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Background:
Systematic Review

The Impact of Outdoor Environmental Factors on Indoor Air Quality in Education Settings: A Systematic Review

by
Jan Rožanec
1,
An Galičič
1 and
Andreja Kukec
1,2,*
1
National Institute of Public Health, Trubarjeva ulica 2, 1000 Ljubljana, Slovenia
2
Faculty of Medicine, University of Ljubljana, Vrazov trg 2, 1000 Ljubljana, Slovenia
*
Author to whom correspondence should be addressed.
Atmosphere 2024, 15(12), 1403; https://doi.org/10.3390/atmos15121403
Submission received: 8 October 2024 / Revised: 8 November 2024 / Accepted: 20 November 2024 / Published: 22 November 2024
(This article belongs to the Section Air Quality)
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Abstract

:
Poor indoor air quality (IAQ) in schools is associated with pupils’ health and their learning performance. This study aims to provide an overview of the outdoor factors that affect the IAQ in educational settings in order to develop public health measures. We conducted a systematic literature review to investigate the outdoor factors that affect IAQ in educational settings. The selection of articles included 17,082 search string hits from the ScienceDirect database published between 2010 and 2023, with 92 relevant studies selected based on the inclusion and exclusion criteria. Based on a systematic review of the literature, we identified the following outdoor factors: proximity to busy roads, commercial and industrial establishments, meteorological conditions, compounds from the natural environment, emissions from heating buildings, atmospheric reactions and secondary pollutants, unpaved school playgrounds, and smoking. This study provides key information on the mentioned outdoor factors and gives recommendations on measures to reduce classroom pollutant concentrations while highlighting educational settings that require special attention. Our study shows that classroom IAQ is affected by many outdoor pollutant sources, the prevalence of which depends on the educational setting’s micro location. Therefore, it is essential to develop an appropriate classroom ventilation strategy for each educational setting.

1. Introduction

Indoor air quality (IAQ) has a major impact on public health, as people spend a lot of time indoors, and exposure to air pollutants has important health effects, especially on the respiratory and cardiovascular systems [1]. The populations potentially at greatest health risk are pre-school and school children. According to the data from the United States Environmental Protection Agency (US EPA), children and adolescents ages 13 years or younger are classified as a vulnerable population group. Children’s greater sensitivity to air pollutants is based on the fact that their lungs, immune systems, and other organs are not yet fully developed, which makes them more susceptible to respiratory diseases and hypersensitivity reactions [2]. Children are also more exposed to environmental pollutants compared to adults because they have a higher respiratory rate and breathe larger amounts of air relative to their body weight [3]. On average, children spend about 65–90% of their time in indoor environments, with a large part of this time spent in educational settings [4]. Therefore, it is important to provide a healthy learning environment in educational settings, which includes good IAQ in the classroom. Poor IAQ in schools further has a negative impact on pupils’ cognitive abilities, development, well-being, concentration, and school attendance [1].
IAQ in classrooms is affected by both indoor and outdoor environmental factors. In addition to indoor emissions from classroom activities, materials and equipment, cleaning processes, etc., a major source of air pollution in classrooms originates from the outdoor air [5]. Due to the various outdoor pollutant sources, outdoor air mainly consists of air pollutants including nitrogen oxides (NOx), sulphur dioxide (SO2), ozone (O3), carbon monoxide (CO), hydrocarbons, and particulate matter (PM) of different particle sizes [6]. Exposure to PM, nitrogen dioxide (NO2), and O3 is an increasingly significant health risk for vulnerable populations, which is most pronounced in urban areas where concentrations of other pollutants are usually also elevated [7,8]. When addressing the IAQ in educational settings and the interventions of the relevant institutions, it is necessary to additionally consider the wider social and building environment. Some schools may be located in highly polluted areas; near highways, power plants, and industrial sites; or even built on old industrial sites [9]. As a good practice example, Davila et al. [10] established an alert system to provide alerts about poor outdoor air quality in Zagreb, Croatia. The system provides information to vulnerable populations about outdoor air quality and raises awareness of environmental and health issues. Brdarić et al. [11] highlighted the problem of elevated concentrations of carbon dioxide (CO2) in classrooms in Croatian primary schools caused by poor ventilation during the heating season. When planning efficient natural ventilation of classrooms, it is important to take into account the outdoor IAQ factors that might otherwise affect the health of school-age children and teachers.
The outdoor environment is thus an important factor that cannot be neglected in IAQ studies, as was also found in a literature review conducted by Leung et al. [6]. Outdoor air pollutants are introduced indoors through natural and mechanical ventilation as well as infiltration through gaps and leaks in doors, walls, and windows. The number of outdoor pollutants introduced to the indoor environment depends on the ventilation and infiltration rate of the outdoor air and depends on the existing indoor and outdoor air quality [6]. This means that outdoor pollutants are continuously transferred to the indoor environment, with most pollutants entering the indoor environment through natural ventilation. The data from the SINPHONIE project [12] shows that between 2010 and 2012, the majority of schools (86%) in Europe used natural ventilation, 7% used mechanical ventilation, and 7% used hybrid ventilation. Knowing the outdoor factors is thus important because when they are taken into account, fewer outdoor air pollutants enter the classroom during the ventilation period.
The field of IAQ in educational settings is well researched, reflecting the growing awareness about the need to provide a healthy environment for children [13]. Most of the studies that have investigated outdoor pollutant sources and indoor air pollutants in educational settings have been conducted in urban environments and in countries where the problem with outdoor air quality is already known, and consequently, there is a greater awareness about the importance of IAQ in different indoor environments. IAQ measurements in educational settings in European Union countries have been carried out less often, suggesting the lower attention to this issue, despite the awareness of its importance. In addition, the lack of measurement of IAQ in classrooms makes it difficult to assess the actual situation of IAQ in educational settings and the prevalence of this problem in Europe.
Several literature reviews have been conducted on the topic of IAQ in educational settings [4,13,14,15], where studies have not only summarised the findings of other studies on IAQ in school buildings but also explored the related social and health impacts on pupils and staff. Previous literature reviews have focused mainly on the individual air pollutants in the classroom, where although IAQ factors are mentioned, they are not fully discussed. In addition, indoor IAQ factors are better understood compared to outdoor IAQ factors, as their effect is easier to associate with air pollutants in the classroom. For this reason, this study aims to provide an overview and description of the outdoor factors that affect IAQ in educational settings in order to develop public health measures for a population that is vulnerable from an air quality perspective. This study is important because understanding the impact of these factors gives us important insights into the issues discussed and provides a focus on the most important issues related to reducing indoor pollutant concentrations in educational settings.

2. Materials and Methods

Outdoor factors that affect the IAQ were identified based on a systematic literature review in the ScienceDirect database. The purpose of the literature review was to identify the outdoor factors that affect the IAQ in education settings, which include preschool, primary, and secondary education. The search term used was “IAQ” OR “IAP” OR “indoor air” AND “school” OR “classroom” OR “kindergarten” OR “primary education” AND “risk factors” OR “environmental factors” for the period from 2010 to 2023. The literature search was conducted twice following the same procedure. First, we conducted a systematic literature review in 2020 for the period from 1 January 2010 to 1 December 2020. To obtain the latest available evidence, we conducted another systematic literature review in 2023, where we additionally included studies in the review for the period from 1 January 2020 to 14 December 2023. In a second search, we included the whole year 2020, despite its previous inclusion, as new articles were published at the end of the year that we did not want to miss in our systematic literature review. The literature was selected based on inclusion and exclusion criteria, which were designed according to the purpose of the literature review. The screening of the results of the selected search term was performed in five steps and according to the Preferred Reporting Items for Systematic review and Meta-Analysis (PRISMA) [16]. The first step included all hits for the search string from the selected database, with each following step introducing additional inclusion and exclusion criteria. The inclusion and exclusion criteria for each step are shown in Table 1.
The selection process was carried out by the three authors of this study, where each author rated the article with a score of “Yes”, “No”, or “Maybe”. This was done for each step of the systematic literature review, and between the steps, all three authors would meet and discuss their decisions with a common consensus on which articles to include in the review. The quality of the selected articles was not evaluated during this selection process.
The articles selected for the review were then extracted to obtain the data on the study, outdoor IAQ factor, country, study area, observed pollutants, monitoring timeline, data collection methodology used to identify sources, and the main findings. The themes in the discussion are based on the outdoor IAQ factors found in the educational settings’ environment from the studies, which we identified in our systematic literature review.

3. Results

The selection of articles for the systematic literature review from the ScienceDirect bibliographic database included 17,082 search string hits published between 1 January 2010 and 14 December 2023. Of these, 92 relevant studies were selected based on the inclusion and exclusion criteria. A PRISMA flow chart of the selection of articles for the systematic literature review is shown in Figure 1.
The most frequently identified outdoor factor that affected IAQ in educational settings was the proximity to busy roads (n = 78), which was followed by commercial and industrial establishments (n = 43), meteorological conditions (n = 30), compounds from the natural environment (n = 26), emissions from heating buildings (n = 25), atmospheric reactions and secondary pollutants (n = 13), unpaved school playgrounds (n = 7), and smoking (n = 6). Compared to Europe (n = 15), the majority of studies were conducted in countries outside the European Union (n = 77), with most studies conducted in Portugal (n = 14) and Spain (n = 11). Among the 92 studies included in the literature review, the majority of the studies investigated the urban area (n = 82) followed by the suburban area (n = 23), the rural area (n = 17), and the industrial area (n = 13). Studies investigating classroom IAQ and potential factors that affect IAQ were conducted during all seasons; however, almost no studies were conducted during the summer holidays, i.e., July and August. All studies measured classroom IAQ, while 80 studies also measured outdoor air quality in the surrounding area of the educational setting. Each study combined multiple methods to identify the sources. The most common method used to support the identification of factors was statistical analysis of the collected data (n = 81) followed by element/component analysis (n = 56), source-directed modelling (n = 22), visual inspection (n = 15), questionnaire (n = 6), interview with employees (n = 6), and traffic count (n = 4). However, the frequency of methods used to determine the source may not be precise, as the authors of the studies did not always explicitly mention all the methods used. The results and backgrounds of the studies included in the literature review on the outdoor factors that affect IAQ are shown in the online Supplementary Materials, Table S1.

4. Discussion

Our systematic literature review shows that outdoor air has a significant impact on IAQ in educational settings [5,17,18,19]. Outdoor air quality depends on the location of the educational building, its surroundings, and meteorological conditions [5,20,21]. The most common pollutants that affect the outdoor air quality of schools’ environments include pollution from traffic, industrial establishments, and building heating, while unpaved school playgrounds and smoking were also identified to be important sources of pollutants, despite their low prevalence. The number of epidemiological studies on each outdoor factor that affects IAQ in educational settings might reflect the mostly urban environment where most studies were conducted. Emissions from these sources can impact the outdoor air quality of educational settings and consequently impact the IAQ in the classrooms. Outdoor air pollutants are introduced indoors by the ventilation of the rooms through windows or ventilation systems; infiltration of pollutants through cracks in the building; and by the transfer of outdoor particles/dust to the indoor environment on shoes, clothes, hair, etc. [5,22,23,24].

4.1. Traffic

The proximity of busy roads to educational settings is a growing problem, as the traffic on the roads is constantly increasing year after year, while educational settings are often located in larger urban areas, which makes them highly exposed to traffic pollution. In educational settings, researchers have associated traffic pollutants with increased concentrations of particles with an aerodynamic diameter of 10 μm (PM10); particles with an aerodynamic diameter of 2.5 μm (PM2.5); UltraFine Particles (UFPs); NO2; CO; Total Volatile Organic Compounds (TVOCs); Polycyclic Aromatic Hydrocarbons (PAHs); black carbon (BC); and elements in PM such as copper (Cu), antimony (Sb), tin (Sn), zinc (Zn), and iron (Fe) [1,5,17,18,19,20,21,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93]. Elevated concentrations of traffic pollutants mainly occur during the morning and afternoon traffic rush hours and when parents bring/take their children from educational settings [5,34]. During peak hours, Reche et al. [35,36] measured 45–85% higher concentrations of BC and the number of particles concentration in outdoor air compared to the concentrations measured before peak hours. Reche et al. [35] found that indoor concentrations of BC increased by 50–85% 1 h after the beginning of heavy traffic due to the infiltration of UFP from the outdoor air. Similar findings were also reported by Buonanno et al. [37], who found that indoor concentrations of UFP increased during peak hours due to the infiltration of particles and the formation of secondary particles indoors. At the end of heavy traffic, outdoor concentrations of PM decrease rapidly, while indoor concentrations can remain elevated for longer periods due to low air exchange [37].
To assess the impact of traffic pollutants on IAQ in classrooms, it is necessary to evaluate the effect of three different factors: the distance between the educational setting and the nearest road, the traffic density, and the orientation of the observed classroom relative to a busy road [35]. Rim et al. [19] associated schools that were located closer to a busy road with higher concentrations of PM2.5, PM10 and BC. Similar findings were reported by Majd et al. [18], who associated a lower distance of schools from a busy road with higher indoor concentrations of CO and NO2. For each increase in the length of all roadways within 100 m of the school in the interquartile range, indoor NO2 exposure increased by 66%. Moreno et al. [38] and Reche et al. [35,36] found that schools located near roads with a higher traffic density tended to have 14–67% higher indoor concentrations of traffic pollutants in the classroom compared with schools in locations with lower traffic density. Higher concentrations of traffic pollutants may also occur when traffic moves more slowly due to traffic congestion and nearby traffic lights [39,40]. Based on the orientation of the classroom, higher indoor concentrations of traffic pollutants occur in classrooms oriented toward the road compared to those oriented toward the schoolyard or playground. Reche et al. [36] observed 26–78% higher nitrogen (N) concentrations in schools with classrooms oriented directly towards the street compared to classrooms oriented towards the schoolyard. However, it is important to consider that most of these schools, which had classrooms directly oriented towards the street, also had the highest impact of traffic emissions. Moreover, Amato et al. [26] found significantly higher PM concentrations in classrooms oriented directly towards the street compared to classrooms oriented towards the interior of the block and towards both unpaved as well as paved playgrounds. The researchers also found an association between the micro location of the school and the concentration of traffic pollutants. Higher concentrations of traffic pollutants were measured in educational settings in urban areas compared to those located in suburbs or rural areas [5,35,37,41]. Portela et al. [5] found that outdoor concentrations of nanoparticles and BC were four and three times higher in urban areas than in rural areas. Concentrations of traffic pollutants were higher mainly due to the proximity to busy roads such as highways and denser traffic. Concentrations of traffic pollutants within an urban area vary considerably depending on the traffic density at the location. The highest particle number concentrations were measured by Reche et al. [36] in the city centre (the main source of traffic pollutants), which decreased with distance from the city centre. In lower-traffic-density urban areas, Becerra et al. [27] associated lower outdoor concentrations of traffic pollutants with better IAQ in classrooms. Compared to urban schools, schools in suburbs showed lower and more constant concentrations of traffic pollutants [37].
The selection of an educational setting’s location therefore has a significant impact on the concentrations of traffic pollutants in classrooms. Special attention should be given to educational settings located in the city, where the effects of traffic pollutants are the greatest. Educational buildings should be located as far away as possible from busy roads, highways, and roads where traffic moves more slowly due to congestion and nearby traffic lights. Classroom ventilation should be adjusted depending on the traffic rush hours and the traffic density in the vicinity of each educational building. We suggest ventilating classrooms early in the morning before the start of traffic rush hours and about a half hour after the end of the rush hour. Ventilation after the end of rush hour is very important, as indoor concentrations of traffic pollutants can remain elevated for long periods due to low levels of air exchange. Besides classroom ventilation during rush hours, we do not recommend ventilating classrooms when parents bring/take their children from educational settings. Classrooms oriented towards the schoolyard compared to classrooms oriented on a busy road can be ventilated more frequently due to lower traffic pollutant burden; however, we still do not recommend ventilating during traffic rush hours.

4.2. Industrial Establishments

The expansion of residential areas, including educational settings, in the proximity of industrial establishments, and the locating of new industries in these areas, can contribute significantly to the occurrence of very harmful pollutants in the classrooms. Industrial pollutants have been associated in education settings with increased concentrations of PM10; PM2.5; UFP; CO; TVOCs; PAHs; and elements in PM such as arsenic (As), Cu, chromium (Cr), manganese (Mn), and Sb in the classrooms [5,18,20,21,23,24,26,27,29,30,32,33,42,43,44,46,48,49,50,51,53,55,57,58,62,63,67,69,70,72,73,74,75,76,80,84,87,90,92,94,95,96,97]. The presence of different types of industrial establishments in the surroundings of educational settings contributes to some specific pollutants. Tran et al. [43] found higher elemental concentrations of indoor and outdoor PM1, PM2.5, and PM10 in a school about 1.5 km away from several industries such as steel industries, fuel and lubricant refineries, power plants, and cement plants. The petrochemical industry was associated with higher PM10 concentrations of Ni and V. Ferromanganese (MnO2) production plants as well as the steel and cement industries have been associated with higher PM10 concentrations of Fe and Mn in the PM [43]. Petrochemical plants 5 to 6 km away from city schools were associated with higher concentrations of benzene, although the measured maximum benzene concentration was generally low [21]. Coal-fired power plants have been associated with pollutants such as SO42−, aluminium (Al), fluoranthene, and pyrene [30,44]. Heavy industry has been associated with emissions of Zn and Sb [20]. The cellulose industry has contributed to PM emissions [5]. A bakery industry located 110 m from the school, which was based on the combustion of wood biomass, contributed to the emission of potassium (K), Zn, and bromine (Br). The proximity of the school to the bakery resulted in higher PM concentrations, especially at night, while PM concentrations in the classrooms during occupied periods were consistently above the WHO guidelines due to the combined effect of other factors [94]. Depending on the micro location, studies [21,43] measured the higher concentrations of industrial emissions in the indoor air of schools in industrial and urban areas compared to rural areas. This was due to a higher number of industrial establishments within 1 to 2 km of urban schools [18]. However, due to the long-range transport of industrial pollutants, Portela et al. [5] found that industrial pollutants can also be detected in rural areas. A greater distance of schools from industrial establishments has a significant impact on improving classrooms’ IAQ and providing a healthier environment for children [18].
The selection of an educational setting’s location relative to existing industrial establishments is also very important to ensure a healthy environment for children. Special attention should be given to educational settings located in the city and the vicinity of industrial areas, as there are usually a greater number of potential industrial establishments located there. Further, we do not recommend locating an educational setting on old industrial sites, as dust from contaminated soil can also be transported into the classroom. However, it is not only the number and distance of industrial establishments from educational settings that is important but also the type of industry that has an important role in the IAQ of classrooms. Some industrial establishments may have only a small impact on classroom IAQ, despite their proximity to educational settings, while others may be problematic due to their harmful emissions, despite their greater distance. When planning the ventilation of educational settings, we recommend avoiding classroom ventilation during periods when the identified industry of concern is producing the highest concentrations of pollutants. Special attention should be given to industrial establishments that are located less than 5 km from educational settings, depending on their activity.

4.3. Unpaved School Playgrounds

The use of unpaved school playgrounds in education settings has been associated with increases in both outdoor and indoor concentrations of PM [45]. PM originating from unpaved playgrounds contain a high content of mineral elements such as Al, sodium (Na), Mn, magnesium (Mg), lithium (Li), Fe, calcium (Ca), Cu, titanium (Ti), rubidium (Rb), and Sn [26,38,45,46,74,75,98]. During activity on the playground, mineral particles from the playground are suspended in the air and can be infiltrated indoors by ventilating or transported indoors on footwear, clothing, and hair [38,46]. The location of the unpaved school playground and the distance from the classroom also have a significant impact on the indoor concentrations of the mineral particles. As the distance of the unpaved playground from the classroom increases, the concentration of mineral particles in the classrooms significantly decreases. The decrease in concentration is even more pronounced on paved playgrounds, where as compared to unpaved playgrounds, 80% fewer mineral particles are suspended [26,38]. Amato et al. [26] found that the presence of unpaved playgrounds contributed on average to an increase of 5–6 µg/m3 in mineral particles in classrooms compared to schools with paved playgrounds.
The presence of unpaved school playgrounds, as well as other unpaved areas (e.g., unpaved roads), contributes to elevated PM concentrations in classrooms. By replacing gravel surfaces with grass or asphalting the surfaces around schools, we can contribute to improving the IAQ of educational settings. We recommend that classrooms of educational settings not be ventilated at times when increased use of outdoor unpaved surfaces near the classroom takes place, as this may increase indoor concentrations of PM due to dust generation and infiltration into the classrooms. It is also important that children should change their shoes as soon as possible when entering the educational building after outdoor activities to reduce the transmission of PM inside the educational setting.

4.4. Emissions from Heating Buildings

The most significant impact of pollutants originating from building heating in education settings has been observed by researchers during the autumn and winter heating seasons [5,18,20,23,29,33,39,47,48,49,52,54,55,59,68,72,78,83,93,99]. Emissions from the heating of buildings in areas with wood biomass have been associated with increased concentrations of PM and elements in PM such as K, Zn, and Br and have an impact on the formation of secondary particles [20,94]. The combustion of wood biomass, heating with coal, peat, and natural gas also has an impact on PAH emissions [29]. Emissions from wood biomass combustion were associated with three-ring PAHs such as fluorene, acenaphthene, phenanthrene, and anthracene [30,44]. The incomplete combustion of biomass results in the emission of BC [5]. Furthermore, household coal stoves have an impact on concentrations of toluene and benzene [47]. The association between micro location and pollutants from heating buildings was observed by Canha et al. [20], who found that concentrations of K, which is the typical source of wood biomass heating emissions, were 10–18% higher in rural areas compared to urban areas. Ward et al. [99] conducted a study on concentrations of PM in school before and after replacing an old school wood stove with a more modern heating system. The results of the study showed that replacing an old wood-burning stove with newer, cleaner heating systems in schools does not have a measurable improvement on IAQ in classrooms. From this, we can conclude that the type of heating systems in the area mainly determines the IAQ in educational settings during the heating season, while the school heating system itself has only a minor impact on IAQ relative to the total emissions from heating systems in the area.
Educational settings located outside the tropical climate are subject to emissions from heating buildings during periods when temperatures are low. The type of building heating pollution and its concentration depend mainly on the predominant heating method of residential and other buildings (e.g., commercial buildings) in the vicinity of the educational setting. We recommend that the installation of newer and cleaner heating systems is promoted in these areas. For classroom ventilation, we recommend that ventilation during the heating period should be more frequent at times when there is no temperature inversion outside and wind speeds are higher, which is when outdoor concentrations of pollutants are lower.

4.5. Other Outdoor Sources of Pollution

Researchers have also identified other outdoor sources of pollution in their studies. Emissions of substances from the natural environment, such as road dust, unpaved roads and areas, pollen (proximity to tree plantations), and long-range aerosol transport (desert particles, dust storms, sea salt aerosols), were all associated with an impact on the indoor concentration of PM [1,17,20,23,24,26,27,30,32,33,43,45,48,49,50,53,58,59,78,100]. Agricultural activity within the proximity of schools also contributes to increases in indoor concentrations of PM2.5; PM10; and elements in PM such as K, soil elements, and heavy metals, with the greatest impact during the sowing and soil covering stages. During the cultivation period, soil dust from agricultural activities could also be associated with pesticides [51]. Further, in educational settings, researchers have detected traces of pollutants from smoking in PM [23,30,33,101,102,103]. Although smoking is prohibited in schools, contaminants from smoking can be transferred indoors on clothing, hair, and skin [101,103]. Cecinato et al. [101] found higher indoor concentrations of caffeine and nicotine in schools where coffee machines were located. The impact of outdoor smoking on IAQ in the classroom is more prevalent in secondary schools compared to primary schools, where Hu et al. [103] detected high concentrations of smoking pollutants at the start of school and after breaks. This reflects the fact that smoking among adolescents is occurring at even younger and younger ages, with individuals from secondary schools already smoking in addition to teachers. Hu et al. [103] also found that residues from smokers’ clothes and hair may further increase indoor formaldehyde concentrations by 55.3% in addition to occupant activity within 1 h. The possible exposure to radon (Rn) in the classroom, located in the basement and at the ground floor of the building, should not be overlooked. The potential occurrence of Rn in educational settings depends mainly on the geographical location of the building, and it is transferred from the soil into the building if the building envelope in contact with the ground is not tight [104]. Outdoor air is also a potential source of fungal aerosols, which tend to have higher concentrations in outdoor air compared to indoor air, especially during warmer weather [105,106].

4.6. Meteorological Conditions

Due to meteorological factors and increases in local emissions, concentrations of outdoor pollutants are usually higher in winter compared to summer. During the winter period, the researchers measured elevated outdoor concentrations of PM10; PM2.5; UFP; particle number concentrations; NO2; SO2; PAHs; BC; and elements in PM such as Ca, Fe, Mn, etc. relative to the rest of the warm seasons [5,28,34,37,39,42,44,52,53,54,55,62]. Elevated outdoor concentrations of pollutants in winter are due to a more stable atmosphere and consequently a lower air mixing layer, which leads to the accumulation of pollutants at ground level [5,39]. Lower relative air humidity and lower precipitation in winter also contribute to higher outdoor concentrations of pollutants, which consequently contribute to higher indoor concentrations of pollutants [55]. In addition to meteorological impacts, during the winter period, higher concentrations of outdoor pollutants are expected due to increased heating demand and increased exhaust emissions from motor vehicles [5,18,39,52]. The reason motor vehicle emissions of pollutants increase during winter is that vehicle catalytic converters take longer to reach their operating temperature [39]. In contrast to the winter period, during the summer period, researchers have detected elevated outdoor concentrations of O3, formaldehyde, and TVOC [28,42,54]. Outdoor concentrations of O3 are higher during summer due to more favourable weather conditions [54]. Higher temperatures and photochemical reactions with O3 in summer lead to elevated outdoor concentrations of formaldehyde [42,54]. The photochemical induction in the summer has been shown to increase the outdoor concentrations of secondary particles [5]. Elbayoumi et al. [17], Majd et al. [18], and Portela et al. [5] found that changes in outdoor concentrations of pollutants due to classroom ventilation also directly affect the seasonal occurrence of pollutants indoors.
The difference between indoor and outdoor temperatures has a significant impact on air movement and consequently on the indoor concentrations of PM. When the indoor air temperature is higher than the outdoor temperature, air with pollutants moves from the inside of the building to the outside, thereby reducing indoor concentrations of pollutants [17,34,45]. However, higher indoor temperatures also lead to higher concentrations of pollutants originating from different materials and equipment, resulting in higher indoor concentrations of formaldehyde and PAHs [21,29]. In the case of reduced room ventilation at lower indoor temperatures relative to the outdoor temperature, Chithra and Shiva Nagendra [34] and Agarwal and Shiva Nagendra [56] detected higher concentrations of PM in classrooms.
Relative air humidity has been associated with lower indoor and outdoor concentrations of PM in the air. Higher relative air humidity has the effect of removing particles from the air and reducing the amount of resuspended dust, as the outside surfaces remain moist [24]. Particle sizes depend on the relative air humidity of the environment, with the particle size increasing with higher air humidity. A pronounced change in particle size was observed at an air humidity above 60% [36]. Higher relative air humidity causes UFP to combine into fine and coarse particles such as PM10, PM2.5, and particles with an aerodynamic diameter of 1 μm (PM1) [34]. In addition to relative air humidity, precipitation also affects outdoor concentrations of pollutants and indirectly indoor concentrations in the classroom. Precipitation reduces outdoor concentrations of pollutants by washing out or absorbing the pollutants from the air [17]. Elbayoumi et al. [45] found that a lack of precipitation in an area, especially during the autumn and spring periods, promotes the elevation of road dust and dust from unpaved playgrounds, which are a permanent reservoir of fugitive dust.
Wind speed has been associated with a decrease in concentrations of PM and CO in the air [5,17,18,34,39]. Higher wind speeds increase the dispersion of pollutants in the outdoor air and consequently have an impact on lower concentrations of pollutants in the indoor environment [18,34]. Majd et al. [18] observed that average indoor concentrations of CO and PM decreased by 3.5–4% with a 1 km/h increase in wind speed outdoors. Elbayoumi et al. [17] and Agarwal and Shiva Nagendra [56] pointed out that increased wind speeds in environments with high concentrations of outdoor air pollution and desert climates could be associated with greater indoor penetration of pollutants. Nevertheless, low wind speeds are associated with the accumulation of pollutants and with more stable atmospheric conditions [39].
In addition to outdoor pollutant sources, meteorology and the wind conditions of the area where the educational setting is located have an important effect on the outdoor air quality and consequently on the classroom IAQ. Special attention should be given to educational settings located in cities and condensed urban areas, where the density of construction and height of the buildings results in worse wind flow compared to other micro locations, leading to an accumulation of outdoor pollutants in the outdoor air. During winter, special attention should also be given to educational settings that are geographically located in valleys and basins, where temperature inversions occur due to more stable atmospheric and other meteorological conditions. The quality of the outdoor air is worse during the winter compared to the summer; however, classroom ventilation during winter is still important as it reduces the concentrations of indoor pollutants caused by IAQ factors. It is important to point out that the frequency of classroom ventilation during the heating season can be significantly reduced due to unfavourable outdoor meteorological conditions. During periods of temperature inversion in winter, we recommend that instead of not ventilating classrooms, classrooms should be ventilated for a shorter duration at normal frequency.

4.7. Strengths and Limitations

Our study provides a systematic approach to the integrated identification and description of the outdoor factors that affect the concentration of indoor pollutants in education settings. In addition, we have also used this information to provide our summary of key information and our recommendations on measures to reduce classroom pollutant concentrations. Furthermore, we have highlighted educational settings in certain micro locations that require special attention regarding specific outdoor IAQ factors. The limitation of our study is that the outdoor IAQ factors discussed are limited to studies that were obtained by a systematic literature review in the bibliographic database ScienceDirect for the selected period. Not including other studies outside the systematic review that do not refer to findings in educational settings may limit the understanding of these factors. Further, the quality of the selected articles was not evaluated in the selection process, which could have an impact on the findings of our study. However, our results are supported by a larger number of reviewed studies included in our systematic literature review with similar conclusions. For the interpretation of the results of this study, it is important to bear in mind that individual factors cannot be directly linked to the measured pollutants in the classroom, as these pollutants depend on many factors, both indoor and outdoor. Due to this limitation, we also did not compare the magnitude of the effect of each factor on concentration, as this would not be reasonable and could be highly biased.

4.8. Current Research Gaps

In our literature review, we found that studies in the field of IAQ factors in educational settings usually only investigate the current state of classroom IAQ, where they compare the concentrations of selected pollutants with IAQ standards while identifying the possible pollutant sources. Current research shows a gap in information about how to improve IAQ in the observed educational settings using the study data. Further, some studies did not measure outdoor air quality [20,28,56,79,94,95,96,97,98,100,103,104], which makes assessing the impact of the outdoor environment on classroom IAQ in these studies difficult. Based on existing studies, it is difficult to compare which factors contribute to higher exposures to air pollutants in classrooms. Therefore, it is necessary to monitor the impact of several factors at the same time; good examples are the studies by Reche et al. [36] and Amato et al. [26]. Another limitation of some existing studies is the short duration of air quality measurements in educational settings or the fact that measurements were only conducted one season [1,21,24,25,27,32,37,41,43,44,50,52,54,56,58,62,63,69,71,76,78,80,81,82,83,86,87,89,92,94,97,98,100,102]. Due to the complex nature of the different outdoor factors, we encourage IAQ studies be conducted in educational settings during different seasons of the year or year round. Only these studies with long-term monitoring of pollutants in educational settings can provide information on the exposure of children and adolescents to pollutants in the classroom throughout the year.

4.9. Further Research

Our study significantly contributes to the understanding and identification of possible outdoor IAQ factors associated with educational settings for the development of targeted public health measures. To provide a complete overview, future research should investigate other outdoor environment factors that affect the indoor environment, such as the effect of intense sunlight, solar radiation, high temperatures, and a high UV index. The synergistic effects of outdoor and indoor factors that affect classroom IAQ and thus children’s health should also be considered. Further research should try to improve the guidelines for educational settings in terms of their placement in a specific micro location and the optimisation of ventilation strategies according to the identified outdoor pollutant sources. Based on this, it would be reasonable to conduct the intervention by developing a ventilation strategy for educational buildings taking into account our findings and analysing the classroom IAQ before and after the implementation of the intervention. This would test the effectiveness and relevance of designing an adequate ventilation strategy in relation to the identified sources of pollutants in the vicinity of educational buildings. Last but not least, the promotion of awareness in educational settings on the importance of designing ventilation according to potential indoor and outdoor sources of pollutants is needed to ensure a healthy environment for children.

5. Conclusions

We conclude that a number of outdoor environmental factors are important for IAQ. The most frequently identified outdoor factor that affects IAQ in education settings was the proximity to busy roads and industrial establishments. In addition to the different outdoor sources of pollution, meteorological conditions have an important impact on the outdoor concentrations of pollutants.
An important public health action is to raise awareness of the fact that outdoor pollutants directly affect IAQ, mainly through classroom ventilation as well as through the infiltration of pollutants indoors and the transfer of particles/dust from the outdoors to the indoor environment. If an education building is located in an environment with high outdoor air pollution or in proximity to outdoor sources of pollutants, the ventilation may have a negative impact on the classroom’s IAQ. However, this does not mean that ventilation is not recommended in such environments but that we should be more cautious when ventilating rooms and consider the characteristics of each micro location. Therefore, an appropriate classroom ventilation strategy should be developed, taking into account the different factors that vary depending on the location. The classroom ventilation strategy needs to be adjusted to each educational building individually since each building is surrounded by different pollutant sources. If information on the outdoor air quality of the educational setting is available, it should be taken into account in the ventilation strategy, where the frequency and length of ventilation should be adjusted accordingly. When planning the location of new education buildings, it is important to consider the sufficient distance from sources of pollutants.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/atmos15121403/s1, Table S1: The results and background of the studies included in the literature review on the outdoor factors that affect indoor air quality.

Author Contributions

Conceptualization, A.G.; methodology, A.G., J.R. and A.K.; statistical analysis, A.G. and J.R.; writing—original draft preparation, J.R. and A.G.; writing—review and editing, A.K.; visualization, A.G. and J.R.; supervision, A.K. All authors have read and agreed to the published version of the manuscript.

Funding

This article was funded by grant No. P3-0429 (Slovenian Research Agency: ARIS).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author(s).

Acknowledgments

The study was conducted as part of an ARIS project entitled “Development of the prognostic model of exposure to indoor air pollutants in schools and preparation of evidence-based measures for planning of efficient natural ventilation of the classrooms (No. V3-1904)”.

Conflicts of Interest

The authors declare no conflicts of interest.

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Atmosphere 15 01403 g001
Figure 1. A flow chart of the selection of articles for the systematic literature review.
Figure 1. A flow chart of the selection of articles for the systematic literature review.
Atmosphere 15 01403 g001
Table 1. Systematic literature review process.
Table 1. Systematic literature review process.
Literature Search StepsInclusion CriteriaExclusion Criteria
Step 1
ScienceDirect database search string hits		All hits for the search term until 1 December 2020 (first search)/14 December 2023 (second search)./
Step 2
Restriction of the search string		All the inclusion criteria from the previous step with the addition of:
-
Search by: title, abstract, or author keywords;
-
Restriction by year: 2010–2020 (first search); 2020–2023 (second search);
-
Restriction by article type: research article;
-
Subject: environmental sciences, medicine and dentistry, engineering.
/
Step 3
Screening by article title		All the inclusion criteria from the previous steps with the addition of:
-
Unit of observation: school or kindergarten;
-
The article investigates indoor air quality or outdoor potential factors that affect the indoor air quality (e.g., traffic, industry);
-
Language: English.
-
Article researchers did not investigate indoor air quality and the unit of observation was not a school or kindergarten;
-
Articles not written in English.
Step 4
Screening by abstract		All the inclusion criteria from the previous steps.All exclusion criteria from the previous step.
Step 5
Screening of the entire content		All the inclusion criteria from the previous steps.All exclusion criteria from the previous steps with the addition of:
-
The study did not investigate outdoor factors that affect the indoor air quality;
-
The article discusses computer modelling in a simulated classroom.
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MDPI and ACS Style

Rožanec, J.; Galičič, A.; Kukec, A. The Impact of Outdoor Environmental Factors on Indoor Air Quality in Education Settings: A Systematic Review. Atmosphere 2024, 15, 1403. https://doi.org/10.3390/atmos15121403

AMA Style

Rožanec J, Galičič A, Kukec A. The Impact of Outdoor Environmental Factors on Indoor Air Quality in Education Settings: A Systematic Review. Atmosphere. 2024; 15(12):1403. https://doi.org/10.3390/atmos15121403

Chicago/Turabian Style

Rožanec, Jan, An Galičič, and Andreja Kukec. 2024. "The Impact of Outdoor Environmental Factors on Indoor Air Quality in Education Settings: A Systematic Review" Atmosphere 15, no. 12: 1403. https://doi.org/10.3390/atmos15121403

APA Style

Rožanec, J., Galičič, A., & Kukec, A. (2024). The Impact of Outdoor Environmental Factors on Indoor Air Quality in Education Settings: A Systematic Review. Atmosphere, 15(12), 1403. https://doi.org/10.3390/atmos15121403

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