Extended Abstract
Introduction
In recent years, exposure to bioaerosols has been of great importance due to its associated health problems. The World Health Organization has warned of the health implications of poor indoor air quality and incidence of infectious, respiratory and allergic diseases. The classroom is an environment where students spend most of their day. Inside the classroom, students are exposed to airborne physical, chemical, and microbial agents. The assessment of indoor air quality and detection of microbial pollutants in classrooms is very important because of the presence of children sensitive to these pollutants. The aim of this study was to determine the concentration and characterization of dominant species of biological aerosols and their relationship with environmental factors in two selected primary schools in Arak, Iran.
Materials and Methods 
This cross-sectional study was conducted at two primary schools in Arak, Iran in Fall 2018. Indoor air sampling was performed using single-stage Andersen microbial sampler (at flow rate of 28.3 liters.min) containing bacterial and fungal culture media. Indoor air sampling was carried out during regular class activity hours at a height of 1.5 m above ground level. At the same time, the effects of suspended Particulate Matter (PM) and environmental parameters (temperature and humidity) on the density of bioaerosols were evaluated. In order to determine the ratio of indoor to outdoor concentrations, sampling of school yard air was performed simultaneously as an outdoor sample. Bacterial media were incubated at 37 ° C for 24–48 h, and plates of fungal samples were kept at ambient air for 5-7 days. The number of colonies was then counted and reported per CFU.m3. Bacterial colonies were identified by biochemical tests and fungal colonies were identified through macroscopic morphological features and microscopic observations. Kruskal-Wallis test was used to compare the concentration of bioaerosols in different environments, and Spearman’s rank correlation test was used to investigate the relationship between different parameters.
Results
The overall mean density of indoor bacteria and fungi was 448 cfu.m3 and 394 cfu.m3, while the mean density of outdoor bacteria and fungi was 210 cfu.m3 and 127 cfu.m3, respectively. The highest and the lowest mean bacterial density belonged to the second floor classroom of School No. 2 (559±141 cfu.m3) and the first floor classroom of School No. 1 (293±170 cfu.m3), respectively. Moreover, the first floor classroom of School No. 1 (63±25 cfu.m3) and the first floor classroom of School No. 2 (132±98 cfu.m3) had the lowest and highest mean fungal density, respectively, in the air of different areas (Figure 1). Statistical analysis showed that there was a significant difference between the indoor concentrations of airborne bacteria and fungi (P <0.05), but not between their outdoor concentrations (P>0.05). Mann-Whitney U test results showed that the mean concentrations of PM10 and PM2.5 in outdoor air were significantly higher than in indoor air (Table 1). There was a positive correlation between indoor density of airborne bacteria and suspected PM concentrations (PM10 and PM2.5), and between PM2.5 concentration and indoor fungal density (P<0.05). Humidity and density of indoor airborne bacteria and fungi were directly correlated with each other, but no relationship between temperature and density of biological aerosols was reported (Table 2). Penicillium (40%), Cladosporium (19%) and Aspergillus (16%) were dominant species of fungi, while Staphylococcus (42%), Micrococcus (28%), Bacillus (21%) were the dominant species of bacteria.

Discussion
A review of past studies shows that a wide range of bioaerosol concentrations have been reported in the classrooms. Some of the studies are consistent with our study [12, 13]. A number of studies have reported higher levels of bioaerosol density. For example, in studies conducted in two girls' high schools of Eslamhshar county in Tehran [2], in 73 classrooms of 20 elementary schools located in Porto, Portugal in winter [10], and in primary schools in Gondar, Ethiopia [14], higher bioaerosol density were reported. Various factors such as: sampling season, environmental and climatic conditions, internal sources of bioaerosol production and differences in study design (sample size, sequence and duration of sampling) are the main reasons for the differences in the results between our study and other studies. In this study, the I.O ratios varied from 1.4 to 5.6 for bacterial aerosols and from 0.4 to 1.2 for fungal aerosols. The highest ratio for bacteria was found in School n. 2 (2.4), indicating that the origin of airborne bacteria in the classroom was internal. The dominant bacterial and fungal species identified in this study are consistent with results obtained from indoor air isolates of school in other studies [4, 22, 23]. Staphylococcus as a natural flora of the skin and nose can cause a wide range of diseases and infections, especially in children. Corynebacterium is also known to be a contributing factor to nosocomial infections, especially in children [24]. The dominant fungal species reported in this study are the most important allergens that have detrimental effects on human health, especially children and students in the classroom [19].

Ethical Considerations
Compliance with ethical guidelines
This study with an ethics code of IR.ARAKMU.REC.1397.76 was approved by the Research Ethics Committee at Arak University of Medical Sciences.
Funding
This study was extracted from a research proposal approved by Arak University of Medical Sciences (code: 3079). The authors would like to thank the Vice-Chancellor for Research and Student Research Committee of Arak University of Medical Sciences for their valuable spiritual and financial support.
Authors' contributions
All authors met the writing standards based on the recommendations of the International Committee of Medical Journal Editors (ICMJE), and they had equal attribution in preparing the paper.
Conflicts of interest
The authors declare no conflict of interest.