1. Introduction
China has been experiencing serious air pollution problems in recent years, due to rapid industrialization, urbanization, and increasing energy consumption [
1,
2,
3]. The Chinese economy is still largely dependent on fossil fuels. In 2011, 69% of total energy consumption was contributed by coal combustion, 18% by oil, and 4% by natural gas. Only 8% of the national energy mix was from renewable sources such as hydroelectric, solar, and wind [
4]. As a result, the ambient air has been heavily affected, especially in urban areas. Only 5 cities out of 367 in China met the World Health Organization (WHO)’s recommended air quality standards in 2012. The primary air pollutant in Chinese cities has been identified as suspended particulate matters (PMs), specifically, PM
2.5 and PM
10. PM10 denotes inhalable particles with diameters that are generally 10 micrometers and smaller. PM2.5 denotes fine inhalable particles with diameters that are generally 2.5 micrometers and smaller [
5]. The readings of PM2.5 concentrations in most urban areas have exceeded acceptable national standards, and are often worse in winter in the north of China due to fuel combustion for heating [
6]. High exposure to particulate matters can bring morbidity problems like asthma, lung cancer, cardiovascular disease, respiratory diseases, birth defects, and premature death. Logue et al. [
7] suggested that PM
2.5 is one of the air pollutants which dominate health impacts in most U.S. residences due to chronic exposure. Such air pollutants lead to greater losses of disability-adjusted life-years (DALYs). Sundell et al. [
8] concluded that the incidence of inflammation, respiratory infections, asthma symptoms, and short-term sickness is enhanced as a result of high exposure to air pollutants. Therefore, urban residents have to reduce the time they stay outdoors and their level of activity. However, it may not be safer to stay indoors, as some studies suggest that indoor air quality closely correlates to the outdoor pollution level, particularly in urban and industrial areas [
9]. Another important factor which affects indoor air quality is building defects. Riley et al. [
10] suggested that the penetration of particles through building cracks would significantly affect indoor air quality, especially for buildings where air exchange through openings or cracks is dominated by pressure differences arising from natural forces. Tian et al. [
11] concluded that for naturally ventilated buildings, the key factors which determine occupants’ exposure to air pollutants were pressure difference, building air tightness, and air flow path. In addition, PM also can be generated by indoor activities, such as cooking, cleaning, cigarette smoking, and the movement of people. Abt et al. [
12] found that cleaning activities and the movement of people significantly increased PM
(0.7–10) concentrations by 0.27 μm
3/cm
3 and 0.25 μm
3/cm
3, respectively, per minute in domestic buildings in Boston. Long et al. [
13] investigated nine homes in Boston and figured out that dusting, vigorous walking, and sautéing were the key factors affecting indoor air quality.
Apart from particulate matters, indoor air quality also can be affected by other parameters, such as carbon dioxide (CO
2) level and indoor air temperature. Seppänen et al. [
14] found that the risk of air-related health problems can be decreased when CO
2 concentrations fall below 800 ppm. Fang et al. [
15] indicated that indoor air temperature has great impact on perceived air quality in office buildings, and high air temperatures can promote the generation of other pollutants, such as volatile organic compounds (VOCs) and formaldehyde.
Children are more sensitive when they are exposed to unhealthy air, because they generally breathe a higher volume of air relative to body weight at a faster rate [
16,
17]. Bakó-Biró et al. [
18] figured out that over 30% of a pupil’s life was spent at schools, and about 70% of their time inside a classroom during school days. Shauhnessy et al. [
19] claimed that poor indoor air quality caused by excessive CO
2 and VOC concentrations have a negative impact on students’ attendance and learning potential, and lead to poor academic performance. Therefore, indoor air quality in school buildings is highly important, and may have a significant impact on a nation’s healthcare system. Haverinen-Shaughnessy et al. [
20] investigated one hundred elementary schools in the southwest of the U.S. and found that 87% of the classrooms had air quality problems. Studies have also pointed out that indoor air pollutant concentrations in classrooms, such as CO
2, PM, and VOCs are greatly influenced by air pollution sources outside the school, primarily traffic and industrial emissions [
21,
22,
23,
24].
The PM concentration inside classrooms is partly determined by infiltration from the outside air. Zwoździak et al. [
25] concluded that there is a positive correlation between outdoor air pollution and indoor air quality, suggesting that urgent action should be taken to improve a building’s air tightness and reduce air infiltration. Amato et al. [
26] investigated 39 primary schools in Barcelona and found that 53% of the indoor PM
2.5 concentration measured could be traced back to seven outdoor sources in the ambient environment. Moreover, schools without paved playgrounds have an increased contribution by 5–6 μg/m
3 on average compared to those with paved playgrounds. Janssen et al. [
27] pointed out that schools close to roadways with heavy traffic usually have higher numbers of children suffering from bronchial hyper responsiveness, positive allergic sensitization, or both, compared with schools located far away from the traffic or industrial areas.
In school buildings, the resuspension of particles by students’ indoor activities also affects the indoor particle concentration in occupied classrooms [
28,
29]. Lefcoe and Inculet [
30] found that indoor activities, such as cleaning and children playing, had significant impacts on the indoor concentration of particles greater than 1 μm. Raunemaa et al. [
31] reported that the average concentration of particles larger than 1.5 μm highly depends on the amount of time spent indoors. Another study from Majumdar et al. [
32] indicated that PM
2.5 and PM
10 also can be produced when writing on the blackboard in a classroom.
Apart from those chemical toxicants and particulate matters, it is found that the CO
2 level may exceed 1500 ppm in classrooms in East London, which is much higher than the recommended level of 600 ppm from the Chartered Institution of Building Services Engineers (CIBSE) [
33]. Ramalho et al. [
34] investigated the indoor CO
2 concentration in 489 mechanically ventilated classrooms in France, and the results indicated that 33% of the samples had a CO
2 concentration above 1700 ppm, which is two times greater than the CIBSE value.
In China, mechanical ventilation systems are not commonly used in public primary schools, and the only way to introduce fresh air into a classroom is to open windows and doors; however, particles and other air pollutants will be brought into the classroom simultaneously. Moreover, in winter, when the air pollution is the most serious in China, doors and windows are often closed to keep the inside warm. This may lead to two consequences: inadequate fresh air supply, and increased indoor air pollution from the combined effect of air penetration and indoor activity.
Currently research related to indoor air quality in China has been focused on chemical toxicants, such as formaldehyde and benzene. Most studies were carried out in office buildings and residential houses. The indoor air quality in primary school buildings and the influential factors affecting indoor air quality (IAQ) have not been extensively investigated. Therefore, this empirical study aims to investigate indoor air quality in primary school buildings in China and identify the opportunities for improvement.
5. Conclusions, Limitations, and Future Research Directions
5.1. Conclusions
In this study, indoor air quality was measured in the four case studies of primary schools in Tai’an city, North of China. Four parameters have been measured and analyzed. The correlations between the indoor air quality and the ambient air pollution, building defects, and internal activities also have been identified and discussed. The results indicated that none of the case studies for primary schools satisfy the indoor air quality standards.
A building’s air tightness and its occupants’ internal activities have a significant impact on indoor PM concentrations in naturally ventilated school buildings. A “tighter” building tends to reduce outdoor air pollutants’ penetration and ensure a higher indoor temperature in winter. Teaching and learning activities also affected the indoor air quality. When the outdoor PM concentration is high, it has been suggested that the windows and doors should be closed to keep particle matters out. However, this might lead to an increase of CO2 concentration due to an inadequate fresh air supply.
Since ambient air pollution is a regional environmental problem, it cannot be solved only by building designers, builders, and school managers. However, it is possible to provide better indoor air quality to primary school students by removing building defects. Some suggestions from this study are listed below.
The current building air tightness requirements are not adequate. This study indicates that around 50% of particles originate from the ambient air even in a school building which has met the highest air tightness level required in China. Thus, the current building air tightness requirement should be improved.
Retrofits are needed to improve the indoor air quality in naturally ventilated school buildings. Cracks and gaps in the envelope can be sealed. For example, the cracks in a masonry-concrete structure and timber frame structure can be removed by gypsum plaster and flake board, respectively.
On the other hand, improved air tightness may cause the increase of CO2 concentration in a naturally ventilated school building, particularly in winter when the doors and windows are closed to maintain reasonable levels of thermal comfort. Therefore, mechanical ventilation systems should be employed in winter to remove the products of respiration, such as CO2 and H2O.
During heavy air polluted days, for example, when the Air Quality Index (AQI) (An air quality index (AQI) is a number used by government to communicate to the public how polluted the air currently is or how polluted it is forecast to become. In China AQI larger than 100 means air is heavy polluted.) is reported 100 or above, it is suggested that a mechanical ventilation system with an air purification function should be installed to ventilate the building with clean air. For days with good ambient air quality, natural ventilation still is the first option for saving the energy consumed by mechanical ventilation systems.
In addition, it is also important to remove particles generated by class activities. For example, chalk should be replaced by using a projector for teaching if possible, or simply substituting chalk by whiteboard markers and the appropriate boards. Moreover, classrooms should be cleaned frequently to reduce the resuspension of particulate matter.
In summary, this empirical research has demonstrated that in naturally ventilated school buildings, improving air tightness might be a way to reduce the penetration of outdoor air pollutants. Mechanical ventilation with an air purification function also could be an option on some severely polluted days.
5.2. Limitations and Future Research Directions
Due to the limited time and resources, four primary schools were selected and the investigations were carried out over three weeks. These four case studies can represent the vast majority of primary schools at a local context, since primary schools are a part of the public educational system, and they therefore have standardized plans and design. They are always designed based on the same building regulations (Code for design of school-GB50099) (Code for design of school-GB50099 is a series of school building regulations which is implemented by the Ministry of Construction China. It defines many common features of primary school buildings, such as the size of classroom, number of students and building orientation, etc.), therefore they have similar features, such as the size of classrooms, the number of students, the number of floors, orientation, positioning of windows and sizing, and the same materials specifications. The biggest differences of primary school buildings are the air-tightness levels and added thermal characteristics, since the China Public Building Energy Saving Standard (GB50189) has been going through updates during the last decades. Several future directions arising from this research have been recognized. First, more school buildings should be investigated in order to extend the number of samples. Second, the seasonal effects on indoor air quality should be further identified. Third, the penetrability of different sizes of particles should be studied, and the effects of the distribution of cracks in the building fabric’s envelope should be investigated.