Carbon dioxide as an indicator of bioaerosol activity and human health in K-12 school systems: a scoping review of current knowledge

Indoor air quality (IAQ) in schools has received attention over the past decades but still lacks specific standards and regulations. This study aimed to review the impact of bioaerosol activity in indoor environments on acute respiratory diseases and explore whether carbon dioxide can be used as an indicator of bioaerosol and respiratory diseases in indoor environments in K-12 school systems. Findings suggest a lack of a consensual approach to evaluate bioaerosols impacting IAQ in indoor infrastructures, particularly in school environments; an elevated CO2 concentration inside the school classrooms was not uncommon, and the evidence of unsatisfactory and degraded IAQ (surpassing ASHRAE standards) at public schools in rural and urban settings in one of the North Central County, Florida. It was found that CO2 levels can be associated with bioaerosol activity, and sufficient ventilation within the space substantially reduces the airborne time of respiratory droplets and CO2 levels. CO2 monitoring can act as an effective, low-cost alternative to surveying or detecting the prevalence of respiratory diseases, which may hold strength through establishing critical CO2 thresholds and, thereafter associating it with the infectious doses of pathogen activity.


Introduction
The coronavirus pandemic (COVID-19 thereafter) has initiated scientific discussions on the societal and economic importance of air quality in the built infrastructure.The K-12 school system is particularly important, which remains vulnerable owing to occupant density, leading to higher person-to-person contact [1].In the early 1970s, investigations were started to determine the cause of adverse respiratory impacts following complaints by occupants in indoor working environments [2].Studies by the U.S. Environmental Protection Agency (EPA) relating human exposure to air pollutants indicated that indoor air contains about two to five times higher (sometimes even more) concentrations of pollutants than outdoor air (EPA report 2000) [3].Indoor environments have multiple and complex sources of pollutants [4,5] such as debris from electronic equipment, products for household cleaning, heating, and cooling systems, burning fuels (wood or petroleum products), the density of people and their behavior like smoking, etc. Besides, outdoor pollution near the population centers, such as vehicular emissions and industrial activities, can infiltrate indoor environments through natural or mechanical ventilation systems [6].
The health impact of indoor air pollutants becomes more critical when occupants spend considerable time in-built indoor environments.In urban settings, residents spend more than 90% of their time indoors [7].In the United States, an individual, on average, spends 87% of their time indoors and an additional 6% in enclosed vehicles [8].Time spent in the indoor environment varies with population age groups, and young children and senior people are likely to spend more time indoors compared to working groups.According to the National Center for Education Statistics, about 49.4 million students are enrolled in the kindergarten to grade 12 (K-12) public school system in the fall of 2020 in the USA.Students in public schools constitute approximately 15% of the total population of the USA and on average, each student in the USA spends 35 h every week in school.
According to the EPA, 'indoor air quality (IAQ) refers to the air quality within and around buildings and structures, especially as it relates to the occupants' health and comfort.'Several studies have shown that indoor environments present increased chances of exposure to pollution [9][10][11], with levels of some pollutants (CO 2 , VOCs, etc) several folds higher indoors than outdoors.Exposure to indoor pollutants may exert a harmful impact on public health; as such, indoor pollutants can give rise to respiratory and cardiopulmonary diseases and asthma [12], especially among children [13,14].Due to developing immune systems, children are considered more vulnerable to adverse effects following environmental exposure [15].IAQ can cause increased absences in schools due to respiratory infections, increased allergic diseases from biological contaminants, or adverse reactions to school chemicals.From 2010 to 2016, 13% of students suffered from severe to moderate respiratory infections in the US [16].This represented a mean of school absenteeism of 4.5 d per year per student [17].
Classrooms are more congested than other workplaces [18], but maintaining good IAQ is essential for children's comfort, health, performance, and ability to concentrate.Every school environment has unique characteristics, and the IAQ inside the school building is likely to depend on factors such as the age and location of the buildings, chemical reactions in indoor air, and heterogeneous processes at the air-solid interfaces and pollutants transport from outdoor [19].Outdoor pollution is primarily caused by various factors such as proximity activities, vehicular emissions, and industrial activities.Proximity activities encompass human activities that occur in close proximity to residential or commercial areas, such as construction sites, road repairs, and waste disposal sites, which release pollutants into the air.Indoor pollution is primarily caused by occupancy, electronics, cleaning products, and combustion activities such as cooking and heating.Occupancy refers to the presence of people, while electronics like computers, printers, and televisions can emit pollutants.The design and age of the school building lead to significant variations in indoor pollutant levels and, hence, highly individualized exposure [11,20].Previous studies have identified several key indoor air pollutants, including carbon monoxide (CO), carbon dioxide (CO 2 ), ozone (O 3 ), nitrogen oxides (NOx), sulfur dioxide (SO 2 ), volatile organic compounds (VOCs), and particulate matter (PM) [9,[21][22][23].Figure 1 provides a summarized overview of the health effects resulting from indoor air pollutants, major sources of indoor air pollutants, mechanisms such as control and ventilation that improve IAQ, and the benefits resulting from improved IAQ.The arrows indicate the flow and their effects on IAQ are color-coded, with green indicating positive effects, red indicating adverse effects, and blue indicating neutral effects.This aims to provide a comprehensive understanding of the complex relationship between environmental factors and air quality, facilitating the identification of potential issues and solutions for maintaining a healthy environment.
Prior to the COVID-19 pandemic, more than 1 billion acute respiratory infections (including bacterial and viral pneumonia, influenza, tuberculosis, and measles) were reported every year in the United States [24].Ever since COVID-19 acquired the global pandemic status, several research and application questions related to respiratory infections have been on the horizon for planning or decision-making activities for the well-being of school students.COVID-19 is likely to persist and become seasonal [25], and its impacts on school-age children with respect to safe breathing environments need appropriate attention.
Bioaerosols, a term encompassing a wide range of airborne particles, have gained significant attention due to their potential impact on human health.Bioaerosols are biologically originated tiny airborne particles from plants/animals that can contain living microorganisms, including bacteria, viruses, fungi, and protozoa.In the context of respiratory infections, the term bioaerosol primarily refers to viable particles such as viruses, bacteria, and fungi that have the potential to cause diseases.However, it is important to note that bioaerosols also encompass non-viable or inactive microorganisms, fragments (especially of fungi), and non-infectious but potentially allergenic substances like pollen [26].Recently, the importance of bioaerosols, such as those from respiratory droplets, has been at the center stage of discussions from the standpoint of understanding the transmission of respiratory infections.Breathing, talking, coughing, and sneezing are modes for the release of infectious airborne [27,28].These omnipresent particles can include a variety of living and non-living entities [29].Particle sizes can range from less than 10 nm to greater than 100 µm and can vary with relative humidity [30].In indoor conditions, particles less than 5 µm in size can remain airborne for extended periods [31,32].Certain bioaerosols with pathogenic microorganisms can infect surrounding human populations, as they are suitable for a host range of pathogens (e.g.M. tuberculosis, influenza virus A, etc) [33].The broadly understood mechanism for respiratory infections is that the infected individual can form the aerosol-containing microorganisms through coughing or sneezing, and the air stream disperses the microorganisms into the air when around infected sources.Next, the viable microorganism inside the airborne particle is inhaled and deposited on the susceptible host's respiratory tract, which then becomes the new host for pathogens.Bioaerosols, thus, play an essential role in the transmission of respiratory diseases, such as influenza [34], tuberculosis (TB), and measles [35], and viruses like SARS-CoV-1 and SARS-CoV-2 [36,37].Medium-sized aerosol droplets (10 µm-100 µm) can remain suspended in the air for at least 30 s [38].Evidence of the presence of SARS-CoV-2 RNA in the sampled air in indoor spaces was reported in the early stage of the COVID-19 pandemic [39][40][41], and later, it was shown that viable SARS-CoV-2 was present in the air of a patient room in a hospital environment [36,41] showing the transmission potential through aerosolization of pathogens.It is estimated that humans generate 1000 virion-containing droplet nuclei when speaking loudly, and their droplets can remain airborne for an excess of 8 min [38].Thus, bioaerosols containing SARS-CoV-2 exhaled by a COVID-19 patient may accumulate in indoor environments, thereby increasing the risk of virus transmission through accumulated virus-laden aerosols [40,42].
In the built environment, a measure of carbon dioxide (CO 2 ) concentration is directly proxied to ventilation quality and thus provides an indicator for monitoring IAQ [22,[43][44][45][46][47].Bioaerosols play a significant role in indoor air pollution [48] and cannot be completely eliminated or eradicated from the natural or built environment.However, it is possible to minimize the presence of bioaerosols and improve IAQ through the implementation of mechanically or naturally enhanced ventilation systems [42].A study on TB outbreak in a university building showed that CO 2 levels <1000 ppm were associated independently with a decrease in TB incidence among contacts by 97% [49].CO 2 , thus, can be used as a marker for potential respiratory infectious disease transmission risk, as reported by previous studies [27,50,51].Few recent studies have even identified CO 2 as a proxy variable of interest for assessing the COVID-19 infection in the human population [52,53].This review was conducted to evaluate the state of using CO 2 as a potential environmental marker of IAQ in school buildings and for the assessment of using CO 2 as a respiratory disease indicator through the association of CO 2 and bioaerosol activity.

Methods
A systematic literature search was conducted using databases such as PubMed and Google Scholar. Figure 2 presents a flow chart that outlines the review process conducted in this study.The flow chart provides a visual representation of the steps followed to identify relevant articles and select studies for inclusion in the review.Keywords used in the search included various combinations of 'school, ventilation, bioaerosol, human health, K-12, IAQ, infectious diseases, and carbon dioxide' as an indicator.The search was limited to articles published in English after 2000.The selection process involved an initial screening of titles and abstracts to determine relevance.Relevant papers were then thoroughly reviewed, and additional papers were identified in the reference sections of these papers.The quality of the articles was assessed using the PRISMA guidelines.While several articles were found that addressed the associations between occupant health outcomes and the type of mechanical ventilation system, this specific issue was not reviewed in detail.To facilitate a synthesis of the published information, tables were created to summarize the characteristics of the studies and their findings.
The conclusions drawn from the analysis took into account the consistency of the findings across different studies, the number of studies with consistent findings, and indicators of study quality, such as the size of the study and the extent to which potential confounding factors were controlled.This approach aimed to provide a comprehensive overview of the available information and identify any patterns or trends in the research findings.

Association of indoor bioaerosol activity with respiratory infections
The pathogen's infectivity determines the transmissibility of any infection, the infected individual's contagiousness, the exposed individual's susceptibility, contact patterns between the infected and the exposed, and the environmental stress exerted on the pathogen during transmission [74].Abiotic factors like relative humidity, water content, temperature, moisture, insulation, air circulation equipment, and duct maintenance regulate the indoor air and survival of biological contaminants.There is no consensus on the relationships between airborne pathogens and environmental factors such as temperature, relative humidity, and CO 2 [75,76]; however, reported cases exhibited a strong association with environmental variables [77].
It has been a subject of recent introspection that the variation in indoor temperature, relative humidity, and carbon dioxide is shown to impact microbial growth and spore dispersal regardless of positive or negative correlation [76,78,79].Indoor environments were found to have significantly higher levels of bacteria [80], fungi [81], and viruses [34] than outdoor environments.The emission of bioaerosols may occur from normal breathing and speaking, and the emission rate varies based on the type of activity.These activities in the case of human gatherings (hospitals, schools, offices, etc), increase contact patterns, enhancing the chances of aerosol transmission of respiratory infectious pathogens.Table 2 summarizes the evidence where pathogenic aerosols were collected from exhaled breath of infected individuals.A significant percentage of participants or patients spread diseases like Influenza A, Influenza B, HRV, RSV, and Parainfluenza.The particle size of the aerosols was predominantly less than 5 µm, suggesting that these diseases can be transmitted through fine aerosols.This highlights the significant role of bioaerosols in indoor environments in the transmission of various diseases.These tiny airborne particles carry biological substances, which can include bacteria, viruses, or fungi.These particles can be inhaled by individuals, potentially leading to various health issues, particularly respiratory infections.Bioaerosols are reported to play a key role in the transmission of respiratory diseases, such as influenza [34], TB and measles [35], SARS-CoV [36,37] and MERS [87].Epidemiological studies on hospital patients demonstrated a positive association between human respiratory infections and pollution exposure [88,89].Evidence of respiratory infections due to bioaerosols in indoor settings is compiled in table 3.
The findings suggest that indoor environments can be a significant source of bioaerosol exposure and that certain populations, such as patients in hospitals and students in schools, may be at increased risk.These disease microbes/pathogens can remain stable in aerosols for extended periods, increasing the likelihood of their inhalation and subsequent infection.Indoor environments, especially those with people present or with poor building conditions, have elevated levels of bacteria and fungi.The presence of SARS-CoV-2, SARS-CoV-1, MERS-CoV, and other bacteria was detected in these environments.Some studies also highlighted the potential for these diseases to be transmitted through HVAC systems and the stability of these viruses in aerosols in indoor environments.The presence of people and poor building conditions can exacerbate the levels of pathogens in indoor environments.Schools, which are typically densely populated and often poorly ventilated, can also be significant sources of bioaerosols, increasing the risk of exposure for students.These findings emphasize the importance of understanding and managing bioaerosol exposure in indoor environments to protect public health.

Carbon dioxide concentrations and school ventilation
The interest in CO 2 as a proxy indicator of IAQ [96] stems from the fact that the measurement of this relatively inert gas is fairly cheap, and most of the existing mechanical air systems have inbuilt capabilities to measure this gas.Yet, the association of CO 2 with bioaerosols is an emerging research interest that needs to be emphasized.The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) maintains and updates IAQ standards and guidelines [97].For safe and acceptable air quality, ASHRAE Standard 62-2001 suggests indoor CO 2 levels be maintained below 1000 ppm in classrooms and 800 ppm in offices.Occupational Safety and Health Administration (OSHA) has set a permissible exposure limit for CO 2 of 5000 ppm over an 8 h workday with respect to worker safety.The American Conference of Governmental Industrial Hygienists also set the threshold limit value to 5000 ppm for an 8 h workday, with a ceiling exposure limit of 30 000 ppm for a 10 min period based on acute inhalation data (OSHA 2014).Literature, where measurement for CO 2 concentrations was performed inside the school environment, was gathered for IAQ analysis in the school environment.Few studies have noted that the CO 2 levels of 600-800 ppm indicate inadequate ventilation rates [98,99].Exposure to higher CO 2 concentrations is associated with asthma [100,101], reduced work performance, and increased harmful health symptoms [102].Shendell et al reported the association of student absences with CO 2 concentration where the value of 1000 ppm CO 2 is associated with a 10%-20% increase in student absence [46], thus indicating CO 2 concentrations to be an important parameter in the health risk assessment of school students [27].CO 2 concentrations were also associated with breathlessness [103], allergies [104], asthma [103,104], and dry cough [13].One of the major sources of CO 2 in the indoor environment is perhaps the occupants, and CO 2 concentration levels can vary with occupancy level, room structure and design, ventilation, and air exchange rate.
Seasonal variations in CO 2 concentration were observed in school environments, with slightly higher levels in the winter compared to the summer season [56].This variation was accounted for by the lack of proper ventilation with reduced use of natural ventilation through windows in the winter season due to coldness.The ratio of indoor to outdoor CO 2 greater than one indicates the presence of a prominent internal source, with insufficient ventilation from the outdoor environment.Table 4 provides an overview of the studies that have examined CO 2 concentration levels in schools and reported its association with students' health.Most studies reported average and median values of the peak CO 2 concentration exceeded 1000 ppm, with many instances exceeding 2000 ppm.The maximum peak CO 2 concentrations ranged from approximately 3000-6000 ppm.While the association between CO 2 levels and health may not be linearly correlated, the overwhelming observation from the table suggests that increased CO 2 levels can impact human health.This emphasize the importance of monitoring and maintaining appropriate CO 2 levels in schools to safeguard respiratory health, attendance, and academic performance.
These findings highlight the importance of monitoring and maintaining appropriate CO 2 levels in school indoor environments to ensure the well-being and academic performance of students.Inadequate ventilation and elevated CO 2 concentrations can have adverse effects on respiratory health and may impact students' attendance and educational outcomes.These insights underscore the need for improved ventilation in classrooms to ensure optimal CO 2 levels.
To evaluate the current state of CO 2 concentrations in school indoor environments, measurements were conducted in two elementary schools located in urban and rural settings in North Central County, FL, in February of 2021.Both schools use mechanical ventilation systems.The rural elementary school was constructed in 1953, with the addition of buildings in 1965 and 1970, whereas the urban school was opened in 1999.Both schools were situated away from heavy outdoor pollution.The results showed significant differences in indoor CO 2 levels between the two elementary schools (figures 3(a) and (b)).The mean CO 2 concentrations in the classrooms of both schools exceeded the recommended level of 1000 ppm set by ASHRAE 62 standards for schools [120].The urban school had an average CO 2 concentration of 1152 ppm, while the rural school had an average of 1981 ppm.It was also observed that the maximum CO 2 concentrations in classrooms occasionally exceeded 3200 ppm in the rural elementary school.However, the CO 2 concentrations in the cafeteria and reception areas were significantly lower than those in the classrooms of both schools.Overall, CO 2 levels at the two elementary schools exceeded the safe and healthy limit of 1000 ppm in 66.6% and 90.9% of measurements recorded at the urban and rural schools, respectively.A detailed description of the sampling campaign can be found in the supplementary section.The findings from the CO 2 measurements in the two elementary schools highlight the potential concerns regarding IAQ and ventilation in educational settings of North Central County, Florida.The elevated CO 2 levels observed in the classrooms indicate inadequate ventilation and poor air exchange, which can have implications for the health and well-being of students and staff.The results of this study align with previous research that has shown the impact of inadequate ventilation on CO 2 levels in schools.These findings reinforce the concerns regarding inadequate ventilation and poor air exchange in educational settings.The elevated CO 2 levels observed in the classrooms highlight the potential implications for the health and well-being of students and staff.

Association of carbon dioxide with microbes
CO 2 concentration in an indoor environment is an excellent indicator of ventilation effectiveness i.e. the existence of adequate air renewal and the presence of sufficient fresh air inside buildings.CO 2 is exhaled by the infected individuals along with aerosols containing microbes, making it an indirect measure of the risk of microbial exposure within enclosed spaces [52].In other words, if CO 2 is accumulated in an indoor setting, this indoor environment has a higher probability of accumulating other contaminants generated indoors (including microbes).Improved ventilation has been shown to reduce the airborne lifespan of bioaerosols [121].When indoor spaces have inadequate ventilation, CO 2 levels can rise due to the accumulation of exhaled breath from occupants.This increase in CO 2 concentration can create a favorable environment for bioaerosol suspension, particularly in areas with high humidity or moisture [122].The persistence of small  bioaerosols (shown in tables 2 and 3), in insufficiently ventilated space could contribute to the spread of respiratory infectious diseases.Several studies (table 5) have also identified a significant association between CO 2 concentration and microbial concentration.Bioaerosols spread airborne respiratory diseases within susceptible individuals and between the infected individuals and their environment.CO 2 concentrations were found to be associated (table 5) with microbial concentrations, ventilation, and bioaerosols concentration.A study conducted by Myatt et al [125] revealed a significant positive correlation between the probability of detecting airborne rhinovirus and a weekly average CO 2 concentration exceeding approximately 100 ppm.Thus, the elevated CO 2 levels may indicate inadequate ventilation and stagnant air, creating conditions that facilitate the transmission of bioaerosols.Therefore, CO 2 can be indirectly used as an indicator for bioaerosol activity.Accurate measurement of CO 2 concentrations in small spaces or rooms has become feasible with the use of low-cost infrared sensors [126].As a result, monitoring CO 2 concentration can be adopted as a cost effective measure and an initial indicator of transmission risk of respiratory diseases.This capability eliminates the need for more sophisticated devices and time-consuming procedures.By monitoring CO 2 concentrations, it becomes possible to assess the ventilation effectiveness and potential bioaerosol transmission risk in indoor environments.

Discussion
The COVID-19 pandemic has sparked scientific discussions on the societal and economic importance of air quality in built infrastructure, particularly in K-12 schools.Indoor pollutants, including bioaerosols, can have a significant impact on public health, leading to respiratory and cardiopulmonary diseases, and asthma, particularly among children who are more vulnerable due to their developing immune systems.Poor IAQ can lead to increased absences in schools due to respiratory infections, allergic diseases from biological contaminants, or adverse reactions to school chemicals.Classrooms, which are often more congested than other workplaces, require good IAQ for children's comfort, health, performance, and ability to concentrate.The IAQ inside a school building can depend on various factors, and with the likelihood of COVID-19 becoming a seasonal occurrence, the impact on school-age children and their breathing environment needs appropriate attention.This study was conducted to evaluate the state of using CO 2 as a potential environmental marker of IAQ in school buildings and for the assessment of using CO 2 as a respiratory disease indicator through the association of CO 2 and bioaerosol activity.
The vulnerability of school students to CO 2 concentrations exceeding 1000 ppm poses an unchartered threat to their respiratory health and associated impacts.According to a study conducted at Washington and Idaho schools, increased CO 2 levels were postulated to be responsible for reduced school attendance [46].Our observations (figures 3(a) and (b)), in addition to several other studies [22,45,47,62,63,69], show elevated CO 2 concentrations, indicating the inefficiency of the ventilation systems in school buildings.NIOSH and OSHA suggest a time-weighted average of CO 2 concentration to 5000 ppm in workplaces, which is an 8 h threshold limit value, but there is a lack of specific standards for schools.The effect of continuous exposure to CO 2 concentrations among children is not mapped yet and require further investigation.Table 4 suggests that variations in CO 2 concentration and their elevated levels inside the school's buildings are not uncommon.
Bioaerosols, which are tiny airborne particles of biological origin, can contain living microorganisms, including bacteria, viruses, fungi, and others.These particles can remain airborne indefinitely and can infect surrounding human populations, especially when they contain pathogenic microorganisms.The association of indoor bioaerosol activity with respiratory infections is significant.Pathogens' infectivity determines the transmissibility of any infection, the contagiousness of the infected individual, the susceptibility of the exposed individual, and contact patterns.Indoor environments have been found to have significantly higher levels of bacteria, fungi, and viruses than outdoor environments.The emission of bioaerosols can occur from normal breathing and talking of individuals, potentially leading to various health issues, particularly respiratory infections.
The association of indoor bioaerosol activity with respiratory infections is found to be significant.Pathogens' infectivity determines the transmissibility of any infection, the contagiousness of the infected individual, the susceptibility of the exposed individual, and contact patterns.Indoor environments have been found to have significantly higher levels of bacteria, fungi, and viruses than outdoor environments.The emission of bioaerosols can occur from normal breathing and talking of individuals, potentially leading to various health issues, particularly respiratory infections.If CO 2 is accumulated in an indoor setting, it suggests that the indoor environment has a higher probability of accumulating other contaminants generated indoors, including microbes.Several studies have reported a positive correlation between CO 2 concentration and microbial concentration.A study by Peng and Jimenez [52] utilized the role of CO 2 concentration as an indirect measurement of COVID-19 infection.One of the major issues in bioaerosol research is the lack of standardization in the methodology, from air sampling strategies and sample treatment to the analytical methods applied [127].Like other pollutants, pathogenic bioaerosols can cause harm to humans.
This review highlights the importance of monitoring and maintaining appropriate CO 2 levels in school indoor environments to protect public health.CO 2 can serve as an indirect indicator for bioaerosol activity, and its measurement can provide valuable insights into ventilation effectiveness and potential transmission risks of respiratory diseases.To ensure a healthy and conducive learning environment, it is crucial to address ventilation issues and maintain appropriate CO 2 levels in schools.This can be achieved through regular monitoring of IAQ, implementing effective ventilation strategies, and considering the use of CO 2 sensors as a screening method to identify areas with potential ventilation problems.Accurate measurement of CO 2 concentrations in small spaces or rooms has become feasible with the use of low-cost infrared sensors.While the spread of an airborne respiratory infectious disease in a given setting can be determined by exhaustive methods such as environmental sampling and epidemiological or clinical studies, a screening method based on these low-cost monitoring of CO 2 concentrations can be used to identify potentially infection prone areas.Drawing parallels between CO 2 and pathogens sampling studies [52], future research may establish critical CO 2 thresholds that indicate the probability of infectious doses of pathogen, further establishing the utility of CO 2 monitoring as a low cost and safe alternative to sophisticated virus sampling.This information can help guide interventions and control measures to mitigate the spread of respiratory diseases.However, it is important to note that CO 2 concentration alone may not provide a comprehensive assessment of transmission risk, and other factors such as humidity, temperature, and the presence of other respiratory pathogens should also be considered.
In general, IAQ in schools has been lacking when compared to other buildings like offices [97,128].The review also highlights a lack of consensual approach in evaluating and addressing bioaerosols-related IAQ issues poses challenges in accurately assessing health risk.This approach refers to the absence of universally agreed-upon methodology or framework for evaluating and addressing the impact of bioaerosols on IAQ in various indoor environments.Standardization of methodologies, guidelines, and protocols is crucial to ensure consistent and reliable assessment of bioaerosol impacts and to protect the health and well-being of individuals in indoor environments, particularly in schools.Bioaerosol monitoring is largely neglected in the IAQ guidelines, which can lead to the rise of indoor microbial pollutants thus resulting in reduced attendance and performance of the students.Investigations need to be conducted to examine bioaerosol levels, their association with CO 2 and indoor air pollution at school, and their effect on children's mental and physical health.

Figure 1 .
Figure 1.Summary figure: variables affecting the indoor air quality and their effects on occupants.

Figure 2 .
Figure 2. Flow chart for the review process.

Figure 3 .
Figure 3. (a) CO2 levels in a rural elementary school in northcentral FL (Black bar indicates >1000 ppm).(b) CO2 levels in an urban elementary school in northcentral FL (Black bar indicates >1000 ppm).

Table 1 .
Evidence of adverse health effects of Indoor Pollutants on humans.

Table 2 .
Summary of studies reporting pathogenic aerosols collected from exhaled breath of infected hospital patients.

Table 3 .
Summary of studies reporting indoor bioaerosols and respiratory infection.

Table 4 .
Global studies with CO2 concentration levels in schools.

Table 5 .
Carbon dioxide association with microbes.