlights
• 	 The importance of airborne transmission of SARS-CoV-2 has been recognized by the World Health Organization rather late into 
the pandemic. 
• 	 During the second wave of the pandemic by the end of December 2020, the prevalence of COVID-19 was 3 to 6 times higher 
among nurses at the Division of Internal Medicine, UMCL, than the national average, indicating that airborne transmission in 
poorly ventilated hospital rooms with infected patients had been taking place.
• 	 Relying on improving ventilation by instructing personnel to open windows for at least 10 minutes every hour does not work 
consistently, as evidenced by CO
2
 measurements at the Department of Vascular Diseases, UMCL.
• 	 Airborne virus transmission should be recognized as a serious public health threat that should be systematically addressed, just 
as contaminated water and food have been addressed and successfully dealt with in the past.
0  INTRODUCTION
During the second wave of the COVID-19 pandemic 
in the autumn and winter 2020/21, Slovenia reported 
one of the worldwide highest incidences per million 
inhabitants. In November 2020, Slovenia suffered 
a more than 100 % increase in all-cause mortality 
compared with the monthly average of previous years 
(Fig. 1) [1]. To a slightly lesser degree, the situation 
repeated itself in the autumn and winter 2021/22, 
a year after effective anti-Covid-19 vaccines had 
already been available.
More than a year into the COVID-19 pandemic, 
the World Health Organization (WHO) still considered 
Covid-19 to be spreading mainly by droplet 
transmission – through ”large“ (> 5 µm in diameter) 
infective droplets, exhaled, sneezed of coughed 
out by an infected person, reaching the respiratory 
tract or eyes of the susceptible next person in close 
contact, at distances of up to 1.5 m to 2 m.  The WHO 
long acknowledged airborne (aerosol) transmission 
only under special circumstances of aerosol-
generating procedures, such as noninvasive high-flow 
oxygenation, and added transmission by aerosols in 
poorly ventilated or crowded rooms to their website at 
the end of April 2021 [2]. Accordingly, until the end of 
December 2020, the recommendations to health care 
personnel of the University Medical Centre Ljubljana 
(UMCL) dealing with “non-Covid” patients, many of 
whom were in fact infected with SARS-CoV-2, only 
advised on wearing a surgical mask and eye protection 
- goggles or a face shield. However, a surgical mask 

Strojniški vestnik - Journal of Mechanical Engineering 68(2022)4, 265-271
266 Blinc,	A .	–	Butur o vić	Po nikv ar ,	J.	–	F ras,	Z.
only shields others against droplet transmission 
from the wearer, but not the wearer against inhaling 
infective aerosols. By the end of December 2020, all 
medical personnel at the UMCL were encouraged to 
wear FFP2 masks when dealing with possibly infected 
patients, and instructions were given to improve 
hospital room ventilation by opening windows for at 
least 10 minutes every hour. 
In this paper, we provide the comparison of 
Covid-19 prevalence among health care personnel 
at the Division of Internal Medicine of the UMCL 
with the prevalence among the general population 
of Slovenia by December 24, 2020. As a surrogate 
marker of the adequacy of room ventilation in the 
UMCL, we present data on CO
2
 content in ambient 
air of two hospital rooms between April 2021 and 
February 2022.
1  MATERIALS AND METHODS
1.1  Prevalence of Covid-19
Data on the number of employees who took a 
mandatory sick-leave because of confirmed infection 
by SARS-CoV-2 from January 1 to December 24, 
2020, before instructions on up-grading of personal 
protective equipment and instructions on regular 
window-opening were issued, were obtained from the 
records of the Division of Internal Medicine, UMCL. 
The prevalence of confirmed infection by 
SARS-CoV-2 in the general population of Slovenia 
by December 24, 2020, was calculated from data 
obtained from [3].
The prevalence of confirmed Covid-19 among 
nurses, registered nurses and medical doctors of the 
Division of Internal Medicine, UMCL, was compared 
with the prevalence of Covid-19 among the general 
population of Slovenia in the same period by the chi-
square test.
1.2  CO
2
 Content in Ambient Air
Air-quality sensors WA VEplus (Airthings, Norway) 
were kindly provided by MIK d.o.o., Slovenia, and 
installed in selected rooms of the Department of 
Vascular Diseases, Division of internal Medicine, 
UMCL.  Data from the sensors were obtained from 
[4].
In the Intensive Care Unit (ICU) of the Department 
of Vascular Diseases the CO
2
 content of ambient 
Fig. 1.  Ex cess	mo r tality	in	N o v ember	2020	and	Januar y	2022	in	Slo v enia	e x ceeded	 
that of the United States of America, United Kingdom and Germany [1]

Strojniški vestnik - Journal of Mechanical Engineering 68(2022)4, 265-271
267
The Prevalence of COVID-19 Among Health Care Personnel in a University Hospital by the End of 2020,  
and Ambient Air CO
2
 in Hospital Rooms Ventilated by Window-Opening in 2021/22
room air was measured between April 4, 2021, and 
February 18, 2022, and 91,048 measurements were 
taken.  The ICU room had an area of about 36 m
2
, a 
ceiling height of about 3.6 m, housed up to 4 patients 
and up to 3 nurses or medical doctors, was connected 
via a large opening in the wall to a symmetrical ICU 
room, and was equipped by air-conditioning. The ICU 
had windows that could be opened.
In the Outpatient Angiology Clinic of the 
Department of Vascular Diseases the CO
2
 content 
of ambient room air was measured between April 
15, 2021, and February 18, 2022, and 87,192 
measurements were taken. The Outpatient Angiology 
Clinic room had an area of about 20 m
2
, a ceiling 
height of about 3.6 m, housed up to 1 patient, 1 nurse 
and 1 medical doctor, did not have air-conditioning, 
but had windows that could be opened.
2  RESULTS
The prevalence of Covid-19 by December 24, 2020, 
among personnel of the Division of Internal Medicine, 
UMCL, compared with the general population of 
Slovenia is presented in Table 1. 
The average, standard deviation, minimum and 
maximum values of CO
2
 in ambient air of the ICU 
and Outpatient Angiology Clinic are given in Table 2. 
Table 1.  The prevalence of Covid-19 by December 24, 2020, 
among nurses, registered nurses and medical doctors at the 
Division of Internal Medicine, UMCL, compared with the general 
population of Slovenia [3]
Total 
number
Confirmed 
cases of 
Covid-19
Prevalence 
of Covid-19 
(%)
p-value
(compared to 
the population 
of Slovenia)
Nurses 292 122 42 < 0.00001
Registered 
Nurses
471 99 21 <0.00001
Medical 
Doctors
282 47 17 0.0008
Population of 
Slovenia
2,079,000 113,886 5.5 -
Table 2.  CO
2
 [ppm] measured between April 2021 and February 
2022 in the Intensive Care Unit and in the Outpatient Angiology 
Clinic of the Department of Vascular Diseases, UMCL
CO
2
 [ppm]
Intensive Care Unit 
(ICU)
Outpatient Angiology 
Clinic
Average 633 552
Standard deviation 198 199
Minimum 376 380
Maximum 1540 1910
Large-scale graphs of the 10-month measurements 
of CO
2
 and room temperature are shown in Fig. 
Fig. 2.  10-month measurements of CO
2
 [ppm] and room temperature [°C] in the Intensive Care Unit and  
the Outpatient Angiology Clinic of the Department of Vascular Diseases, UMCL

Strojniški vestnik - Journal of Mechanical Engineering 68(2022)4, 265-271
268 Blinc,	A .	–	Butur o vić	Po nikv ar ,	J.	–	F ras,	Z.
2.  Since occasional peaks of CO
2
 cannot be seen 
on the large-scale graph, an example of day-to day 
measurements is presented in Fig. 3.
3  DISCUSSION
The prevalence of COVID-19 by December 24, 2020 
was 3- to 8-times higher among health-care personnel 
of the Division of Internal medicine than among the 
general population of Slovenia. It seems unlikely that 
lack of adherence to protective protocols has been the 
only reason for the high infection rates. The highest 
prevalence of COVID-19 among nurses, who spend 
the most time in patient rooms, speaks in favor of 
airborne transmission of SARS-CoV-2 in hospital 
rooms with infected patients.  Aerosol scientists have 
long known that there is no clear boundary between 
droplet transmission and aerosol transmission [5]. 
People exhale droplets with a continuous distribution 
of sizes, most of which are smaller than 1 µm in 
diameter [6].  With air-flow velocities typical of indoor 
spaces, droplets up to 5 µm in diameter may travel 
several tens of meters before falling to the ground 
[7]. An invited commentary for Clinical Infectious 
Diseases by Morawska and Milton [8], endorsed by 
239 scientists from all over the world, summarized 
these facts in July 2020, explicitly warning of SARS-
CoV-2 transmission in poorly ventilated indoor spaces. 
Their message was presented to the lay public by the 
New York Times [9] and to the scientific community 
by Nature [10]. In October 2020, airborne transmission 
of SARS-CoV-2 was addressed by a letter published 
in Science [11], claiming that droplets not only up 
to 5 µm but up to 100 µm in diameter contributed to 
infective aerosols, and that airborne transmission was 
the major route of spreading COVID-19 [11].  Poorly 
ventilated indoor spaces where an infected individual 
resides for a longer time are prone to become sites 
of airborne COVID-19 transmission [11] and [12]. A 
review of studies testing air samples from hospitals 
for the presence of SARS-CoV-2 RNA found the 
highest concentrations in toilets, bathrooms, inner 
rooms for personnel and in hallways /waiting rooms 
without windows [13].  Weighing the evidence for and 
against the probability of airborne transmission has 
given SARS-CoV-2 a high probability score of 8- on a 
9-point scale, together with tuberculosis and influenza 
[14].
Authors from the Massachusetts Institute 
of Technology (MIT) [15] have developed a 
mathematical model predicting the probable time 
to airborne infection in confined spaces with an 
infected individual, considering 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. Based on this model, an 
Fig. 3.  Day to day measurements of CO
2
 [ppm] in the Intensive Care Unit and  
the Outpatient Angiology Clinic of the Department of Vascular Diseases, UMCL

Strojniški vestnik - Journal of Mechanical Engineering 68(2022)4, 265-271
269
The Prevalence of COVID-19 Among Health Care Personnel in a University Hospital by the End of 2020,  
and Ambient Air CO
2
 in Hospital Rooms Ventilated by Window-Opening in 2021/22
interactive internet application was developed [16]. 
The authors assumed an infective dose of a few tens 
of SARS-CoV-2 virus particles [15] and [16], which 
is in accordance with recent estimates [17] and [18] .  
Admittedly, we do not know the precise infective dose 
of SARS-CoV-2 in humans, since all assumptions rely 
on animal data and on modeling [17].   However, our 
experience at the Department of Internal Medicine, 
UMCL, agrees with predictions of the MIT model:  in 
a poorly ventilated room, where an infected individual 
has resided log enough to create stationary conditions 
of the infective aerosol, another person entering the 
room without protective equipment becomes infected 
after only 3 minutes on average [15].  
At the request of the Medical Director of the 
UMC Ljubljana, a lecture on airborne transmission 
of SARS-CoV-2 was presented to the Medical Board 
of the UMCL on December 21, 2020 [19]. The lecture 
was summarized in [20], and a mini-review has been 
published [21].
Immediately following the lecture to the Medical 
Board of the UMCL, all medical personnel were 
encouraged to wear FFP2 masks when dealing with 
possibly infected patients, and instructions were given 
to improve hospital room ventilation by opening 
windows for at least 10 minutes every hour. However, 
the initial interest in improving room ventilation 
and installing air- filtration systems in the UMCL 
decreased immediately after effective COVID-19 
vaccines became available. 
Paralleling developments in the community, 
efforts were mainly focused on promoting 
vaccination, which was important and commendable, 
but not sufficient to bring the COVID-19 pandemic 
under control. In the autumn of 2021, the numbers 
of SARS-CoV-2 infected residents of Slovenia, 
hospital patients and personnel at the UMCL rose 
again sharply.  Especially with the overwhelming 
wave of the omicron variant, vaccination against the 
original Wuhan strain of SARS-CoV-2 no longer 
protected against symptomatic infection, although it 
still strongly protects against severe illness requiring 
hospitalization [22].
In parallel with the decreasing efficacy of 
existing anti- Covid-19 vaccines against symptomatic 
infection, the enthusiasm for ventilating patient rooms 
by regularly opening the windows every hour also 
decreased, as evidenced by the occasionally excessive 
concentrations of CO
2
 at the Department of Vascular 
Diseases of the UMCL (Fig. 3). The concentration of 
CO
2
 in indoor air approximates the concentration of 
exhaled aerosols and the recommended “safe level” of 
CO
2
 in the air is less than 750 parts per million [23] 
and [ 24].  
Vaccines against SARS-CoV-2 have been 
developed in record time and especially mRNA 
vaccines have proven to be very safe and effective 
against the early strains of SARS-CoV-2 [25] and 
[26].  Unfortunately, at first due to the limited supply 
of vaccines, and later due to vaccine hesitancy, 
vaccination has been proceeding much too slowly to 
result in herd immunity.  Vaccination rates in many 
parts of the world remain very low, which creates the 
opportunity for infectious mutant virus strains, such 
as omicron, to rapidly spread around the globe [27]. 
Additionally, there are other respiratory pathogens 
besides SARS-CoV-2 that are transmitted by aerosols, 
among which influenza is the most well-known [28] 
and [29].   It is therefore prudent to address prevention 
of airborne spread of infective respiratory diseases in 
addition to promoting large-scale vaccination against 
Covid-19. 
From the public health perspective, it is wise 
to promote outdoor activities, which however is not 
a reasonable alternative for hospitals and nursing 
homes that urgently need adequate ventilation and 
air filtration systems. Morawska and co-workers [30] 
have appealed in their science paper that airborne 
virus transmission should be recognized as a serious 
public health threat which should be systematically 
addressed, just as contaminated water and food have 
been addressed and successfully dealt with in the past.
4  CONCLUSIONS
During 2020, before the instructions for the use of 
personal protective equipment were up-graded and 
before regular window-opening was advised, the 
prevalence of Covid-19 among health care personnel 
at the Division of Internal medicine, UMCL exceeded 
the national average by 3- to 8-fold. We have not 
proven a causal relationship between airborne 
transmission and the high number of Covid-19 cases 
because many potential confounding factors have not 
been accounted for. However, the association remains 
highly suggestive. After regular window-opening was 
advised in UMCL, the peak CO
2
 levels still often 
exceeded the recommended “safe” level of 750 ppm. 
Adequate ventilation /air filtration systems should be 
installed and maintained in hospitals, nursing homes 
and other densely populated public buildings in order 
to assure good air quality and minimize airborne 
hazards to human health.

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270 Blinc,	A .	–	Butur o vić	Po nikv ar ,	J.	–	F ras,	Z.
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and Ambient Air CO
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 in Hospital Rooms Ventilated by Window-Opening in 2021/22
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