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
[2][3][4][5][6][7]. 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
[8][9][10]. This implies that minimal or no electrical energy is required during its operation
[10][11]. 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
[9] and a remarkable 78% reduction in cooling energy
[12]. This invariably promotes environmental sustainability, as the system emits a limited amount of carbon dioxide (CO
2) and requires only a small space for its operation
[12]. The system, however, has its drawbacks, including a reliance on outdoor wind speed to ensure ventilation adequacy
[13][14], sensitivity to climatic conditions affecting thermal comfort and ventilation rates
[11][15][16], and its inability to condition outdoor air before introducing it indoors due to the lack of temperature and humidity control
[9]. 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
[17][18]. 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
[19][20]. 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
[6]. 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
[21][22].
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
[23]. Ji et al.
[24] 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)
[25][26]. 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
[26]. 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.
[27] found that mechanically ventilated rooms had an overall better IEQ when compared to naturally ventilated rooms. Additionally, Yang et al.
[28] 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 CO
2 [29]. 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.
[30], 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 CO
2-controlled mechanical ventilation system. After the intervention, the average classroom CO
2 concentration decreased from 1335 ppm (range: 763–2000 ppm) to 841 ppm (range: 743–925 ppm)
[31]. 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
[28]. A high concentration of CO
2 in classrooms reflects inadequate ventilation
[32][33]. The number of occupants in a building has been associated with CO
2 concentration. For example, a school study by Scheff et al.
[34] found a continuous relationship between classroom occupancy and CO
2 concentration; there was an increase in CO
2 concentration associated with high occupancy. Indoor CO
2 concentration is related to outdoor concentration and the metabolic CO
2 exhaled by occupants
[35][36]. The concentration of CO
2 in exhaled air is 100 times that of inhaled air
[37]. Occupant-generated CO
2 may sometimes result in indoor CO
2 concentrations exceeding the outdoor levels, especially in highly occupied buildings such as schools
[32][38][39].
A common ‘rule of thumb’ has been to keep the indoor CO
2 concentration below 1000 ppm (e.g.,
[40]). While CO
2 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
[41] associates CO
2 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 CO
2 levels
[31][33], and the higher the CO
2 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
[41], while that for Finnish schools is 6 L/s per person
[42][43]. The ventilation recommendations for classrooms for some countries is based on classroom CO
2 concentration. The recommended limit for indoor CO
2 is 700 ppm above the outdoor level in Singapore
[44]. Further information regarding ventilation, temperature, and other IAQ parameters for various countries can be found in the summaries provided by Toyinbo et al.
[45] and Dimitroulopoulou et al.
[46]. In addition, some countries only gave recommendation on how best to construct classrooms for better ventilation. For example, the National Building Code of Nigeria
[47] 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
[48]. ASHRAE standard 55
[49] 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
[50] and Daghigh
[51], 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
[52][53]. 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
[54][55].
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
[51][56]. 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 CO
2 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
[18].
Mechanically ventilated and air-conditioned rooms offer improved thermal comfort for occupants compared to naturally ventilated rooms
[16][51][57]. This is primarily because controlling and adjusting natural ventilation systems can be challenging due to their dependence on outdoor wind speed and weather conditions
[17][51]. For instance, a study conducted by Yang and Zhang
[58] 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
[59] found that 60% of building occupants expressed thermal discomfort in naturally ventilated rooms. Even when Prajongsan and Sharples
[60] 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.
[48] showed that 76% of respondents in naturally ventilated buildings preferred a cooler environment. Daghigh et al.
[61] 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.
[23], 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
[62][63]. 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
[64][65][66]. A school study conducted by Cho et al.
[67] 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
[68], while similar problems were identified in 20%, 41%, and 24% of schools in three other European countries (the Netherlands, Spain, and Finland, respectively)
[65]. Additional investigations of schools in the Netherlands, Spain, and Finland revealed a high prevalence of microbial secondary metabolites in damp schools
[69] and reported respiratory health symptoms among school children
[70][71].
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.