Internal particulate matter pollution in educational building

The authors undertook research on the proper strategy of operation of educational building with gravitational ventilation in historic city center with high concentration of particulate matter PM10 and PM 2.5. In this facility the momentary increase in carbon dioxide concentration is often very high, and at the same time health requirements regarding atmospheric aerosol should be absolutely met. That is why long-term measurements of PM concentration outside and inside, as well as carbon dioxide concentration inside were carried out. CO2 was used also as a tracer gas for measurement of air change intensity. The article presents the first results of these tests and a correlation that occurs between the external and internal concentration of particulate matter PM 10 and PM 2.5. Due to a significant filtration effect of the external building envelope and particle deposition a potential conflict between required gravitational ventilation intensity and internal air pollution with particulate matters was partially reduced.


Introduction
Two things work against Krakow's air quality: pollution and geographical factors that prevent the dispersal of pollution. Krakow, a city of great historical value and the former capital of Poland, is located in a very large valley. This unfavourable location makes natural ventilation of the city very difficult and for many days of the year the air speed is very low here. Due to the wide use of coal for apartment heating and intense urban traffic in the historic city centre, emission of pollutants is high, which causes a high concentration of atmospheric particulate matter PM10 and PM2.5. Effective ventilation of building interior is to maintain good indoor air quality by, among others, limiting the concentration of carbon dioxide, air humidity, chemical pollution, etc. At the same time, during periods of very high atmospheric PM concentration, the intended air exchange should be significantly reduced. The authors undertook research on the proper strategy of operation of buildings with gravitational ventilation in these conditions, and in particular educational buildings. In these facilities, in densely occupied spaces, the momentary increase in carbon dioxide concentration is often very high, and at the same time health requirements regarding atmospheric pollutions should be absolutely met. The mechanisms of diffusion of suspended particles in the atmosphere into the building are not yet recognized, as well as the relationship between particles concentration and ventilation or air infiltration. That is why long-term measurements of PM concentration outside and inside, as well as carbon dioxide concentration inside are carried out in the educational building. In addition, the infiltration tightness of the monitored room was tested by means of the pressure method and random air exchange measurements by means of the trace gas method are conducted. The article presents the first results of these tests and the correlations between the concentration of particulate matter PM 10 and PM2.5 in the interior and surroundings of the building, and the concentration of carbon dioxide. The purpose of the research is to indicate the rules of space use that will limit users' exposure to adverse conditions.

Building ventilation
Building ventilation system provides the fresh and removes contaminated and humid air to and from the interior environment which is crucial for the internal air quality. Indoor air quality is significantly associated with ventilation. Low ventilation air exchange can negatively affect the inside air quality and people's health. On the other hand the same process of air exchange supplies the external contaminated air to the internal environment. Intentional air exchange in form of ventilation should be considered in conjunction with infiltration (uncontrolled and unintentional airflows) through the leaks in the building envelope.
The efficiency of air exchange can be evaluated using gas tracing method. One of the gases which may be used as a medium in those measurements is carbon dioxide. The CO2 generated by users can be treated as the tracer gas. It makes the tests simpler and cheaper as no artificial gas emitters are required. Gas tracing measurement with CO2 allows for simultaneous evaluation of ventilation efficiency, building airtightness and also partially air quality. This approach to the problem was suggested by Persily [1], who presented some techniques to evaluate the building ventilation efficiency and indoor air quality analyzing carbon dioxide concentration within the internal space. The similar problems were described by Bulinska [2], who used metabolically generated carbon dioxide to determine efficiency of ventilation systems in the buildings. Zhang [3], Nowak-Dzieszko [4] used the same method while testing the ventilation rates of dormitories and offices. Air quality measurements in the university facilities were described by Benedettelli [5] and Cichowicz [6]. The measurements were conducted using gas tracer concentration decay method. The methodology of measurements is described in the ISO standard PN-EN12569 [7]. This method was also described by Sherman [8]. It is based on the analysis of natural decay of concentration of a tracer gas mixed with the internal air.

CO2 concentration
Polish legal acts do not specify admissible concentrations of carbon dioxide in atmospheric air and in rooms intended for permanent residence, for example for dwellings. Because of that, the standards and recommendations of other European countries and the United States (ANSI/ASHRAE) [9], [10] are most commonly used to determine acceptable concentrations. According to those requirements [9] the upper level of carbon dioxide concentration in spaces for permanent residence of people should not exceed 1000 ppm. To keep this requirement about 27 m 3/ h of fresh air per person should be supplied per hour [9].
In every enclosed space where a human resides, the concentration of carbon dioxide increases because of depletion of the oxygen due to process of breathing. The carbon dioxide concentration in exhaled air is about 40 000 ppm [10]. However, the exact amount of CO2 produced by human body may differ depending on its weight and level of metabolic activity [9], [10]. The CO2 exhaled stream is related to the amount of oxygen consumed. The amount of oxygen consumed by one person in m 3 /h can be determined from the equation (1) [7]: where: AD is DuBois surface (surface of human body) measured in m 2 , M is a metabolic rate, met (1 met is equal to 58.2 W/m 2 ), Rq is a respiratory quotient which can be assumed depending on the intensity of performed activity. The DuBois surface area can be taken as 1.8 m 2 for an average adult person. Polish regulations define the permissible concentrations and intensities of harmful factors in the working environment [11]: 1. Permissible exposure limit (NDS) -the weighted mean value of concentration, the impact on an employee of 8 hours per day and the average weekly working time during his/her working activity should not result in negative changes in his/her health and health of his/her future generations; 2. Short term permissible exposure limit (NDSCh) -mean value of concentration which should not cause negative changes in the worker's health if it is present in the work environment for not more than 15 minutes and not more than 2 times during the work shift, with the a time interval not shorter than 1 hour. 3. Maximum exposure limit (NDSP) -value of concentration which due to health or life threat of the worker, cannot be exceeded in the work environment at any time. According to this regulation NDS of carbon dioxide is 5000 ppm (9000 mg/m3), and NDSCh cannot exceed 15 000 ppm (27 000 mg/m3).
Permissible levels of carbon dioxide in nonresidential spaces are regulated by the standard PN-EN16798-3 [12]. It introduces four acceptable levels of carbon dioxide concentration depending on the selected level of indoor air quality. The table 1 shows the permissible values of the carbon dioxide concentration according to the air quality category.
In Poland there are no formal and obligatory requirements regarding the acceptable levels of CO2 concentration in the university educational buildings. Polish designers use the guidelines of the European standard in their work, attempting to maintain CO2 concentration not higher than 1000 ppm. Category of air quality in the room

Increment of CO2 concentration above CO2 level in external air, [ppm]
High quality of air 350 Medium quality of air 500 Moderate quality of air 800 Low quality of air 1200

Atmospheric particles
Understanding of the relationship between indoor and outdoor particles concentration is very important from the occupants' perspective, especially when the levels of particulate matters outside are very high.
In the developed countries individuals spent the majority of time indoors, being exposed to the pollution in the used spaces. Epidemiological studies consistently demonstrate adverse effects of particulate matter exposure on human health. Internationally PM2,5 is estimated to cause about 3% of all mortality from cardiopulmonary disease, about 5% of mortality from cancer of the trachea, bronchus, and lung, and around 1% of mortality from acute respiratory infections in children under 5 years Cohen (2005) [13], Taylor (2014) [14]. The above E3S Web of Conferences 1 0 (2020) 72, 6008 NSB 2020 ttp://doi.org/10.1051/e3sconf/20201720 h 0 6 08 mentioned data makes the analysis of indoor air quality very important.
In the modern buildings with mechanical ventilation systems equipped with filters, the air entering the building through infiltration and ventilation is partially cleaned. The effects of using the filters were described by Quang (2013) [15] and Spilak (2014) [16]. The usage of HEPA filters or PFU (particle filtration units) can reduce the internal concentration of particles up to 60%.
In buildings with gravitational ventilation the amount of particles transported to the inside is strictly connected with air infiltration mechanism. The correlation between indoor and outdoor particle concentration can be described by the widely used parameter (I/O ratio) indoor/outdoor ratio [17].
Where Cin and Cout are the indoor and outdoor particle concentration, respectively. conducted the measurements in 17 houses in Switzerland, in clean environment and estimated the I/O ratio. In houses without any indoor sources the PM I/O ratio was lower than 0.7 and in buildings with internal sources it was higher than 1.8.
Estakova (2010) [20] conducted similiar analysis in one flat in Kosice, Slovakia, and the conclusions were similar, concentration was the highest in the kitchen due to the internal PM sources.
The room tested in this paper is free of any iternal PM sources like smoking, open fire, cooking etc.

Measurement methods
Measurements with the gas tracing method, using metabolically generated carbon dioxide, were conducted using 3 Rotronic meters CP11 (Fig.1) allowing the continuous measurements and recording of CO2 concentration, temperature and relative humidity. Measurement precision of Rotronic meter is ±30 ppm.
Concentration of particulate matters was measured and recorded by means of "looko2" device with laser sensors. The analysed room #11 is located on the ground floor of the three-storey educational building at the main campus of Cracow University of Technology, in the old city centre. It is a computer laboratory, used by maximum 15 people. The room has only one external wall with three large (2,55 m width and 1,8 m height) windows at south elevation of building. Fresh air is supplied in natural way by the air vents, located in PVC window frames, and exhausted by two ventilation ducts 10 cm by 10 cm located 3,0 m above floor level. There is no mechanical air handling unit and no filter system in this building. Volume of the room is 149,6 m 3 .
Tests were conducted between 10 th of December 2019 and 15 th of January 2020. The simultaneous measurements of CO2, PM10, PM2,5 and PM1 concentration have been conducted. Measuring devices were located on the internal wall, two meters above floor.
The same device was located outside the building on the south elevation of building to record the external PM concentration, Figure 1.

Analysis of CO2 concentration
Two CO2 detectors were located in two different places of the tested room in a 2 meters distance from users to avoid direct contact of the exhaled air with the sensors, according to ISO standard [7]. The concentration of carbon dioxide in a representative locations should not differ by more than 10% from the mean concentration value in the tested zone [7]. Before performing the test all the sensors were calibrated in accordance with the manufacturer's instructions.
Measurements were carried out for one month but only some representative periods were selected to emphasize the analysed phenomena. Concentration of particulate matters both inside and outside was measured continuously but only the average hourly data were recorded.
Analysis of CO2 concentration will be divided in two parts, first one while room was occupied by students and the second part when students left (analysis of CO2 concentration decay).  Fig. 2 presents changes of CO2 concentration during one day when room was occupied by users. A sharp increase of carbon dioxide concentration can be observed when students entered the room at 12.00, then the decline in concentration was caused by a longer break in class and next at 3:30pm. Presence of 10 people in this room resulted in the final increase of CO2 concentration up to 1400 ppm within ca. 3 hours. Around 2.00 pm windows were open and the concentration drop was observed. At 3:30 14 people entered the room which resulted in the increase of CO2 concentration to more than 2000 ppm within 1.5 hour. Based on the received data the quality of internal air was classified. The external concentration of CO2 during entire measurement time was assumed at the level of 400 ppm. The internal concentration of carbon dioxide in the specific periods of time was higher than 1700. Per the classification of standard [12], that was presented in Table 1, the indoor air in tested room can be qualified as low, which is very unsatisfactory result for university educational utility.
Concetration decay data were used later to calculate the natural air flow rate by means of gas tracing method [7]. The expmple of CO2 concentartion decay curve is presented on Figure 3. Similar analysis and charts as above were prepared for several other selected measurement periods.
The specific airflow rates were calculated separately for five different days by means of a two point decay method. As decay period time is not strictly determined in the standard [7] different periods were analysed: one, two, and three hours from initial concentration levels. In order to calculate the air exchange rate, equation concerning 2 point decay was used. (2) where: N-time-mean specific airflow rate (1/h) t -time (t1: measurement start point, t2: measurement end point) (h); The selected results of measurements are presented in Table 2. The values of specific airflow rate do not differ by more than 5% between each other. The external temperature conditions were similar in the analysed periods, temperature fluctuated between 3°C and 6°C.
For further considerations of the ventilation efficiency of the office room, the average airflow rate value n=0.5 [1/h]

Relation between indoor and outdoor particles concentration
As the most of people spent about 85-90% [13] of their time indoors, the understanding of relationship between indoor and outdoor particles concentration becomes very important, especially when the external pollution concentraion level outside is very high. The widely used parameter describing this relation is I/O (indoor/outdoor) ratio which varies considerably due to the air change rates. In many countries when the topic of air quality is mainly connected with the usage of the building (internal conditions) and where the external air quality is high it is above 1. In Poland during winter months it's lower than 1 as the concentration of particulate matters outside is high. The external air quality in Poland is classified by the six Air Quality Indexes (AQI), the appropriate values of PM10 and PM2.5 concentration levels are presented in table 3. Those ranges are much higher in eastern countries and for example in China the Hazardous value of PM10 concentration is in the range 300-500.    According to a review conducted by Chen (2011) [13] linear regression can be used for simplified assessment of the infiltration factor. Fig. 8 shows a relationship between indoor air concentration of PM10 against concentration of PM10 in outdoor air of the tested room. Since there are no significant PM sources in the room, the graph practically starts at the beginning of the coordinates, the slope of the regression line corresponds to the value of the infiltration coefficient. Over 70% reduction of external PM concentration may be expected inside of tested space in average winter conditions. The actual value of infiltration coefficient depends in reality on the air change intensity, penetration coefficient and deposition coefficient. In buildings with natural ventilation three major deposition mechanisms may be observed: gravitational setting, Brownian diffusion and inertial impaction. These effects are, in turn, strongly dependent on the shape and size of the gaps in the building envelope, on the material characteristics of the room and its equipment and also on the size of particles suspended in the air. In case of the tested space E3S Web of Conferences 1 0 (2020) 72, 6008 NSB 2020 ttp://doi.org/10.1051/e3sconf/20201720 h 0 6 08 infiltration coefficient is practically not sensitive to particles' size, Fig. 9. According to Chen (2011) [13] significant differences in the values of infiltration coefficient are observed for very fine particles (below 1 ȝm).
Because a particularly high concentration of PM is associated with the so-called low emission during winter, it can be stated that a massive building envelope and standard windows are efficient barrier for particulate matter transfer and prevent excessive pollution of internal air with PM particles.

Conclusions
Metabolically generated carbon dioxide, used as a tracer gas, can be a source of a unique and useful information on the space characteristics. The decay of the tracer gas allows for calculation and evaluation of ventilation system efficiency. The results of measurements can be a specific source of information about actual air change intensity.
Simultaneous measurements of particulate matters concentration inside the room and in external environment allowed to confirm that I/O (indoor/outdoor) ratio varies considerably due to the external conditions fluctuations. The high PM concentration outside the building has a decisive impact on indoor air quality, especially when there are no sources of contamination inside.
Linear regression can be used as a tool of the simplified assessment of the infiltration factor. This quantity allows an assessment of the expected indoor conditions due to external air pollution.
The conducted measurements and calculations of CO2 concentration in the classroom proved that the efficiency of ventilation system is not sufficient and the air quality is poor. In the same time the low air exchange rate may be regarded as a form of protection of the internal space against the intake of particulate matters from external environment. Filtration effect of the external building envelope and PM deposition in the room reduce a potential conflict between ventilation intensity and internal air pollution with particulate matters in building with gravitational ventilation.