KEY FACTORS DETERMINING INDOOR AIR PM10 CONCENTRATIONS 
IN NATURALLY VENTILATED PRIMARY SCHOOLS IN BELGRADE, SERBIA
Branislava MATIC1*, Uros RAKIC1, Verica JOVANOVIC1, Snezana DEJANOVIC1, Nela DJONOVIC2
1Institute of Public Health of Serbia “Dr Milan Jovanovic Batut”, Environmental Health and School Hygiene Department, 
Dr Subotica 5, 11000 Belgrade, Serbia
2University of Kragujevac, School of Medicine, Institute of Public Health of Kragujevac, Serbia
Received: May 31, 2016
Accepted: Jul 27, 2017 
Original scientific article
Introduction. Indoor air quality (IAQ) is rated as a serious public health issue. Knowing children are accounted 
as more vulnerable to environmental health hazards, data are needed on air quality in schools.
Methods. A project was conducted from 2007 until 2009 (SEARCH, School Environment and Respiratory Health 
of Children), aiming to verify links between IAQ and children’s respiratory health. Study was conducted in 
ten primary schools on 735 children, in 44 classrooms. Children were randomly selected. Research tools 
and indicators used for children’s exposure to school environment were indoor and outdoor pollutants, two 
standardized questionnaires for school and classroom characteristics. In both classroom air and ambient air in 
front of them we measured, during a 5-day exposure period for continuous 24h measuring: carbon monoxide, 
carbon dioxide, indoor air temperature, relative humidity, and PM10 during classes. 
Results. PM10 concentrations were significantly most frequent in an interval of ≥80.1µg/m
3, that is, in the 
interval above 50µg/m3. Mean PM10 value was 82.24±42.43 µg/m
3, ranging from 32.00µg/m3 to of 197.00µg/m3.
Conclusion. The increase of outdoor PM10 concentration significantly affects the increase of indoor PM10. A 
statistically significant difference exists for average IAQ PM10 concentrations vs. indicators of indoor thermal 
comfort zone (p<0.0001); they are lower in the classrooms with indicators within the comfort zone. Moreover, 
dominant factors for the increase of PM10 are: high occupancy rate in the classroom (<2m
2 of space per child), 
high relative humidity (>75%), and indoor temperature beyond 23°C, as well as bad ventilation habits (keeping 
windows shut most of the time).
Uvod. Kakovost notranjega zraka (IAQ) predstavlja resen javnozdravstveni problem. Ker so otroci bolj ranljivi 
za okoljska zdravstvena tveganja, so potrebni podatki o kakovosti zraka v šolah.
Metode. Med leti 2007 in 2009 se je izvajal projekt (SEARCH, School environment and respiratory health of 
children – Šolsko okolje in respiratorno zdravje otrok), katerega namen je bil dokazati povezave med IAQ 
in respiratornim zdravjem otrok. Raziskava se je izvajala med 735 otroki v 44 učilnicah desetih osnovnih 
šol. Otroci so bili izbrani naključno. Zunanji in notranji onesnaževalci ter dva standardizirana vprašalnika o 
značilnostih šol in učilnic so služili kot raziskovalna orodja in kazalniki izpostavljenosti otrok šolskemu okolju. 
V zraku pred učilnicami in v njih so v 5-dnevnem neprekinjenem obdobju merjenja merili: ogljikov monoksid, 
ogljikov dioksid, temperaturo notranjega zraka, relativno vlažnost in PM
10
.
Rezultati. Koncentracije PM
10
 so bile pomembneje najpogostejše v intervalu ≥80,1 µg/m3, torej v intervalu 
nad 50 µg/m3. Povprečna vrednost PM
10
 je bila 82,24±42,43 µg/m3 in je segala od 32,00 µg/m3 do 197,00 µg/m3. 
Zaključek. Povečanje koncentracij PM
10
 v zunanjem zraku pomembno vpliva na povečanje PM
10
 v notranjem 
zraku. Pojavljajo se statistično pomembne razlike med koncentracijami PM
10
 IAQ glede na kazalnike območja 
notranjega toplotnega ugodja (p<0,0001); nižje so v učilnicah s kazalniki znotraj območja udobja. Poleg tega 
so glavni vzroki za večje vrednosti PM
10
 sledeči: velika zasedenost učilnice (<2 m2 prostora na učenca), visoka 
relativna vlažnost (>75 %) in temperatura notranjega zraka nad 23 ºC, poleg tega pa tudi slabe prezračevalne 
navade (zaprta okna večino časa).
ABSTRACT
Keywords: 
indoor air quality, 
classrooms, particulate 
matter, PM10, primary 
schools, exposure, 
Serbia
IZVLEČEK
Ključne besede: 
kakovost notranjega 
zraka, učilnice, trdni 
delci, PM
10
, osnovne 
šole, izpostavljenost, 
Srbija 
*Corresponding author: Tel: + 381 641 771 746; E-mail: damjanko98@yahoo.com
10.1515/sjph-2017-0031 Zdr Varst 2017; 56(4): 227-235
227
KLJUČNI DEJAVNIKI, KI VPLIVAJO NA KONCENTRACIJO DELCEV PM10 
V NOTRANJEM ZRAKU NARAVNO PREZRAČEVANIH OSNOVNIH ŠOL 
V BEOGRADU V SRBIJI
Matic B, Rakic U, Jovanovic V, Dejanovic S, Djonovic N. Key factors determining indoor air PM10 concentrations in naturally ventilated primary schools in Belgrade, Serbia. 
Zdr Varst 2017; 56(4): 227-235.
© Nacionalni inštitut za javno zdravje, Slovenija. 
1 INTRODUCTION
The quality of indoor air of homes, offices, or other public 
or private dwellings could be accounted as one of the 
essential determinants of a healthy life and wellbeing 
of each individual (1). School indoor air quality (IAQ) is 
expected to have a key role in the assessment of children’s 
personal exposure to air pollution, concerning the fact 
that they spend at least a third of their time inside school 
buildings, approximately 7 hours a day (2-5). Children are 
particularly vulnerable to all types of pollutants, because 
their breathing and metabolic rates are high. In a school, 
they have much less floor space than adults working in a 
typical office. Their breathing zone tends to be closer to 
pollutant sources, such as a new carpet, and less likely to 
be well ventilated, as it is below the window level. The 
immune system of young children is yet immature, and 
exposure to pollutants can mean allergic reactions or ill 
health (6). 
Poor indoor environment in schools may be attributed to 
three primary causes: inexistence or inadequate operation 
and maintenance of ventilation systems, infrequent and 
unthoroughly cleaned indoor surfaces, and a large number 
of students in relation to room area and volume, with 
constant re-suspension of particles from the surface (1). 
Furthermore, IAQ can be attributed to various phases of 
the building process, including poor site selection, choice 
of materials, roof design, poor construction quality, 
improper installation, or any number or combination of 
these factors (7). Therefore, it is of utmost importance 
to provide good IAQ in classrooms, to help minimize these 
effects (8, 9). Sources of IA (indoor air) pollution could 
be: furnishings, IT equipment, bio-effluents, and external 
pollutants, such as nitrogen-dioxide and carbon-monoxide 
(3, 10-12). In the indoor environment, in which people 
spend most of their time, both indoor and outdoor sources 
contribute to PM levels. Indoor PM is affected by ambient 
concentrations, air exchange rates, penetration factors, 
as well as deposition and re-suspension mechanisms. 
In this complex microenvironment, activities such as 
cleaning, walking, playing, and, particularly, smoking 
cause the formation of PM in indoor air (13,14).
The main objective of this paper was to study the 
difference of PM10 concentration values, sampled both 
inside classrooms of ten Belgrade primary schools and in 
ambient air in front of them, and relate it to different 
classroom and school characteristics, using SEARCH 
Project methodology. A specific objective was to study 
the difference of concentration of the PM10 measurements 
inside and outside the chosen classrooms. In addition, we 
aimed to correlate a specific quantitative indicator of the 
thermal comfort zone, as given by the ASHRAE Standards 
(4), that is, space occupancy (<2m2 of indoor space per 
child, not suitable), to the measured values of indoor 
PM10. 
2 METHODOLOGY
2.1 Sampling Site Description
The cross-sectional SEARCH (School Environment and 
Respiratory Health in Children) study has involved 6 
European countries (Albania, Bosnia and Herzegovina, 
Hungary, Serbia, Slovakia, and Italy). In Serbia, the 
research was undertaken in the capital city of Belgrade. 
The project has defined that a sample of 10 schools per 
country shall be enough to get a clear picture on the 
level of indoor exposure of children in primary schools. 
They were chosen (sampled) to be with heterogeneous 
characteristics. Primarily, schools were grouped by their 
location, i.e. the level of their exposure to potential 
sources of air pollution in their vicinity (be it traffic or 
industrial facility), as suburb schools, schools in broader 
urban area, and downtown ones, shown in Table 1 
(15). As for the vicinity of busy traffic, it can be easily 
comprehended by consulting the GIS map of their address, 
together with traffic characterisation (Figure 1). The 
criterion for the choice of a classroom was its orientation, 
either towards the street or to the school yard.
10.1515/sjph-2017-0031 Zdr Varst 2017; 56(4): 227-235
228
Figure 1. GIS map of 10 schools’ geographic distribution 
on the Belgrade City map  
10.1515/sjph-2017-0031 Zdr Varst 2017; 56(4): 227-235
229
windows. Measuring instruments were looked after by the 
teachers (indoor) and authors who were mostly present in 
the school to check upon the equipment. The following 
measurements took place in the chosen classrooms: 
Combination of diffuse sampling during a 4-day exposure 
period for formaldehyde (HCHO), nitrogen dioxide (NO2), 
BTX, and continuous 24h measuring for carbon monoxide 
(CO), carbon dioxide (CO2) and PM10, during school hours. 
Air temperature and relative humidity were measured as 
well. Parallel to these IAQ monitoring activities, outdoor 
air quality was followed for the same specific pollutants, 
close to school building (selected classroom). Besides 
air sampling procedures, the study protocol included 
two standardized questionnaires, namely: for school 
characteristics (filled in by the school administrator); for 
classroom characteristics (filled in by the teacher holding 
classes in it).
2.3 Statistical Methodology
Simple descriptive statistics, such as mean ± standard 
deviation, was used for continuous variables, IAQ and OAQ 
PM10, the number and % of IAQ interval distributions, by 
schools and schools’ position, while numbers (percentages) 
were used for categorical variables. The Kolmogorov–
Smirnov test was used to check if IAQ and OAQ PM10 
Table 1. Characteristics of sampling sites.
“Aca Milosavljevic”
“Kosta Abrašević”
“Nikola Tesla”
“I. G. Kovačić”
“Skadarlija”
“Stevan Sremac”
“Drinka Pavlović”
”Petar Petrović Njegoš”
“Radojka Lakić”
“Ivan Gundulić”
1
2
3
4
5
6
7
8
9
10
44°41’01,10” N
20°26’20,86” E
44°52’20,13” N
20°27’18,7” E
44°42’33,95” N
20°27’06,52” E
44°49’18,19” N
20°24’15,75” E
44°44’30,27” N
20°25’44,89” E
44°48’17,27” N
20°29’01,08” E
44°48’16,20” N
20°27’43,91” E
44°48’21,72” N
20°27’21,81” E
44°49’06,12” N
20°27’49,41” E
44°48’48,32” N
20°27’54,66” E
Suburb Rušanj village, in the valley downhill the regional highway;
Suburb Resnik, residential area, trees in between the street and school
Rakovica, suburban municipality, ex-industrial zone, ordinary urban 
traffic mode
Broad city centre, isolated from dense traffic, residential area;
Downtown, high density traffic, backyard towards pedestrian zone, 
bus stop in front
Borča III, urbanized suburb, with no heavy traffic
Downtown, close to two busy streets, 1 tunnel
Downtown, high traffic density
Squeezed between two streets with high density traffic (lots of heavy 
traffic), downtown, next to the Central Rail Station
New Belgrade, broader urban zone, frequent traffic
School name School code GIS coordinatesSpatial characteristics of the surroundings
2.2 Monitoring Campaign
For both indoor and ambient air PM10 sampling, a portable 
HAZ-DUST EPAM-5000 particulate monitor (Ambient 
Portable Direct Reading Aerosol Monitor for Measuring 
Lung Damaging Airborne Particles) was used. It uses 
light scattering to measure particle concentration and 
provide real-time determinations and data recordings of 
airborne particle concentration in mg/m3. By the model 
specification, its accuracy is ±10% to filter gravimetric SAE 
fine test dust; sensing range: .001-20.0 mg/m3 or optional 
.01-200.0 mg/m3 or 0.1-2000.0 mg/m3; particle size range 
is .1-100 µm, and precision ±.003 mg/m3 (3 µg/m3) (16). 
For indoor air monitoring, it was positioned in classrooms, 
at 1.5 m height, away from the walls, to prevent influence 
of chipping. The monitor was positioned outside of the 
school building, in front of the indicated classroom, for 
ambient air PM10 sampling. Monitoring lasted for one whole 
working week, in both school shifts, while the children 
were present indoors, only. Monitoring lasted for 4 days 
during heating season in February 2008, simultaneously 
with the procedures undertaken in Albania, Bosnia and 
Herzegovina, and Slovakia. During each of the 4 measuring 
days, authors would fill in the checklist of the questionnaire 
for the classrooms, with details on the presence of pupils, 
and conditions concerning natural ventilation through the 
10.1515/sjph-2017-0031 Zdr Varst 2017; 56(4): 227-235
230
had a normal distribution. Quantitative variables were 
compared using ANOVA F test, and categorical variables 
were compared using contingency tables and Chi-Square 
or Kruskal Wallis test. Chi-square test was used to 
compare IAQ PM10 between groups – schools or schools’ 
position. For correlations between variables, we used 
Pearson Correlation for the linear relationship between 
two variables, by schools. A P-value less than 0.05 were 
considered statistically significant. 
3 RESULTS 
3.1 Geographic Positions of Schools
Table 2 shows comparative values of IAQ PM10 and OAQ 
PM10 by groups of schools. Both indoor and outdoor PM10 
measured values are significantly higher in suburban 
schools than in those located in the broader urban zone: 
(for PM10 IAQ: K-W test=107.86, p<0.0001; PM10 OAQ: 
K-W test=39.43, p<0.0001). A similar level of significance 
appears when correlating values measured in suburban 
schools, with the values in schools located in the strictly 
urban zone: (K-W test=93.01; p<0.0001), and for PM10 
OAQ, (K-W test=27.74; p<0.0001). 
Table 2.
Table 3.
IAQ and OAQ PM10 concentration related to school’s geographic positions (µg/m3).
Interval distribution IAQ PM10 Concentration >50  or ≤49.9 µg/m3 by schools 
Suburb schools
Schools in broader
urban area
Downtown
Total
Suburb schools
Schools in broader
urban area
Downtown
Total
PM10 
IAQ
PM10 
ranges
PM10 
QAQ
244
220
271
735
244
220
271
735
0.0
100.0
39.7
60.3
0.0
100.0
32.1
67%
57.8
42.2
0.0
100.0
23.4
76.6
67.2
32.8
0.0
100.0
37.8
62.2
≤49.9 µg/m3
>50 µg/m3
109.18
66.08
71.09
82.24
153.90
83.46
77.30
104.57
47.66
37.36
26.90
42.43
130.39
55.77
28.53
89.85
103.17
61.12
67.88
79.17
137.46
76.05
73.89
98.07
115.19
71.05
74.31
85.31
170.34
90.87
80.72
111.08
96/164
53/126
70/79
70/165
116/515
55/168
80/89
82/527
33
32
32
32
34
22
30
22
197
158
111
197
549
190
119
549
Type of schools 
by location
No. of exposed 
children
School 1 School 5School 3 School 7 School 9School 2 School 6School 4 School 8 School 10
Mean SD Median/
Range
Min Max95% Cor Mean
Lower Upper
Indoor PM10 concentrations are significantly lower in schools 
located within a broad urban zone, when correlated to ones 
in a strictly urban zone, i.e.downtown (K-W test=12.943, 
p<0.0001). On the other hand, it does not count in the 
case of outdoor PM10 values (K-W test=2.228, p=0.135). 
PM10 IAQ measured values are significantly highest in 
suburban schools, (K-W test=133.454, p<0.0001), together 
with PM10 OAQ, (K-W test=69.86, p<0.0001).
Among schools, a statistically significant difference is 
proved for the distribution of  IAQ PM10  concentration 
(p<0.0001).  School 4 has significantly higher frequency of 
measured values IAQ PM10 in the range lower than 50 g/m
3. 
On the other hand, schools No. 1, 2, 3 and 8 had highest 
average values, and in all of them each of measured 
indoor concentrations was in the interval beyond 50 µg/
m3 (Table 3).
10.1515/sjph-2017-0031 Zdr Varst 2017; 56(4): 227-235
231
Table 4.
Table 5.
IAQ PM10 (µg/m
3) concentration by schools.
Outdoor PM10 (μg/m3) concentration by schools.
Suburban
Broad urban area
Downtown
Suburban
Broad urban area
Downtown
„A.Milosavljevic“
„K. Abrasevic“
„S. Sremac“
„Nikola Tesla“ 
„I. G. Kovacic“
„I. Gundulic“
„Skadarlija“
„D. Pavlovic“
„P.P. Njegos“
„R. Lakic“
„A.Milosavljevic“
„K. Abrasevic“
„S. Sremac“
„Nikola Tesla“ 
„I. G. Kovacic“
„Ivan Gundulic“
„Skadarlija“
„D. Pavlovic“
„P.P. Njegos“
„R. Lakic“
109.16
162.12
62.47
99.70
44.72
50.40
58.24
65.68
103.87
52.31
105.84
320.82
68.27
134.00
80.33
38.33
83.64
69.85
98.00
51.17
105.48
151.97
57.11
89.35
41.53
49.03
54.12
58.26
101.92
49.51
100.46
287.47
60.04
121.43
71.20
36.21
76.88
61.05
94.83
50.31
18.440
41.926
23.592
45.620
12.479
6.224
18.276
26.899
8.523
11.222
26.945
137.797
36.277
55.399
35.626
9.648
30.003
31.935
13.862
3.444
112.84
172.27
67.82
110.06
47.92
51.77
62.36
73.09
105.82
55.12
111.21
354.18
76.51
146.57
89.45
40.45
90.41
78.65
101.17
52.03
105.00
183.00
52.00
72.00
38.00
53.00
70.00
51.00
110.00
44.00
106.00
309.00
41.00
141.00
75.00
42.00
96.00
52.00
101.00
51.00
46.00
106.00
63.00
107.00
30.00
16.00
46.00
63.00
18.00
26.00
78.00
380.00
85.00
135.00
84.00
26.00
87.00
83.00
46.00
9.00
School Groups
School Groups
School 
School 
Median
Median
Range
Range
PM10 (µg/m
3)
PM10 (µg/m
3) OAQ
IAQ Ci 95% 
Ci 95% 
Mean
Mean
Lower
Lower
SD
SD
Upper
Upper
The following Table 4 presents indoor PM10 mean 
concentrations (μg/m3), standard deviation, median 
and range. The maximum concentration values of PM10 
(162.12±41.93 μg/m3) was registered in school No. 2, 
while a significantly lower concentration value of PM10 
(44.72±12.48 μg/m3) was at school 4 (p<0.001), and its 
value is below 50 μg/m3.
Ambient air mean concentration of PM10 (μg/m
3), together 
with standard deviation, median and range by groups and 
schools, is given in Table 5. The maximum concentration 
values of PM10 in ambient air (320.82±137.79 µg/m
3) were 
in front of a suburban school No.2, while significantly lower 
concentration values of PM10 OAQ (38.33±9.65 μg/m
3) were 
outside school 6, located in the broad urban area, slightly 
hidden away from frequent traffic flow (p<0.01), with a 
value below 50 μg/m3. The highest average values of both 
IAQ PM10 and OAQ PM10 concentration are accounted to 
school 2 (suburb schools).
10.1515/sjph-2017-0031 Zdr Varst 2017; 56(4): 227-235
232
Descriptive statistical analysis of the data for particulate 
matter mass concentrations (PM10) measured outdoors and 
in the classrooms, by schools is given in Table 6. None 
of the 10 schools satisfies the World Health Organization 
(WHO) standard for PM10 annual average of 20 µg/m
3 
(17, 18). However, they meet the National Ambient Air 
Quality Standards (12) and WHO standards for PM10 24-
hour average, which have been set at 150 µg/m3 and 50 
µg/m3, respectively (19).
Table 7 presents the distribution of the occupancy rate 
(according to ASHRAE) for each school, in m2 per present 
child in the indicated classroom. Values are given as 
the percentage of children exposed to overcrowdedness 
(<2m2/per child), or to convenient spatial conditions 
(>2m2/per child). Indicators of crowdedness are the 
number of children in the classroom (less/more than 20), 
and space available in the classroom, per one child, of 
less/more than 2m2 (4). Statistically significance is proven 
for the distribution of occupancy rate (m2/per child), for 
each school, χ2=340.70, p<0.0001.
Table 6.
Table 7.
Correlations between IAQ and OAQ by school (signif.)
Child occupancy rate per school distribution (m2/per child) vs. PM10 values (µg/m
3), by schools.
„A.Milosavljevic“
„K.Abrasevic”
„N.Tesla”
„I.G.Kovacic”
„Skadarlija”
„S.Sremac”
„D.Pavlovic”
„P.P.Njegos”
„R.Lakic”
„I.Gundulic”
„A.Milosavljevic“
„K.Abrasevic”
„N.Tesla”
„I.G.Kovacic”
„Skadarlija”
„S.Sremac”
„D.Pavlovic”
„P.P.Njegos”
„R.Lakic”
„I.Gundulic”
1
2
3
4
5
6
7
8
9
10
106.00
309.00
141.00
75.00
96.00
41.00
52.00
101.00
51.00
42.00
105.00
183.00
72.00
38.00
70.00
52.00
51.00
110.00
44.00
53.00
0.799
0.756
0.457
0.956
0.598
0.744
0.937
0.725
0.160
0.453
77.8
/
/
16.4
38.6
/
/
47.4
/
/
22.2
100
100
83.6
61.4
100
100
52.6
100
100
109.16
162.12
99.70
44.72
58.24
62.47
65.68
103.87
52.31
50.40
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.207
0.000
99
68
77
61
78
77
53
76
65
82
School
School nameSchool 
No.
PM10 outdoor median 
(µg/m3)
PM10 indoor median 
(µg/m3) R
<2 m2/child >2 m2/child IAQ PM10 mean
P N
4 DISCUSSION
As an outcome of our study, when differentiating between 
OAQ PM10 and IAQ PM10 concentration values in relation 
to some classroom and school characteristics, we singled 
out the following moments: the highest average values of 
IAQ PM10 and OAQ PM10 concentration were measured in 
the school No. 2 (suburb school), while only in one school 
measured values IAQ PM10 were below 50 µg/m3, that is 
in the school No. 4. The school No. 10, located in New 
Belgrade, in a broader urban zone with frequent traffic, 
had PM10 outdoor mean value below 50 µg/m3, which 
could be explained with the terrain’s topography. New 
Belgrade is, in geographical means, a flat terrain, with 
broad boulevards and widely spread buildings, belonging 
to the geographical entity of the Pannonian plain, enabling 
the build-up of high ambient air concentrations of traffic-
related pollutants. This particular school is located in a 
residential block, built in the 1960s. School building is 
encircled by greenery and residential buildings, acting as 
10.1515/sjph-2017-0031 Zdr Varst 2017; 56(4): 227-235
233
a physical barrier to three streets with very busy traffic, of 
which one is the E-75 highway. It was, moreover, reported 
by the City of Belgrade Institute of Public Health, that 
in 2008, a series of days in a row were characterized 
with SE (south-east) wind (‘Košava’), with episodes of 
wind speed reaching 12m/s, which caused a decrease of 
concentration of all ambient air pollutants measured by 
this institution (20).
A statistically significant correlation exists between 
PM10 indoor and outdoor concentration for each school 
(p<0.0001), except for the school No. 9, ‘Radojka Lakic’ 
(p=0.207), although it is located in a strictly urban zone, 
close to the juncture of two streets with very busy traffic. 
The increase of outdoor PM10 concentration is significantly 
correlated to the increase of indoor PM10 values (except 
for the school No. 9).
Considering this school, located close to the heavy traffic 
and Central rail station, with no statistical significance 
between indoor and outdoor measurement, we can 
conclude that, in this case, indoor concentration could 
be influenced by activities and movements of occupants, 
allowing re-suspension of previously deposited particles or 
their delayed deposition or settling. Fromme et al. found 
an average indoor particulate matter concentration higher 
than corresponding outdoor level in a German primary 
school (21). Similar was confirmed in a Belgian survey (22). 
Oeder et al. detected indoor PM10 concentration even 
5-fold higher than outdoor ones in six schools in Munich 
(23). In addition, the presence of children, together with 
their movements, could affect indoor PM levels through 
the interception of personal clouds (primarily comprising 
of course particles), recorded by sampling devices (1).
In the school No. 1, which is both in the suburb and 
has all its measured values of IAQ PM10 beyond 50 µg/
m3, with an average PM10 IAQ of 109.16±18.44 µg/m3, 
a significantly higher number of children is exposed to 
classroom indoor environment in a space with less than 
2 m2 per child. Concerning the fact that there is no busy 
traffic close to the school, high occupancy rate, together 
with bad ventilation habits and cleaning practice could 
be the reason for such results. This may be due to large 
number of students in relation to the room area and 
volume, whose movements cause re-suspension of settled 
particles (24). 
In the school No. 8, 47.4% of its pupils have less than 2 
m2 per child, while all measured IAQ PM10 were above 
50 µg/m3, in which case the high occupancy rate could 
be accounted for one of the reasons for it. Some authors 
have determined that re-suspension is a significant factor 
affecting indoor particle concentration with suspension 
rates increasing with the particle size (25).
The school No. 4 is worth mentioning, with only its 16.4% 
pupils residing in the space with <2 m2 per child, located 
in a broad urban zone of the city. IAQ PM10 values were 
below 50 µg/m3, with the average PM10 IAQ concentration 
being the lowest compared to other schools, 44.72±12.48 
µg/m3. 
On the contrary, the school No. 3 has 100% PM10 IAQ 
measurements in space with >2 m2 per child, located in a 
broader urban zone, with all measured values of IAQ PM10 
beyond 50 µg/m3. The average indoor PM10 concentration 
in it is 99.70±45.62 µg/m3. This school is cleaned mostly 
with the broom, and combined with the use of chemicals. 
This cleaning practice, being predominant as a cleaning 
pattern in this school, obviously facilitates the process of 
particle re-suspension (24).
As one of the defined elements of the thermal comfort 
zone, occupancy rate was chosen to be one of the 
key indicators in this study, together with CO2 indoor 
concentration, relative humidity, and classroom air 
temperature. Statistically significant smaller chances 
exist for formaldehyde indoor concentrations to be below 
1.01 mg/m3 in classrooms with more than 2 m2 of space 
per child. For indoor CO2 concentration to increase above 
1000 ppm, the number of children in the classroom above 
20 (N>20) is a statistically significant predictor. Namely, 
chances for CO2 concentrations to be higher than 1000 
ppm are 3.6-fold bigger in classrooms hosting more than 
20 children. For indoor (classroom) air temperature 
within a comfort zone, a statistically significant predictor 
is classroom crowdedness, i.e. space in square meters 
per child. In fact, chances for indoor temperature to 
be within the comfort zone are less likely to occur in 
classrooms with less than 2 m2 of space per child (4). 
For indoor values of relative air humidity, a statistically 
significant predictor is classroom crowdedness, i.e. space 
in square meters per child. In other words,, chances for 
this parameter to be within the comfort zone are close to 
18-fold higher in classrooms hosting less than 20 children.
Chances for PM10 to be below 50µg/m3 are smaller, with 
high statistical significance, in classrooms hosting more 
than 20 children, having a blackboard, with chalk to write 
on.
The average indoor PM10 concentrations are lower in 
the classrooms where indicators of thermal comfort zone 
are satisfactory. In the school No. 4, the average indoor 
PM10 concentration is lower in classrooms with achieved 
standards for indoor comfort zone indicators, with high 
statistical significance Z - test=6.540, p<0.0001, while 
in the schools No. 5 (Z=0.105, p=0.916), No. 8 (Z=1.614, 
p=0.107) and No. 10 (Z=0.948, p=0.343) it is lower, but 
with no such high significance. Significantly highest 
mean values of IAQ PM10 concentration was measured in 
the schools 1, 2, 3 and 8, where comfort zone was not 
achieved (p<0.001). 
10.1515/sjph-2017-0031 Zdr Varst 2017; 56(4): 227-235
234
Values of classroom indoor PM10 concentrations, from 
measurements in similar studies in different countries, 
during the heating season, are close to the values reported 
in this study in Serbia: in three schools in Portugal, PM10 
average concentrations ranged from 30 to 146 µg/m3; in a 
German study implemented in 64 urban and rural schools, 
it was 105 µg/m3 (16.3-313.2 µg/m3), while in HESE study, 
IAQ was monitored in 21 schools, both urban and rural, 
with the average PM10 concentration 112 µg/m3 (91-133 
µg/m3) (1, 10, 26). 
In most of the SEARCH1 countries, ambient PM10 
concentrations were significantly increased in school 
zones close to frequent traffic streets, compared to those 
located further from such sources of air pollution. On the 
other hand, this difference in the traffic frequency of 
streets surrounding schools, has not significantly influence 
IAQ PM10 concentrations measured in classrooms, pointing 
that key sources of this pollutant are mainly in classrooms 
themselves (15). In the case of Belgrade study, as a part 
of the mentioned research, 36.2% of pupils study in 
classrooms with I/О PM10 ratio beyond 1.0, where the key 
source of PM10 is within the room; 3.7% is in classrooms 
with the ratio equal to 1.0., while 60.1% attend classes 
in rooms with the ratio below 1.0, where the source of a 
particulate matter is in ambient air, mostly from traffic in 
the vicinity of school buildings.
Although the city is located close to two coal burning 
power plants (Obrenovac, Kolubara), traffic is seen as the 
most powerful source of air pollution. Air back trajectories 
analysis showed that the prevalence of stagnant or week 
flow regimes (calm conditions) favours the suspension and 
accumulation of particles produced locally, resulting at 
the elevation of suspended particles levels (20, 27). 
If we compare the location of Belgrade, on the banks of 
Sava River, with another region on its banks in Zasavje 
Region in Slovenia, with PM10 ambient air concentration, 
we spot some differences. Firstly, it is an issue of 
topography, as Zasavje is surrounded by steep hilly 
terrain, while Belgrade spreads in parts of the Pannonian 
Plain. Secondly, sources of air pollution are different in 
these two cases. In Zasavje, it is the case of industrial 
PM10 emissions (36.3µg/m3), while in SEARCH1 Belgrade 
study, traffic was attributed as the key source for high 
PM10 levels (104.7 µg/m3) (15, 28).
5 CONCLUSIONS
The majority of surveyed children is exposed to high indoor 
PM10 concentrations (560/735; 76.2%). Maximum PM10 
values were measured in suburban schools, away from 
busy traffic. The increase of outdoor PM10 concentration 
significantly affects the increase of indoor PM10 values. 
Concerning an insufficient achievement of standards for 
indicators of indoor thermal comfort zone, dominant 
factors for the increase of PM10 are: high occupancy rate 
in the classrooms (<2 m2 of space per child), high relative 
humidity (>75%) and indoor temperature beyond 23ºC; 
bad ventilation habits (keeping windows shut most of the 
time). 
As the authors suggest, measures for the improvement 
of conditions in classrooms are as follows: schools should 
be built in places not directly affected by heavy traffic 
or industry, or any other polluting establishments in 
the neighbourhood; crowdedness should be avoided in 
the classrooms; appropriate ventilation regime of the 
classrooms should be introduced in order to provide good 
indoor air quality during the whole period of teaching 
hours, especially in classrooms directed towards crowded 
streets with presumably high ambient air pollution; in 
schools being ventilated only through natural means, but 
located close to traffic-induced air pollution, installing 
air-conditioning units should be taken into consideration. 
Cleaning practices should be standardized.
ACKNOWLEDGMENTS
The financial support for the SEARCH project by the 
Italian Ministry for the Environment, Land and Sea is 
highly appreciated by the authors.
CONFLICTS OF INTEREST
The authors declare that no conflicts of interest exist.
FUNDING
Italian Ministry for the Environment, Land and Sea.
ETHICAL APPROVAL
In the case of the Republic of Serbia, if there is no 
direct contact with body fluids, tissues, personal contact 
concerning individual dignity, through questionnaires, 
researchers are not required to apply for the permission 
to research at the Ethical Committee. This study was not 
implemented on individual subjects, but was concerned 
only with the positioning of the standardized air quality 
measuring equipment in the classrooms and outside the 
same classrooms, for which, the REC project office had 
direct communication with school directors. Especially, 
when the moment of the equipment turning on/off was 
performed while classrooms were empty, with no physical 
contact with the pupils whatsoever.
10.1515/sjph-2017-0031 Zdr Varst 2017; 56(4): 227-235
235
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