Indoor and Outdoor Air Quality Assessment of SO 2 and NO 2 in Suburban Schools in Imphal, Manipur

Air quality assessments for NO 2 and SO 2 levels in indoor and outdoor environments were conducted in three suburban schools in Imphal. Season-wise, the highest NO 2 and SO 2 concentrations were observed during winter, while the lowest occurred during the monsoon. The concentration of SO 2 in the indoor air was comparatively cleaner than in the outdoor air. Similarly, indoor air was found to be cleaner than outdoor air with respect to NO 2 . The overall concentration obtained for the entire study period for SO 2 was 4.9±1.5 µg m -3 , and for NO 2 , it was 9.2±3.0 µg m -3 . The concentrations of the two gases obtained during the study were below the NAAQs and WHO permissible limits. Seasonal variation of SO 2 concentrations between the pre-monsoon period and monsoon months and also between the monsoon period and winter months ( at p<0.001 ) was significant. For NO 2 , seasonal variation in the concentrations between the monsoon and winter (at p<0.001 ) was found significant. The estimated I/O ratios for the two studied gases, which were below one, plus the positive correlation between the classroom and campus environment concentrations of NO 2 (r=0.96), and SO 2 (r=0.96), reflected the influence of the indoor air quality by outdoor pollution sources. Data on similar studies for the country’s northeastern region, in particular, is scanty, and that’s why it may provide valuable insights to the scientific world.


Introduction
Intensification of air pollutant levels in urban areas has become a significant problem with fast economic growth, especially in metropolitan cities. 1 Majority of air pollution fatalities occur in developing countries, where laws are not adequately enforced and emission standards are less stringent. 2In India, traffic emissions are a significant source of air pollution and associated damage to human health. 3ehicle emissions are problematic for human health, road safety, and the global environment. 4However, there is uncertainty in vehicle emission estimations in India due to the lack of reliable emission factors representing real-world driving data. 5The complicated impacts of urban-scale urban forms on air pollution are poorly understood. 6Air quality in the indoor environment is a critical determinant of the level of comfort, health status, and well-being of the building residents. 7Understanding the importance of good Indoor Air Quality (IAQ) in school is the backbone of developing an effective IAQ program. 8ood air quality in classrooms becomes an essential need for children in their learning ability and benefits their health.Students, teachers, and staff working in classrooms with poor air quality may develop chronic and acute health issues.Although clean air may be a fundamental human right, many school children worldwide do not access it, posing an increased risk of sickness and pollution. 9Microclimate in the classroom is one of the crucial indicators of whether the environment is healthy or unhealthy. 10Children are highly vulnerable to air pollutants that impact their respiratory system, leading to asthma, allergic rhinitis, and eczema due to their air-breathing pattern and lower body mass index. 11As per the World Health Organisation (WHO) report,the primary air pollutants, such as SO 2 , NO 2 , CO 2 , O 3 , and Particulate Matter (PM), affect human health, causing severe diseases among various age groups. 12Sulphur dioxide is one of the primary gaseous effluents released from the combustion of sulphur-containing fuels, and the pollutant is the main issue of air pollution problems in developing countries. 13Sources of air pollutants in urban areas include transportation sectors, power plants, industrial boilers, incinerators, petrochemical plants, aircraft, and ships. 14Oxides of nitrogen are detrimental to the human respiratory system, and NOx is directly or indirectly involved in ozone and particulate formation. 15Exposure to high levels of air pollutants poses a higher risk of developing chronic illnesses like cardiovascular disease in children, creating complicated health issues later in life. 16Children who attend schools near roadways face the potential risk of exposure to various contaminants arising from roads, thereby, are more prone to hazardous health impacts. 17As a result of a poor public transport network, residents in Imphal increasingly rely on private vehicles in and around Imphal city. 18The current investigation aimed to evaluate the concentrations of NO 2 and SO 2 in three suburban roadside schools situated along the National Highways passing through Imphal city.

Sampling and Analysis
Measurements of NO 2 and SO 2 were conducted parallelly inside the classroom and in the outside environment of the schools.The NO 2 and SO 2 concentrations in the air were measured using portable gaseous samplers (Envirotech APM 433).The concentration of SO 2 was analysed following the standard methods described by the Improved West and Gaeke method. 17The airborne NO 2 level was analysed following Modified Jacob and Hochheiser method. 19The respective absorbing solution (30 mL) for the two gases was lodged in separate midget impingers for carrying out the sampling.
The sampling air was bubbled through the respective absorbing agent at the rate of 1 Litre Per Minute (LPM).The NO 2 and SO 2 content in the air was determined colourimetrically (Genesys 180) at 540 nm and 560 nm wavelengths respectively.

Statistical Analysis
The variation in the concentrations of NO 2 and SO 2 during the study period was evaluated using the One-way ANOVA with Tukey (HSD) Post Hoc test.SPSS software was employed for the statistical calculations.

Results and Discussion
The results obtained during our assessment of SO 2 and NO2 in the suburban schools of Imphal are presented in Tables 2 and 3.In the three suburban schools, the concentrations of indoor SO 2 for the entire study period ranged between 2.5 and 7.7 μgm -3 , and the mean was 4.7±1.1 μg m -3 .The corresponding level of SO 2 in outdoor air during the same period ranged between 2.7 and 8.4 μgm -3 with a mean of 5.2±1.2μgm -3 .For NO 2 , the indoor concentrations ranged between 5.0 and 13.9 μgm -3 , while the outdoor concentration ranged between 5.0 and 15.1 μg m -3 .The mean SO 2 and NO 2 content in the indoor and outdoor air for the three schools for the entire study period were 4.9±1.5 μg m -3 and 9.2±3.0μg m -3 .Dambajamts et al.(2019). 20reported that the two-year mean levels of SO 2 in schools in Ulaanbaatar, Mongolia ranged between 4.4 ppb and 10.1 ppb, while the highest concentrations ranged between 36 ppb and 63 ppb.Salonen et al.( 2019). 21concluded that the average levels of NO 2 in schools ranged between 6.00 and 68.5 μg m -3 , and concentration in office environments ranged between 3.40 and 56.5 μg m -3 .The levels of the two studied gaseous pollutants in the classroom reported by these authors are comparatively higher than the observations obtained in the current study.
Clean air is a concern in the mega cities in India, as the content of fine particulates, SO 2 , and NO 2 , are frequently found to exceed the limits of the National Ambient Air Quality Standards (NAAQS). 224][25] Children are most at risk for health problems related to air pollution. 26The mean concentrations of the two air pollutants were recorded as highest during the winter, and the concentrations decreased consecutively in the following two seasons.During the winter, the SO 2 in classrooms ranged between3.0and 6.7 μg m -3 .The NO 2 content in the classrooms during the same season ranged between 5.0 and 13.9 μg m -3 (Table 2 & 3).The outdoor concentrations of the two gases during winter ranged between 3.0 and 8.4 μg m -3 (SO 2 ) and 5.0 and 15.1 μg m -3 (NO 2 ).Seasonal concentration of the two respective gases during winter was 6.0±1.4 μg m -3 and 11.3±3.4μg m -3 .The content of SO 2 and NO 2 in the classroom during the pre-monsoon was between 2.8 and 7.7 μg m -3 and 5.4 and 12.8 μg m -3 .The corresponding levels of the two gases in the campus ambient air during pre-monsoon were between 3.1 and 8.4 μg m -3 and between 5.0 and 14.5 μg m -3 , and the average gaseous content in the air was 5.7±1.5 μg m -3 and 9.7±2.9μg m -3 .In monsoon, the content of SO 2 in the air ranged between 2.5 and 5.7 μg m -3 , and for NO 2 , it ranged between 5.1 and 11.1 μg m -3 .In the same period, the SO 2 in the outdoor environment was from 2.7 to 5.4 μg m -3 ) while the content of NO 2 ranged between 5.1 and 14.5 μg m -3 with respective mean values of 4.0±0.8μg m -3 and 8.1±2.1 μg m -3 .Lower gaseous pollutants in the schools during monsoon and pre-monsoon seasons might be due to persistent rainfall and the prevailing turbulent meteorological conditions.In contrast, ambient air is considerably drier during winter, and the washout of pollutants by precipitation is almost non-existent.Mukta et al.(2020)., 27 in a study at Gazipur, Bangladesh, observed mean seasonal NO 2 values of 45.1 ppb, 33.5 ppb, and 13.9 ppb, respectively, during winter, pre-monsoon, and monsoon.The researchers claimed that the washout effect of precipitation lowered the gas concentration during the rainy season. of SO 2 and NO 2 concentrations in indoor and outdoor environments are presented in Figure 1.

Fig. 1:Temporal variations of: [A] SO 2 and [B] NO 2 concentrations
The monthly variations of the two pollutants in the classrooms and the ambient air of the three schools are depicted in Figure 1.The highest indoor concentration of SO 2 (7.7 µg m -3 ) occurred during March.However, the peak outdoor concentration (8.4 µg m -3 ) occurred in two months (March and February) in SU-1.The comparatively higher SO 2 and NO 2 concentrations obtained in the current study during the pre-monsoon and winter might be due to high temperatures and low humidity.A significant seasonal variation in the concentrations of SO 2 was observed between the pre-monsoon and monsoon samplings(at p< 0.001) and also between the monsoon months and winter months (at p< 0.001).
For NO 2 , a significant variation in the concentrations of the gas was observed only between the monsoon and winter (at p< 0.001).
The lowest SO 2 concentrations in the schools were observed during September for the indoor (2.5 µg m -3 ) as well as outdoor (2.7 µg m -3 ) environments, both were observed in the SU-1.Similarly, for the NO 2 , the highest concentration was observed during January in both the indoor (13.9 µg m -3 ) and outdoor air (15.1µg m -3 ) of the SU-1.During the study, the lower most indoor air pollutant levels (5.0 µg m -3 ) occurred during February in the SU-3, while the lowermost outdoor pollutant levels (5.0 µg m -3 ) occurred in January and April in SU-3.The SU-1 is located along the road, while the school SU-3 is situated in a rural residential area.Dandotiya et al., (2019) 31 observed that the adverse health consequences of air contaminants were lower in well-vegetated areas than in high-traffic metropolitan areas.It is noteworthy to mention here that the overall mean concentrations of the SO 2 (4.9±1.5 µg m -3 ), as well as NO 2 (9.2±3.0 µg m -3 ), were well below the prescribed limit recommended by NAAQS (SO 2 -50 µg m -3 annual and 80 µgm -3 24-hrs mean, NO 2 -40 µg m -3 annual and 80 µgm -3 24-hrs) 19 and the WHO 32 (40 µg m -3 24-hrs mean for SO 2 , and 25 µg m -3 24-hrs mean for NO 2 ).Salonen et al.( 2019) 21 reported that the concentrations of NO 2 in the school and the office were well below the WHO-recommended concentrations.

I-O Relationship
The indoor and outdoor relationship of the two gases was deduced with the help of linear regression (Figure2).The mean I/O ratio was well below one for both the SO 2 (0.89±0.07) and NO 2 (0.90±0.09).Analysis of the classroom and campus air contaminations of the two gases exhibit a positive correlation, SO 2 (r= 0.96), and NO 2 (r= 0.96), and was significant (p < 0.001), showing that outdoor air pollutant sources influenced the air quality in the classrooms.The concentrations of NO 2 in the outdoors may be considered a reliable predictor of indoor NO 2 levels. 21Majd et al.( 2019) 33 opined that pollutants significantly contributing to indoor exposure were mainly related to outdoor sources, indicating a strong correlation between indoor concentrations with outdoor sources.

Conclusion
In modern times, proper monitoring of air quality in schools has become highly necessary due to a host of multiple underlying factors.Some of the pertinent reasons compelling air quality analysis in schools are the ever-increasing traffic volume near schools, mushrooming of schools near the roadside, children's sensitivity to air contaminants, the time children spend in the schools, the crowded working space in classrooms, etc.The current study investigated the status of air quality in the schools in the suburban area of Imphal city with respect to the content of SO 2 and NO 2 .The peak SO 2 (7.7 µg m -3 ), as well as NO 2 (13.9 µg m -3 ) in classroom air, were observed in a school (SU-2) located very close to a heavy-traffic road.In contrast, the lowest concentration of the two gases in the classroom was observed in a school (SU-3) in a suburban residential area.Air pollutants released from transportation remain the primary cause of air contamination in the city and its surrounding suburban areas.Seasonwise, the classrooms' highest concentrations of SO 2 and NO 2 were observed in winter, while the lowest concentrations of the gases were observed during the rainy season.The higher concentrations of gaseous pollutants in the schools during winter might be due to decreased humidity and the prevailing atmospheric conditions.The levels of gaseous pollutants in the campus and classrooms of the schools were found to be positively correlated.
The I/O ratio indicated that the outdoor polluting sources of the two gases influenced indoor air concentrations.However, the levels of the two gaseous pollutants observed during the current study were within the safe concentration range recommended by the WHO and NAAQS Standards.

Fig. 2 :
Fig. 2: Gaseous concentration in the classroom versus outdoor air of SO 2 [I] and NO 2 [II]

Table 1 : Some characteristics of the schools under study Parameters SU-1 SU-2 SU-3
Imphal serves as the gateway city, connecting mainland India with the Southeast Asiatic countries.The present study was conducted in three suburban schools in Imphal.The schools taken up for the current study are SU-1: Manipur Public school, Koirengei, SU-2: Standard Robarth Higher Secondary school, Canchipur, and SU-3: Children's Training school, Langol in Imphal.Out of the three, the first two schools are situated along the National Highway (SU-1 and SU-2 are located along the NH-2 and NH-102, respectively), while the third is located in a residential area.The study was conducted for eleven months (March 2018 -February 2019), covering three seasons, viz. the pre-monsoon, monsoon and winter.The instrument was run for eight hours on each air sampling day, starting from 8 am to 4 pm.Sampling campaigns were reported twice a month, covering 22 samplings in each school in the eleven months study period.The main air pollution sources in the suburban areas of Imphal include vehicular emissions, small enterprises, diesel-powered electric backup generators (especially in commercial and settlement zones), diesel-runmobile tower generators, domestic activities, etc.

Table 3 : Seasonal NO 2 concentrations in the suburban schools
30concluded that higher temperature and relative humidity favour the condensation of SO 2 and NO 2 into sulfate and nitrate aerosols.The temporal variations (monthly and seasonal)