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Indoor environmental quality (IEQ) relates to the conditions that exist within a building; these include indoor air quality (IAQ), thermal conditions, visual (e.g., lighting) and aural (e.g., noise) comfort, and their potential effects on building occupants. Classrooms have more students per square meter than other buildings such as offices, making them more crowded. Poor IEQ has been shown to influence learning and concentration, as well as impact the long-term health of students. A continuous classroom IEQ research review and appraisal are needed to summarize existing knowledge, which can help other researchers avoid research duplication and identify research gaps for future studies.
- school
- ventilation
- thermal comfort
- mold and moisture
- cleanliness
- indoor environmental quality
1. Introduction
Primary school education is essential for children; this basic education is compulsory for all children in most countries of the world and requires them to spend several hours in schools. It is important to investigate the school environment, to know what might affect students’ learning, health, and wellbeing. Pupil health encompasses the physical, mental, and emotional well-being of students, which is vital for optimal learning and personal development, while academic performance pertains to a student’s attainment and accomplishments in educational pursuits, encompassing their grades, test results, engagement in class, and overall grasp of the curriculum [1].
2. Ventilation and IEQ in Schools
Ventilation involves the removal of noxious indoor air and the supply and distribution of fresh (outdoor) air into the indoor environment. Ventilation is important for ensuring a favorable IEQ as it dilutes and removes pollutants, odors, and excessive moisture, while providing occupants with fresh air to breathe [20,22,23,24,25,26]. Ventilation is achieved through the implementation of various types of ventilation systems, including natural ventilation, mechanical ventilation, or a combination of both, known as hybrid/mixed-mode ventilation.
Natural ventilation relies on outdoor wind conditions and thermal buoyancy to direct air into a building through specifically designed openings, such as trickle ventilators, doors, and windows [27,28,29]. This implies that minimal or no electrical energy is required during its operation [29,30]. This reduces the energy cost associated with the day-to-day operation of buildings, especially public facilities like schools, resulting in potential savings of up to 30% of total energy [28] and a remarkable 78% reduction in cooling energy [31]. This invariably promotes environmental sustainability, as the system emits a limited amount of carbon dioxide (CO2) and requires only a small space for its operation [31]. The system, however, has its drawbacks, including a reliance on outdoor wind speed to ensure ventilation adequacy [32,33], sensitivity to climatic conditions affecting thermal comfort and ventilation rates [30,34,35], and its inability to condition outdoor air before introducing it indoors due to the lack of temperature and humidity control [28]. Additionally, they might introduce raw outdoor air with high (in tropical climates) or low (in temperate climates) levels of temperature, humidity, and particle loads, while offering less control over the airflow rates [36,37]. Mechanical ventilation comes in two forms: a mechanical exhaust-only ventilation system, where polluted or spent air is extracted mechanically while fresh air is introduced naturally into the indoor environment; or a mechanical supply and exhaust ventilation system that employs mechanical systems for both introducing fresh air and removing polluted air [38,39]. The use of mechanical systems helps increase ventilation rates, and they can be designed or adjusted to deliver a specific flow rate. Such systems can also include options for conditioning and purifying incoming air with cooling, dehumidification, and filtering equipment. However, it is important to note that mechanical ventilation is associated with energy consumption, which comes at a cost [25]. Another challenge with the use of mechanical ventilation in schools is the need for ongoing maintenance and adequate control by specialists to ensure that the required classroom ventilation rate is consistently met [40,41].
A hybrid or mixed-mode ventilation system combines both natural and mechanical ventilation, where mechanical ventilation is utilized only when natural ventilation falls short of meeting the required standards [42]. Ji et al. [43] suggested that employing hybrid ventilation in buildings is a practical approach to attaining a desirable IEQ while minimizing energy consumption. Additionally, some studies have associated mechanical ventilation with a higher incidence of sick building syndrome (SBS) [44,45]. SBS might not be as prevalent in naturally ventilated rooms, where a continuous exchange of outdoor and indoor air occurs. Mechanical ventilation systems that recirculate air can contribute to an increase in indoor microorganisms and other pollutants [45]. Additionally, they may also have dirty or contaminated units, such as those with as mold and bacteria growth in vent pipes. However, some other research has concluded the opposite. For example, Wallner et al. [46] found that mechanically ventilated rooms had an overall better IEQ when compared to naturally ventilated rooms. Additionally, Yang et al. [47] recommended the use of a mechanical ventilation system to improve ventilation rates in schools for better classroom IEQ. In another study, classrooms with natural ventilation exhibited poor air quality due to inadequate ventilation, resulting in a high concentration of CO2 [48]. The study found that ventilation adequacy was linked to the type of ventilation system, with mechanical supply and exhaust ventilation systems exhibiting the highest ventilation rates.
In a Finnish school study by Toyinbo et al. [19], none of the classrooms with natural ventilation and mechanical exhaust-only ventilation systems met the Finnish building code ventilation rate of 6 L/s per student. Meanwhile, 52% of the schools with a mechanical supply and exhaust ventilation system type met the recommendation. In a Netherlands intervention study, classrooms that relied on natural ventilation had their IEQ improved with a CO2-controlled mechanical ventilation system. After the intervention, the average classroom CO2 concentration decreased from 1335 ppm (range: 763–2000 ppm) to 841 ppm (range: 743–925 ppm) [49]. Improving ventilation comes at a cost, but the resultant benefit of enhanced ventilation may outweigh the amount paid in terms of improved health outcomes, productivity, and reduced absenteeism.
Physical, biological, and chemical factors, as well as ventilation and the extent of human activities, all act to contribute to the levels of pollution in any given indoor environment [47]. A high concentration of CO2 in classrooms reflects inadequate ventilation [50,51]. The number of occupants in a building has been associated with CO2 concentration. For example, a school study by Scheff et al. [52] found a continuous relationship between classroom occupancy and CO2 concentration; there was an increase in CO2 concentration associated with high occupancy. Indoor CO2 concentration is related to outdoor concentration and the metabolic CO2 exhaled by occupants [53,54]. The concentration of CO2 in exhaled air is 100 times that of inhaled air [55]. Occupant-generated CO2 may sometimes result in indoor CO2 concentrations exceeding the outdoor levels, especially in highly occupied buildings such as schools [50,56,57].
A common ‘rule of thumb’ has been to keep the indoor CO2 concentration below 1000 ppm (e.g., [58]). While CO2 concentrations lower than 5000 ppm are not associated with direct health effects, ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) Standard 62.1 [59] associates CO2 levels greater than 700 ppm above outdoor air levels (i.e., usually around 1000 ppm) with inadequate ventilation in terms of the removal of human bio-effluents (body odors). This can result in dissatisfaction among the people entering such rooms. Improving ventilation reduces indoor CO2 levels [49,51], and the higher the CO2 concentration indoors, the lower the ventilation rate per student. Some countries have recommended ventilation rates for classrooms. For example, the ventilation rate recommendation for classrooms in the U.S. (United States) is about 7.1 L/s per person [59], while that for Finnish schools is 6 L/s per person [60,61]. The ventilation recommendations for classrooms for some countries is based on classroom CO2 concentration. The recommended limit for indoor CO2 is 700 ppm above the outdoor level in Singapore [62]. Further information regarding ventilation, temperature, and other IAQ parameters for various countries can be found in the summaries provided by Toyinbo et al. [63] and Dimitroulopoulou et al. [64]. In addition, some countries only gave recommendation on how best to construct classrooms for better ventilation. For example, the National Building Code of Nigeria [65] suggests positioning windows directly opposite each other to facilitate cross-ventilation and ensure adequate airflow.
3. Thermal Comfort and IEQ in Schools
Thermal comfort relates to how people feel about the thermal conditions of their environment [72]. ASHRAE standard 55 [73] defines thermal comfort as a state of mind regarding a person’s thermal environment. In a typical application of the standard, it recommended that at least 80% of building’s occupants should feel thermally satisfied with their thermal environment, while 90% satisfaction is encouraged when a higher level of thermal satisfaction is desired in indoor spaces. According to Fanger [74] and Daghigh [75], six factors can influence people’s responses to their thermal environment: air temperature, mean radiant heat, air velocity, relative humidity, clothing thermal resistance, and metabolic rate. Thermal comfort is also influenced by factors such as the seasons of the year and location, age, gender, and individual adaptive characteristics [76,77]. Given the amount of time students spend in schools and the impact of thermal conditions on health and performance, ensuring their thermal comfort is essential [78,79].
Thermal comfort is closely linked to ventilation adequacy, particularly in temperate regions where energy-efficient buildings with tight construction are common. Increased ventilation has been shown to result in lower indoor temperatures, thus contributing to the thermal comfort of building occupants [75,80]. Ventilation with air conditioning appears to influence thermal comfort in the tropics rather than just adequate ventilation. This is because warm unconditioned air can be introduced into the indoor space, making students uncomfortable. A Nigerian school study confirmed this. In the study, most of the investigated classrooms had adequate ventilation, judging by their CO2 concentration level. However, the lack of proper thermal insulation against solar radiation and the introduction of warm, unconditioned outdoor air into the classrooms adversely affected the thermal comfort experienced by the students [37].
Mechanically ventilated and air-conditioned rooms offer improved thermal comfort for occupants compared to naturally ventilated rooms [35,75,81]. This is primarily because controlling and adjusting natural ventilation systems can be challenging due to their dependence on outdoor wind speed and weather conditions [36,75]. For instance, a study conducted by Yang and Zhang [82] demonstrated that naturally ventilated rooms often fail to provide sufficient comfort to occupants and do not meet the recommended standards set by ASHRAE Standard 55. Similarly, Indraganti’s study [83] found that 60% of building occupants expressed thermal discomfort in naturally ventilated rooms. Even when Prajongsan and Sharples [84] improved natural ventilation in their study, thermal comfort only increased from 38% to 56%, which still fell short of the ASHRAE recommendation. Another study by Lu et al. [72] showed that 76% of respondents in naturally ventilated buildings preferred a cooler environment. Daghigh et al. [85] achieved thermal comfort, meeting ASHRAE Standard 55 in mechanically ventilated rooms, but the results changed when natural ventilation was substituted for mechanical ventilation in the same space. According to Brittle et al. [42], a hybrid ventilation system provides enhanced comfort and enables energy savings ranging from 21% to 39%.
4. Moisture and Mold in Schools
It is believed that the operation and maintenance of school facilities are often not funded as adequately as other types of buildings, such as offices. This lack of funding may lead to persistence of environmental problems [94,95]. Insufficient maintenance can result in the deterioration of building materials and systems, leading to failures in moisture control. Furthermore, school buildings may have areas that are prone to leaks, causing moisture damage that can contribute to the growth of microbial organisms [96,97,98]. A school study conducted by Cho et al. [99] found that classrooms with recent water leakage exhibited higher levels of dampness compared to those with only previous or no history of leakage. The study also found a linear association between dampness and the presence of culturable bacteria, with 63% of the examined classrooms exhibiting both dampness and mold. In a Danish study, moisture damage was reported in 49% of the sampled schools [100], while similar problems were identified in 20%, 41%, and 24% of schools in three other European countries (the Netherlands, Spain, and Finland, respectively) [97]. Additional investigations of schools in the Netherlands, Spain, and Finland revealed a high prevalence of microbial secondary metabolites in damp schools [101] and reported respiratory health symptoms among school children [102,103]. Table 1 shows a summary of some school study results conducted on moisture and mold in school buildings.
Table 1. Summary of results from school studies conducted on moisture and mold.
| Reference | Summary of Result |
|---|---|
| [99] | Classrooms with recent water leakage are damp. There was a linear association between dampness and culturable bacteria, with 63% of the classrooms examined having dampness and mold. |
| [100] | Moisture damage was found in 49% of sampled Danish schools. |
| [98] | Moisture damage in schools is associated with microbial growth. Schools with timber-frame construction were majorly in need of moisture and mold damage repair. |
| [104] | Inadequate school maintenance can result in moisture damage of the school structure. |
| [105] | Effective ventilation and cleaning guard against moisture and mold growth risk. |
5. Cleanliness in Schools
Cleaning and hygiene involve the removal of unwanted substances from the school environment. These unwanted substances may include noticeable dirt on surfaces, such as students’ desks, and disinfecting high-contact objects, such as toilet seats and doors, which may be contaminated with pathogenic microorganisms. It also includes hand washing practices and other forms of sanitation among students and school staff, such as proper waste disposal in schools, eating in the cafeteria, and the use of functioning toilets. Classrooms are usually more crowded than other occupied spaces (such as offices); hence, students meet one another more often. Therefore, it is important that their school environment is clean and that hygiene is enhanced to avoid the start and exacerbation of diseases or their spread. Table 2 shows a summary of some school study results on cleanliness in schools.
Table 2. Summary of results from school studies conducted on cleanliness.
| Reference | Summary of Result |
|---|---|
| [116] | A total of 10% of schools studied in rural India had drinking water problems, while 40% and 50% had waste disposal problems and toilet problems, respectively. |
| [117] | In a study in Bangladesh, 18 to 95 students shared one toilet, 36% of the schools studied had proper sanitation, and 47% had handwashing points. |
| [118] | There was lack of water and proper cleaning. Inadequate toilet systems made students prefer open defecation. |
| [119,120] | Ineffective classroom cleaning was related to absenteeism. |
| [121] | Improper cleaning of high-contact surfaces in schools was related to students’ weekly absences from school due to gastrointestinal illness. |
| [122,123] | Improved sanitation was associated with a reduction in disease transmission. |
6. Effects of IEQ (Thermal Environment and Indoor Air Quality) on Students’ Health Outcomes
IEQ in schools has been identified as a major influencing factor on students’ health outcomes. Apart from their homes, students spend quality time in their classrooms, where they are exposed to their school environment, which studies have shown to be related to their health. This, in part, is due to their inability to fully understand their environment and what might affect it (such as students not knowing the importance of desk cleaning). They are not able to effectively influence decisions concerning their school environment, they have developing organ systems (digestive, respiratory, nervous, and reproductive), breathe more air per body size ratio in comparison to adults, and have a weak immunity that is sensitive to pathogens and other pollutants [137,138]. Therefore, it is important to pay attention to the factors that affect students’ health. Table 3 shows a summary of some school studies conducted on IEQ and students’ health.
Table 3. Summary of school studies conducted on IEQ and students’ health outcomes.
| Reference | Study Population | IEQ Parameters Studied | Health Outcomes/Effect |
|---|---|---|---|
| [139] | 108 schools (401 classrooms) with 6590 pupils in France. | Formaldehyde, PM2.5, Aldehyde acrolein, Nitrogen dioxide (NO2). | Rhino-conjunctivitis was associated with high concentration of formaldehyde (OR 1.19; 95% CI 1.04 to 1.36). The prevalence of asthma was associated with high concentration of PM2.5 (OR 1.21; 95% CI 1.05 to 1.39), aldehyde (OR 1.22; 95% CI 1.09 to 1.38), and NO2 (OR 1.16; 95% CI 0.95 to 1.41). |
| [140] | 8 schools (23 classrooms) with 1014 pupils in Sweden. | Microbial volatile organic compounds (MVOCs), plasticizers. | Wheezing, daytime breathlessness, nocturnal breathlessness, and asthma associated with MVOCs (e.g., Isobutanol) OR 2.74 (0.71–10.57), 2.05 (0.30–13.87), 7.23 (0.60–87.16) and 1.43 (0.35–5.87), respectively. |
| [141] | 10 schools (30 classrooms) with 1414 students in China. | Mold, temperature, CO2, NO2, O3, CH2O | Asthma attacks associated with mold (OR 2.40: p < 0.05). Daytime breathlessness associated with temperature (OR 1.26: p < 0.001). Current asthma associated with CO2 (OR 1.18 for 100 ppm: p < 0.01) and NO2 (OR 1.51 for 10 µg/m3: p < 0.01) |
| [142] | 28 schools (162 classrooms) with 45,000 students in U.S. | Ventilation rate | Students’ illness absences increases with a reduced ventilation rate/student that is below California standard (7.1 L/s-person). |
| [67] | 51 schools (81 classrooms) with 1019 students in Portugal. | IEQ (T, RH, CO, CO2, O3, etc.) | A total of 2.2% of students had chronic bronchitis (p > 0.05), 15.2% had wheezing (p > 0.05), 26.1% had sneezing attacks (p > 0.05), 18.9% had rhinitis (p > 0.05), 16.1% had coughs (p > 0.05), and 10.1% had breathing difficulties (p > 0.05). |
| [143] | 8 schools (8 classrooms) with 252 students | Temperature | 97%, 97%, and 94% of students were tired, lost concentration, and were sleepy due to thermal discomfort. |
| [121] | 34 schools with 15,814 students | ATP as a measure of high contact surface contamination | ATP levels increased the probability of students being absent from school due to gastrointestinal illness (parameter estimate 0.91 (0.84–1.00) p = 0.04). |
Students’ health is affected by the many contaminants that can exist in their classrooms [144,145]. These contaminants may be physical, chemical, or biological in nature, such as particulate matter (PM), VOCs, and mold, respectively [119,146,147,148]. These can be generated indoors: for example, mold growth is associated with moisture damage [100,149] and VOCs with emissions from building materials [150,151,152]. They can also originate from outdoor sources when schools are built near pollution sources, such as incinerators and power plants. For example, Brazilian schools that were closed to potential sources of pollutants such as vehicular traffic and petrochemical plants had high concentrations of pollutants such as PM and VOCs [14]. Exposure to these agents may cause or exacerbate pre-existing health conditions, as presented in the following.
A multi-country school exposure study by Adams et al. [153] linked both bacteria and mold in classrooms to moisture-damaged schools that resulted in health effects for students. Students who were exposed to microbial contamination (bacterial and fungi) in their classrooms due to moisture damage experienced respiratory symptoms that affected their health outcomes. The study by Mi et al. [141] found an association between indoor mold in classrooms and asthmatic attacks (OR 2.40: p < 0.05). This further validates the effect microbial contamination can have on the health and wellbeing of students. In addition to biological contaminants, high concentrations of chemical pollutants in the classroom, often exceeding recommended levels, can also impact students’ health outcomes. A study of French primary schools reported an increased prevalence of rhinitis and asthma in pupils due to a high concentration of VOCs (OR 1.19; 95% CI 1.04 to 1.36) and of PM2.5 (OR 1.21; 95% CI 1.05 to 1.39), respectively, in classrooms [139]. An investigation by Kim et al. [140] found that a higher concentration of microbial volatile organic compounds (MVOCs) resulted in pupils experiencing nocturnal breathlessness, daytime breathlessness, wheezing, and doctor-diagnosed asthma.
Inadequate ventilation (indicated by high classroom CO2) has been associated with children experiencing asthmatic attacks, cough, and rhinitis, as well as increased illness absenteeism [119,142], reduced students’ cognitive performance [154], and reduced levels of concentration [67]. Mi et al. [141] found classroom indoor CO2 to be related to current asthma (OR 1.26: p < 0.001). Daisey et al. [146] also reported that inadequate ventilation in classrooms led to adverse health symptoms in their study. A study by Cartieaux et al. [12] found that IEQ adversely affected students’ health, resulting in respiratory symptoms and other respiratory diseases. This was supported by the study of Sundell et al. [155], which showed that inadequate ventilation reduced respiratory functions and resulted in asthmatic symptoms. While CO2 itself is not considered a pollutant, it is used as a proxy for ventilation adequacy. A high concentration of indoor CO2 serves as an indicator of low oxygen (O2) levels in the indoor space, which is related to ineffective air exchange.
7. Effect of IEQ (Thermal Environment and Indoor Air Quality) on Learning Outcomes
Studies have shown the negative impact of being absent from classes on student learning achievement. An absent student misses out on the complete lesson experience, unlike their peers. This absence could potentially impact their understanding of the subject. In a study by Gottfried [158], absent school days by pupils were found to affect their learning achievement, lowering their mathematical and reading achievement results. Missed school days affected the grades of student negatively in a study by Hidayat et al. [159]. Moonie et al. [160] recorded a similar result, finding a negative relationship between absenteeism and student performance. A meta-analysis performed by Credé et al. [161] found class attendance to have a serious impact on student grades. The impact of IEQ on students’ absenteeism or on their health, which leads to missed school days, can be said to have an indirect impact on their learning outcomes. For example, an increase in classroom CO2 concentration levels was related to reduced ventilation and associated with up to one half-day increase in student absence from school [70]. While missed school days were negatively associated with students’ academic outcomes in a study by Young et al. [162], the association between a lower student grade and IEQ parameters analyzed in the study was not moderated by students’ missed school days.
Exposure to PM was associated with about a 2% increase in absence rates per academic year [163]. As explained by Gaihre et al. [70] and Mendell et al. [142], IEQ may not only affect students’ academic performance but can also result in parents missing work to stay with their kids at home, ultimately having economic consequences.
This entry is adapted from the peer-reviewed paper 10.3390/buildings13092172
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