ABSTRACT
This study investigates the air quality and ventilation effectiveness in a
repurposed museum space with a hybrid ventilation system in Ljubljana,
Slovenia. Focusing on CO₂ and particulate matter concentrations, the aim is
to determine the correlation between these parameters and the suitability of
the ventilation system following a change in space use. Measurements were
conducted over a four-month period, analyzing data during different
occupancy and ventilation scenarios. The study compares observed values
with World Health Organization (WHO) guidelines, specifically targeting
PM₂.₅, PM₁₀, and CO₂ concentrations. Findings reveal inadequate ventilation
in the repurposed museum space, even with hybrid ventilation. CO₂
concentrations correlated with PM₂.₅ and PM₁₀ levels, suggesting CO₂
monitoring as an indirect indicator of overall air quality. Recommendations
include improving ventilation efficiency and limiting occupancy to ensure
adherence to air quality standards.
University of Ljubljana
Faculty of Mechanical Engineering 
Laboratory for Heating, Sanitary, Solar and Air Conditioning
Engineering - Chair of Thermal and Environmental Engineering
Aškerčeva 6
1000 Ljubljana, Slovenia
*Corresponding author:
Assoc. Prof. Uroš Stritih, PhD 
University of Ljubljana
Faculty of Mechanical Engineering
Aškerčeva 6
1000 Ljubljana, Slovenia 
Email: uros.stritih@fs.uni-lj.si
SANITARNO INŽENIRSTVO / International Journal of Sanitary Engineering Research 2024;17(1):
4-14 DOI: 10.2478/ijser-2024-0001
Occupancy Impact on Air
Quality in Repurposed
Museum Space
© 2024 Lucija GRUDEN, Uroš STRITIH. This is an open
access article licenced under the Creative Commons
Attribution NonCommercial- NoDerivs license as currently
displayed on http://creativecommons.org/licenses/by-nc-
nd/4.0/.
Lucija Gruden and Uroš Stritih*  
Keywords: air quality, ventilation, CO₂, aerosols, indoor environment
Original Research Article
Received: 31. 1. 2024 
Accepted: 22. 4. 2024
Published: 24. 5. 2024 
4 International Journal of Sanitary Engineering Research    Vol. 17 | Issue 1 | 2024
INTRODUCTION
Indoor air quality (IAQ) is becoming an increasingly visible parameter of the
internal environment. In addition to indoor temperature and relative humidity,
there has been a lot of research in recent years on adequate indoor and outdoor
air quality. During the SARS-CoV-2 pandemic, adequate and sufficient
ventilation proved to be a key measure in preventing the spread of the disease
[1, 2]. The same is true for other pathogenic particles [3]. Air quality also
depends on other pollutants such as particulate matter and CO₂ concentration
[4]. Some pollutants are not well perceived by humans, even though they have
an impact on health [5]. It is therefore important to control the levels of
pollutants that have a negative impact on humans in space. Standards are
ensured by properly designed HVAC systems. Purpose-built buildings must have
adequate ventilation designed in by design, based on occupancy and use, to
ensure minimum standards. In high occupancy areas, more attention needs to
be paid to maintenance and to assuring the adequacy of the installed systems.
There are several studies carried out in teaching spaces in older buildings where
the ventilation is only natural and is not sufficient to meet the minimum
requirements [6, 7, 8]. Sometimes, years after construction, some buildings
change use and the conditions for a suitable indoor environment change. When
more people are indoors, more pollutants emitted by general human activity are
trapped in the indoor environment. The amount of CO₂ emitted by an individual
depends on the time spent indoors and the level of activity. CO₂ is a known
indicator of the general condition of the indoor air and is an indicator of other
indoor pollutants and thus of the adequacy of ventilation [9, 10]. The amount of
particulate matter or PM in the air depends on the ambient air and the activity in
the room. High levels of PM in the air have a negative effect on the respiratory
system. Research has linked high levels of PM in the air to the development of
respiratory diseases as well as cardiovascular diseases [11-14].
The purpose of the study was to analyse the existing air quality in a showroom
with a hybrid ventilation system. The building has been repurposed for the
current use. The measurements were carried out in a museum space in
Ljubljana. We were interested in the correlation between CO₂ and particulate
matter concentrations and the suitability of the new ventilation with the change
of use. We want to find out if it is possible to assess the air quality of the room
and the adequacy of the ventilation with a known CO₂ value alone. WHO
guidelines dictate particulate matter values for an 8-hour average of 15 µg/m³
for PM₂.₅ and 45 µg/m³ for PM₁₀ [15]. The maximum recommended indoor CO₂
concentration is 1000 ppm. The goal of this study is to assess the effectiveness
of air quality and ventilation in a repurposed museum space with a hybrid
ventilation system and based on the findings, recommendations for
improvement will be given to the museum.
This paper consists of three more sections. The remainder of this paper is
organised as follows. The location and the measurement procedure are
presented in the methods section, where we also present the experimental part.
In the Results and Discussion section we comment on the results of the
measurement. The conclusions are drawn in the final section.
L. Gruden, U. Stritih
The amount of particulate matter or
PM in the air depends on the ambient
air and the activity in the room. High
levels of PM in the air have a negative
effect on the respiratory system.
The maximum recommended indoor
CO₂ concentration is 1000 ppm. 
International Journal of Sanitary Engineering Research    Vol. 17 | Issue 1 | 2024 5
L. Gruden, U. Stritih METHODS
The measurements that we want to use as the basis for the study were carried
out in the semi-basement space of the museum, which is located in Ljubljana.
We chose a room where a large number of people are changing, but the original
design of the building did not foresee this and ventilation was retrofitted in this
room. The room has dimensions of 21m x 6.3m x 2.5m (Figure 1). It has
ventilation with four exhaust fans installed in sets of two under the ceiling of the
room, locations are marked by red crosses (Figure 1). For heating gas
condensing furnace is used and there are four radiators in the room, where the
parameters were measured. Locations are marked by green numbers (Figure 1).
Natural ventilation is provided by twelve hopper windows of  0.75m x 0.85m on
one wall and a 1.95m x 1.3m opening leading to the adjacent room.
The measurements were made over a longer period of time. For further analysis,
we used data over a four-month period in spring. The experiment was carried
out in the showroom during normal operation on two non-consecutive days with
differrent different occupancy and ventilation modes during the heating season
2023. The natural ventilation mode was varied according to the needs, the
mechanical ventilation was switched on all the time when people were present
in the room. The measured parameters were CO₂, PM₂.₅ and PM₁₀ in relation to
the amount of occupants and the ventilation during the heating season. The
occupants in the room were exposed to normal operating conditions. The
number of visitors varied between measurements. The temperature and relative
humidity of the air were also measured. By monitoring the change curves of both
concentrations, we tried to identify the correlation.
Figure 1: Model of the room in which the measurements were made
A Testo 400 sensor was used, with a CO₂ probe placed at a height of 1.6m. The
measurement error of the sensor in the measurement range is ±(0.3 °C + 0.3 %
of mv) for temperature measurements, ±(50 ppm + 3 % of mv) for CO₂
concentration measurements and ±(2 % RH) for relative humidity. Aerosol
quantity and concentration were measured with a Grimm 11-D aerosol
spectrometer at 60s intervals. The minimum counting efficiency in the
measured range is 0.8. The maximum uncertainty for the magnitude in the
measured range is ±(4 % nm). A Testo high precision vane probe was used to
measure the air flow with a measurement accuracy of ±(0.1 m/s + 1.5 % of mv).
The maximum daily outdoor temperature on the first day was 20.4°C, while on
the second day it was 9.2°C. The airborne particulate matter was 16 µg/m³ for
PM₂.₅ and 24 µg/m³ for PM₁₀ on the first measurement day and 10 µg/m³ for
PM₂.₅ and 16 µg/m³ for PM₁₀ on the second measurement day [16, 17].
The experiment was carried out in the
showroom during normal operation
on two non-consecutive days with
differrent different occupancy and
ventilation modes during the heating
season 2023. 
International Journal of Sanitary Engineering Research    Vol. 17 | Issue 1 | 20246
 
9:15-9:30
The entrance door and all the windows in the room connected to the
door are wide open, and in the room where the measurements are
taken, the windows are open.
9:30-10:15 There are visitors in the room, about 15 people in the room.
11:55-12:05
The entrance door and all the windows in the room connected to the
door are wide open, and in the room where the measurements are
taken, the windows are open.
12:05-12:55 There are visitors in the room, about 15 people in the room.
8:40-8:50 Ventilation was carried out with the front door and all windows open. 
9:45-9:55 Ventilation was carried out with the front door and all windows open. 
10:45-10:55 Ventilation was carried out with the front door and all windows open. 
11:45-11:55 Ventilation was carried out with the front door and all windows open. 
13:45-14:25 Ventilation was carried out with the front door and all windows open.
L. Gruden, U. StritihThe measured flow rate of all four outlet fans located at the centre and at the
end of the room was 347 m³/h. The number of air changes is 1.05 per hour. At
the maximum occupancy of 19 persons, the air change is 18.28 m³/h. The main
opening through which the air enters the room is the door leading to the next
showroom. This means that the exit air contains more pollutants than would be
present if fresh outside air entered the room directly.
Experiment
The measurements were divided into two ventilation regimes.
Regime 1: 23.3.2023
Intensive natural ventilation at three intervals per day when the room was
empty. In between, an average of 15 people were in the room at two intervals.
Mechanical ventilation was on at all times.
Regime 2: 5.4.2023
In the second regime, natural ventilation was carried out in accordance with
monitoring the increase in CO₂ concentration in order to keep the value at an
acceptable level. On this day, the room was occupied by between 16 and 19
visitors, with pauses during which intensive cross ventilation took place. Natural
ventilation with the windows open was carried out at all times and cross
ventilation when the room was empty. Mechanical ventilation was on at all
times.
RESULTS AND DISCUSSION
By measuring these parameters, we have confirmed that the ventilation provided
in the building with its current use is inadequate and does not provide
acceptable levels of pollutants in the indoor air. PM₂.₅ levels of 32 μg/m³ on the
first and 27 μg/m³ on the second day of measurement (Table 1) and PM₁₀ levels
of 57 μg/m³ on the first and 59 μg/m³ on the second day of measurement (Table
2) are well above the WHO recommended values, even with intensive hybrid
ventilation [15].
The main opening through which the
air enters the room is the door leading
to the next showroom. This means
that the exit air contains more
pollutants than would be present if
fresh outside air entered the room
directly.
By measuring these parameters, we
have confirmed that the ventilation
provided in the building with its
current use is inadequate and does
not provide acceptable levels of
pollutants in the indoor air. 
International Journal of Sanitary Engineering Research    Vol. 17 | Issue 1 | 2024 7
PM₂.₅ [μg/m³] >15 >25 >35 Average
23.3. [%] 100 100 17 32
5.4. [%] 94 43 21 27
PM₁₀ [μg/m³] >45 >50 >60 Average
23.3. [%] 70 60 41 57
5.4. [%] 63 57 47 59
CO₂ [ppm] >1000 >1500 >2000 >2500
23.3 [%] 57 27 3 0
5.4. [%] 82 55 21 4
CO₂ [ppm] Averagevalue
Standard
deviation Range
23.3 [%]   1162 433  614 - 2060
5.4. [%]   1625 521  576 - 2612
L. Gruden, U. Stritih The CO₂ concentration during the time when visitors are in the room does not
provide the minimum recommended standards. The values of both measured
quantities should be reduced, which would require improving the ventilation
efficiency and installing better devices that would bring outside air into the
space, not the air from the adjacent room. Limiting the maximum number of
people in the room would likewise be helpful.
Table 1: Proportion of PM₂.₅ aerosol measurements above the limit values
during the period considered
Table 2: Proportion of PM₁₀ aerosol measurements above the limit values during
the period considered
Average PM₂.₅ values were 18% higher on the first measurement day. PM₁₀
values were 3% higher on the second measurement day.  The highest value
measured on the second measurement day was 138 μg/m³.
Table 3: Proportion of CO₂ measurements exceeding the limit values during the
period considered [15]
Table 4: Average CO₂ Levels
The highest value measured on the
second measurement day was 138
μg/m³.
The values of both measured
quantities should be reduced, which
would require improving the
ventilation efficiency and installing
better devices that would bring
outside air into the space, not the air
from the adjacent room.
International Journal of Sanitary Engineering Research    Vol. 17 | Issue 1 | 20248
 
L. Gruden, U. StritihCompared to PM values, CO₂ concentration values vary considerably more. On a
day when the occupancy was lower, the values are also significantly lower. Within
the acceptable range, the concentration was applied 43% of the time on the first
day and 18% of the time on the second day (Table 3). The measured minimum
value vas 576 ppm at the time the visitors first entered the space. Maximum
value was 2612 ppm and it was measured on the second day. It can be observed
that the effect of the amount of people in the room has a stronger influence on
the CO₂ concentration than on the particulate concentration and that the
measurements of our study show that the concentrations do not increase linearly
with the amount of people in the room.
The relative humidity and temperature graph of the first day meets the minimum
requirements (Figure 2) and despite the increase in relative humidity with
occupancy, the ventilation present is successful in maintaining the minimum
value. The graph of the measurements on the second day, when the outside
temperature was lower, illustrates the situation of inadequate relative humidity.
At each ventilation the value falls below the minimum required. The minimum
daily reading was 20.8% at the end of the long duration cross natural ventilation
(Figure 2).
Figure 2: CO₂ concentration, temperature and RH of indoor air on 23.3.2023
Figure 3: CO₂ concentration, temperature and RH of indoor air on 5.4.2023
It can be observed that the effect of
the amount of people in the room has
a stronger influence on the CO₂
concentration than on the particulate
concentration and that the
measurements of our study show that
the concentrations do not increase
linearly with the amount of people in
the room.
International Journal of Sanitary Engineering Research    Vol. 17 | Issue 1 | 2024 9
 
L. Gruden, U. Stritih The graphs show the success of cross-flow natural ventilation in removing
pollutants from rooms, but this can only be done when there are no people in the
room due to draughts (Figure 4). On a day when there were more visitors in the
room, the CO₂ content increased sharply, indicating that the capacity of the room
is lower than the occupancy at the current ventilation capacity (Figure 4). We
compared the trends of the concentration curves of aerosols of 0.65 μm and 2.00
μm. From the graphs, we conclude that the concentration of 2.00 μm aerosols
follows a very similar trend to that of CO₂, while the 0.65 μm aerosol curve
follows this concentration more loosely, but still shows a partial pattern. It can
be argued that, under similar conditions with the same ventilation regimes, the
CO₂ value is also an indicator of particulate matter in the air and thus, under
these conditions, an increase in CO₂ is indicative of a general deterioration in air
quality.
Figure 4: CO₂ and aerosol concentration of indoor air on 23.3.2023
Figure 5: CO₂ and aerosol concentration of indoor air on 5.4.2023
From the graphs, we conclude that
the concentration of 2.00 μm aerosols
follows a very similar trend to that of
CO₂, while the 0.65 μm aerosol curve
follows this concentration more
loosely, but still shows a partial
pattern.
International Journal of Sanitary Engineering Research    Vol. 17 | Issue 1 | 202410
L. Gruden, U. StritihThe distribution of aerosol concentrations on the first measurement day before
and during natural cross ventilation illustrates that ventilation removes most of
the larger pollutants from the room and particles in range PM₁ are the most
abundant in the room (Figure 6).
Figure 6: Particle number concentration per unit logarithmic size interval on
measurement day 23.3.2023 at 10:00
Figure 7: Particle number concentration per unit logarithmic size interval on
measurement day 23.3.2023 at 11:30
Ventilation removes some pollutants from the room (Figure 7), even if it does
not meet the minimum standards [15].
The findings of this study are consistent with previous research, offering similar
insights into the importance of monitoring indoor air quality parameters,
especially CO₂, for assessing ventilation effectiveness [6, 7, 8]. These studies
collectively highlight the significance of adequate ventilation in indoor spaces
for maintaining good air quality and protecting occupant health. However, they
also underscore the challenges posed by inadequate ventilation and its
implications for indoor air quality. Despite this alignment in findings, there
remains a need for further research in diverse contexts to deepen our
understanding of the relationship between ventilation effectiveness, indoor air
quality, and occupant health outcomes.
Despite this alignment in findings,
there remains a need for further
research in diverse contexts to deepen
our understanding of the relationship
between ventilation effectiveness,
indoor air quality, and occupant health
outcomes.
International Journal of Sanitary Engineering Research    Vol. 17 | Issue 1 | 2024 11
L. Gruden, U. Stritih CONCLUSION
The variation in airborne particulate matter levels can be estimated from a
preliminary study of the spatial variation of particulate matter in the room. This
method can be used to ensure adequate air quality in the museum. The finding
allows us to assess the overall air quality by simply measuring the CO₂
concentration in a room where people are the main source of pollution and to
take appropriate action when the values are not satisfactory. Air quality was
negatively affected by increased visitor presence and inadequate ventilation.
The maximum recorded CO₂ concentration in the room reached 2612 ppm
during peak occupancy with 19 visitors. The minimum CO₂ level measured was
on the same day, 567 ppm. On the first day, the average CO₂ concentration
stood at 1162 ppm, while on the second day, it rose to 1625 ppm. The CO₂
concentration of the air is higher when there are more people in the room. This
value is also an indicator of other pollutants or aerosols emitted by people. The
highest PM₁₀ airborne value was 138 μg/m³, also at the highest occupancy. This
means that high levels of CO₂ in the air are also indicative of high levels of
particulate matter.
We found that hybrid ventilation is more efficient than mechanical ventilation
alone in the museum space, but it is still not adequate enough. A more efficient
ventilation system needs to be installed in the space, and until then, based on
the findings, we can limit the number of occupants to ensure adequate air
quality with the current capacity.
We analysed the indoor air quality and the impact of human presence on it. CO₂
and aerosol levels vary in a correlated manner, so just by tracking CO₂ levels we
can get indirect information about aerosol levels. When CO₂ values are too high,
aerosol levels in the air are also too high. However, too high values of both
parameters indicate inadequate ventilation of the room. We found that an
adequate indoor environment is not guaranteed if the operation of the museum
is unchanged and no additional interventions are made to the building.
In conclusion, while our study offers valuable insights into air quality and
ventilation in a repurposed museum space, it may lack generalizability due to
its focus on a single location. Additionally, the short duration of measurements
and the absence of longitudinal data limit our understanding of seasonal
variations and long-term trends. Nevertheless, our study provides a
comprehensive analysis of CO₂ and particulate matter concentrations,
compared against WHO guidelines. Implementing practical recommendations
to enhance ventilation efficiency and optimize occupancy management
represents a crucial step towards fostering a healthier indoor environment
within the space investigated in our study.
Future research should address these limitations by expanding sample size,
conducting longer-term measurements, and including longitudinal data to
provide a more nuanced understanding of indoor air quality and ventilation
dynamics.
This means that high levels of CO₂ in
the air are also indicative of high
levels of particulate matter.
We found that hybrid ventilation is
more efficient than mechanical
ventilation alone in the museum
space, but it is still not adequate
enough.
CO₂ and aerosol levels vary in a
correlated manner, so just by tracking
CO₂ levels we can get indirect
information about aerosol levels.
International Journal of Sanitary Engineering Research    Vol. 17 | Issue 1 | 202412
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