4.1. Comparison of PM Concentrations with Historical Data
Overall, the 5-d mean PM
2.5 and PM
10 concentrations measured in the present study (
Table 3) were quite comparable with the results of previous investigations. A Dutch survey identified average indoor PM
10 levels between 51 µg/m
3 and 166 µg/m
3 [
25], but was performed in summer, and higher values would likely be expected during winter. A German study examining 54 classrooms in winter and spring found quite low indoor PM
2.5 concentrations (median = 15 µg/m
3, range: 5–40 µg/m
3) [
26].
The majority of studies, however, evaluated PM concentrations only during teaching hours when pupils attended class.
The 5-d indoor PM
10 concentrations during
S (
Table 4) were comparable to the results of Diapouli
et al. [
27], which measured 8-h PM
10 concentrations of 229 ± 182 µg/m
3. The respective indoor PM
2.5 concentrations (82 ± 56 µg/m
3) were approximately 1.5-fold higher than those measured in our study.
Nevertheless, our 5-d indoor PM
10 levels during lesson hours were generally elevated in comparison with similar studies. Wheeler
et al. measured in the U.K. mean PM
10 winter concentrations of 80 µg/m
3 [
28], while average indoor 8-h PM
10 levels < 50 µg/m
3 were found in Detroit [
29]. Noteworthy is that during re-suspension phenomena caused by human presence and activities the re-suspended particles (primarily coarse particles and PM
10 of crustal origin) can exhibit densities markedly higher than PM
2.5 (primarily elementary and organic carbon), which was used in our study to correct all the PM optical data. Thus, especially during
S, the high indoor PM
10 concentrations registered in the studied classrooms could be overestimated.
The PM 5-d median values during
S (
Table 4) were comparable to those reported in a German study (118.2 µg/m
3 for PM
10 and 37 µg/m
3 for PM
2.5) [
12]. Lower median PM
10 values during teaching hours were found by Janssen
et al. (81 µg/m
3) [
30] and Fromme
et al. (92 µg/m
3) [
13].
4.2. Indoor and Outdoor Fine PM Concentrations
Contrary to the results of previous studies [
12,
16], we identified indoor 5-d PM
2.5 concentrations lower than the associated outdoor levels during both sampling campaigns (
Table 3).
During SC1, a lower 5-d outdoor PM
2.5 was observed only for school S1, primarily because the ambient PM levels were decreased to values close to zero by heavy rains (
Table 2). Overall, however, SC1 was characterized by 5-d PM
2.5 outdoor levels higher than SC2 (
Table 3).
Notably, in the Po Valley the atmospheric stability is high, especially during wintertime, and frequent thermal inversions at low altitude, low mixing layer heights and prolonged foggy periods can occur. Specifically, the sampling during SC1 was affected by a lower rainfall rate and a slightly lower average wind speed in comparison with SC2 (
Table 2), thus promoting the accumulation of outdoor pollutants, as occurred for school S4, which exhibited the highest outdoor and indoor 5-d PM
2.5 levels (106.8 µg/m
3 and 57.9 µg/m
3, respectively) found in our survey.
In contrast, the outdoor concentrations in SC2 were approximately 50% lower than those in SC1. The lowest 5-d ambient concentration (12.0 µg/m
3) was found during a rainy and windy sampling week (S2) (
Table 2). In the same week, the lowest indoor 5-d PM
2.5 concentration (13.5 µg/m
3) was also registered.
4.3. Influence of Indoor Sources on Indoor PM Levels
Although school environments usually lack typical indoor PM sources, such as smoking, wood-burning and cooking, high levels of airborne particles in classrooms are often present nonetheless.
It has been demonstrated that in indoor microenvironments where there is no specific indoor source of pollution (e.g., combustion processes), human presence and activities may represent the main source of indoor particulates [
11,
31].
In our work, the highest mean concentrations of PM
2.5, PM
5, PM
10 and TSP were observed when the classrooms were occupied by students (
Table 4), whose movements and activities led to the re-suspension of settled particles and also affected indoor PM levels through personal clouds.
A chemical characterization of the particles collected in our survey was not conducted, but other studies have reported that PM in classrooms is partly of human origin (skin flakes) and partly of mineral origin (building materials such as cement and gypsum as well as chalk and soil material brought in by children’s shoes) [
12].
The ratios of the 5-d mean concentrations during
S and
N (
Figure 4) highlighted the strong influence of occupancy on indoor airborne particle concentrations. The exponential growth of the
S/N ratios for particles > 1 µm indicated that re-suspension became relevant from particulates of 1 µm in aerodynamic diameter but was predominant for particles in the ranges 2.5–10 µm and > 10 µm, with re-suspension rates increasing with particles size, as demonstrated by other studies [
8,
15]. Overcrowding of classrooms can contribute to increased re-suspension phenomena. The maximum occupancy in a classroom environment, as recommended by the ASHRAE Standard 62-1989 is 50 people/100 m
2 [
24]. In our study, 23.8% of classrooms exceeded the recommended standard during the first sampling campaign, and three rooms were borderline. During SC2, the Standard was exceeded by 27.7% of the classrooms, with one room lying on the threshold (
Table 1).
The high indoor PM levels found during
OA for particles > 2.5 µm (
Table 4) were likely the result of cleaning activities, as the cleaning of rooms was normally performed during this time period. It is well known that cleaning activities, such as dusting or vacuuming, are potentially significant indoor PM sources, especially of coarse-mode particles, causing considerable variations in PM levels over short time periods [
32].
In the absence of indoor PM sources, when the classrooms were empty and no activities were being performed, the indoor PM
10 was primarily composed of PM
2.5 (
Figure 5), as also observed by Branis
et al. [
11]. The influence of human presence and activities on particle levels was also demonstrated by the poor correlation between the fine and coarse PM fractions during
S and
OA (
Table 5), suggesting that various classroom sources exerted different effects on the indoor particle levels, with the greatest influence on the largest particle sizes. In addition, the higher correlations observed during
N (
Table 5) underlined that, without human activities, no particular indoor source could be identified, and the only relevant determinant of indoor concentrations was the particle infiltration from outdoors.
In contrast to the results of other studies [
11,
31], we did not observe a significant correlation between the indoor PM levels, especially for coarse particles and PM
10, and the occupants’ number and density. This observation could be due to the lack of differences in the number of students among the studied classrooms. Further, the various types and intensity of activities performed in the classrooms by the students (whose details were not available) could determine the high indoor particle levels regardless of the number of occupants.
The estimation of the indoor- and outdoor-generated PM
2.5 suggested that slightly less than half of the 5-d indoor PM
2.5 in classrooms originated indoors and 53% infiltrated from outdoors. This results could have arisen because the re-suspension phenomena primarily affected the coarse particle concentrations but also the PM
1-2.5 fraction, which was found to decrease from
S to
N (
Figure 5). The slight reduction in the PM
2.5 I/O ratios when students were not present in the buildings and no activities were being performed (
Table 4) confirmed this behavior. Similar results were reported by Crist
et al., who demonstrated a decline in PM
2.5 indoor concentrations and I/O ratios associated with the absence of students in school [
33].
As discussed below, the most innovative goal of this study was to evaluate the efficacy of the intervention measures suggested to improve the IAQ in classrooms, since, to date, only a limited number of surveys have tackled the problem of assessing PM exposure following risk mitigation interventions.
4.4. Evaluation of the Efficacy of Intervention Measures in School Classrooms
To evaluate the efficacy of the proposed intervention measures, we considered PM2.5 and CO2 as being possibly and beneficially affected by the improved cleaning of surfaces with vacuum cleaner and/or the frequent opening of doors and windows. No time-activity diaries were provided for obtaining information on windows opening and cleaning activities. Thus, no specific detail regarding the recommended intervention was available.
A comparison for PM5, PM10 and TSP would be very important because of the considerable influence of cleaning activities on these size particle ranges. Unfortunately, we were unable to consider all of the PM fractions due to the small number of comparable OPC samples between the two monitoring sessions.
Statistically significant differences for PM
2.5 (
p = 0.001) were found in the classrooms affected by the intervention measures, with lower post-intervention PM
2.5 5-d concentrations. Conversely, no significant differences were found in the classrooms not subjected to the intervention program (
p = 0.150). Nonetheless, this improvement should not be considered as the direct consequence of risk mitigation interventions, as a parallel decrease in the corresponding outdoor PM
2.5 levels was registered for the schools involved in the intervention program, although not in a statistically significant way (
p = 0.164). Furthermore, no statistically significant differences were found between SC1 and SC2 PM
2.5 I/O ratios (
Table 6). We can therefore conclude that the intervention measures proposed to reduce indoor particle levels did not seem to significantly influence the indoor fine PM concentrations.
Conversely, the statistically significant decrease in the indoor CO2 levels reported in SC2 during S (p = 0.006) could be attributed to the more frequent opening of doors and windows. No significant differences were found in the classrooms where intervention measures were not implemented (p = 0.084).
These preliminary findings should be further investigated using a larger number of samples and more detailed information about all the possible factors and confounders of the PM and CO2 measurements (e.g., climatic conditions and the frequency and intensity of cleaning activities and opening of windows). Furthermore, such an assessment would have greater value if performed using parameters that are significantly influenced by the types of interventions proposed (i.e., coarse particles and PM10).
4.5. Comparison of Indoor PM and CO2 Levels with International Guidelines
The indoor AQGs recently proposed by the WHO aimed to provide a uniform basis for the protection of public health from adverse health effects of exposure to indoor air pollutants [
2]. When defining the chemicals to be considered for the development of new indoor guidelines, PM (PM
2.5 and PM
10) was included in Group 1, as all the priority pollutants for which the definition of guidelines for indoor air was recommended. Nevertheless, no convincing evidence of a difference in the hazardous nature of particulate matter from indoor sources as compared with those from outdoors was found. Therefore, the PM ambient AQGs defined by the 2005 Global Update were recommended also for indoor spaces [
2,
23].
In general, the high concentrations that could be achieved in school buildings do not necessarily result in higher health risk to pupils, because the sources and composition of PM in indoor air may differ from those in outdoor air [
34]. Among other things, studies highlighted the need of further investigations of the chemical composition and source apportionment of indoor PM, also to better clarify the toxicological and biological relevance of PM exposure in schools in comparison to outdoors [
12].
67.9% of the total 24-h PM2.5 indoor concentrations found in this study exceeded the WHO AQG limit established to prevent excess mortality and morbidity among human populations.
Furthermore, 40.3% of these cases did not achieve the PM
2.5 Interim target 3 (37.5 µg/m
3), implying an estimated increase of 1.2% in short-term mortality; some of the measurements (31%) also exceeded the PM
2.5 Interim target 2 (50 µg/m
3), which is related to an estimated 2.5% increase in daily mortality compared with the guideline value [
23]. None of the 24-h indoor concentrations exceeded the PM
2.5 Interim target 1 (75 µg/m
3), although during SC1 two classrooms in school S4 achieved indoor 24-h PM
2.5 levels close to that threshold (73.4 µg/m
3 and 73.6 µg/m
3).
PM
10 is less hazardous to human health because coarse particles are unable to reach the deepest parts of the respiratory tree. In the majority of cases (91.2%), the 24-h WHO guideline value was exceeded, with 78.8% of 24-h levels surpassing the PM
10 Interim target 3 (75 µg/m
3). 32.7% of cases exceeding the 24-h limit value also exceeded the PM
10 Interim target 1 (150 µg/m
3), with an estimated 5% increase in short-term mortality [
23].
24-h classroom concentrations were calculated for comparison with the 24-h WHO guidelines in terms of average exposure times, although they are not fully representative of the students’ daily exposure to PM. Pupils, indeed, spent on average of 30% of their week-day time in the school environment, where, moreover, they might have frequented locations and rooms in which the airborne PM exposure could be higher or lower (e.g., gymnasium or library) [
27]. Moreover, students passed the remainder of the day in other outdoor or indoor microenvironments—primarily at home—where the indoor PM concentrations were likely to be high [
35].
With respect to long-term exposure, the collected data were not directly comparable with the 1-year values recommended for PM
2.5 and PM
10 (10 µg/m
3 and 20 µg/m
3, respectively) because our study provided only winter levels. However, an average annual indoor value lower than the mean winter concentration is expected, due to typical PM seasonal trends in European countries [
13,
17].
According to the ASHRAE Standard 62-1989, indoor CO
2 levels should not exceed 1,000 ppm to ensure satisfactory comfort [
24]. Values exceeding this threshold indicate insufficient fresh air and are associated with a higher frequency of health complaints, primarily due to undesirable levels of body odor, although no health impact on humans has been reported at these concentration levels [
22]. Several studies have demonstrated that, during lesson hours, classrooms are often inadequately ventilated, especially in wintertime, with high indoor CO
2 levels reached [
6,
13,
36].
In our study, the CO
2 ASRHAE Standard was exceeded for the majority of the time occupants spent in the classrooms (73%). The closing of windows and doors, inadequate ventilation rates and overcrowded rooms could be responsible for the high indoor CO
2 levels monitored during lessons. Intervention measures that appeared to decrease the indoor CO
2 levels in SC2 were unable, anyway, to reduce the classroom CO
2 concentrations below 1,000 ppm during teaching hours (
Table 4).