Claire Trocquet

Irene Lara-Ibeas

Aurélie Schulz

Pierre Bernhardt

Béatrice Cormerais

Stéphanette Englaro

Stéphane Le Calvé

Continuous aldehydes monitoring in primary schools in France: Evaluation of emission sources and ventilation practices over 5 weeks

Keywords: Aldehydes, indoor air, hexanal, acetaldehyde, schools, ventilation Highlights

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Over the past two decades, indoor air quality (IAQ) has become a major concern among the scientific community, government and even the general public. This is explained by the harmful effects of air pollutants on human health. According to the World Health Organization (WHO), indoor air pollution leads to 4 million premature deaths per year, the vast majority being children [START_REF] Who | Household air pollution and health[END_REF]. Children are more fragile than adults to air pollutant exposure, not only because their immune systems are not fully developed but also because their inhalation rates are higher and they spend longer periods indoors [START_REF] Mendell | Do indoor pollutants and thermal conditions in schools influence student performance? A critical review of the literature[END_REF].

Between the ages of 3 and 16, children spend almost 30% of their day at school, most of the time in a classroom. Consequently, indoor air quality in schools has a strong impact on children's exposure to air pollutants and can also influence their learning capacities [START_REF] Mendell | Do indoor pollutants and thermal conditions in schools influence student performance? A critical review of the literature[END_REF][START_REF] Simoni | School air quality related to dry cough, rhinitis and nasal patency in children[END_REF]. Indoor air quality in schools is usually characterized by a range of parameters including temperature, relative humidity, and levels of carbon dioxide (CO2), ventilation rates and average concentrations of gaseous pollutants [START_REF] Gaihre | Classroom carbon dioxide concentration, school attendance, and educational attainment[END_REF][START_REF] Jiang | A study on pupils' learning performance and thermal comfort of primary schools in China[END_REF][START_REF] Rodriguez | Classroom-comfort-data: A method to collect comprehensive information on thermal comfort in school classrooms[END_REF][START_REF] Verriele | The MERMAID study: indoor and outdoor average pollutant concentrations in 10 low-energy school buildings in France[END_REF] although there is no specific guidelines to assess IAQ in classrooms. These latter are usually determined by a single measurement over 4.5 or 7 days of passive sampling.

Aldehydes concentrations are usually higher than the ones found outdoors since many sources of aldehydes exist in indoor environments: combustion processes involving organic matter (smoking, cooking, domestic wood heating, wood pellets), building and decorating materials, furnishing products and everyday consumer goods (floor cleaners, wooden flooring, laminates, glues, varnishes, paint, etc.) [START_REF] Fortmann | Characterization of emissions of volatile organic compounds from interior alkyd paint[END_REF][START_REF] Marchand | Concentrations and determinants of gaseous aldehydes in 162 homes in Strasbourg (France)[END_REF][START_REF] Rahman | Occupational exposure of aldehydes resulting from the storage of wood pellets[END_REF]. In an air quality study conducted in France, the indoor concentration of acetaldehyde was higher than outdoors in 98% of the investigated homes (ANSES, 2014).

Numerous studies have been carried out in order to monitor the indoor/outdoor levels of Volatile Organic Compounds (VOCs) including aldehydes in schools in France, [START_REF] Breizh | Poor air quality in classrooms related to asthma and rhinitis in primary schoolchildren of the French 6 Cities Study[END_REF][START_REF] Canha | Assessment of ventilation and indoor air pollutants in nursery and elementary schools in France[END_REF][START_REF] Lara-Ibeas | BTEX near real-time monitoring in two primary schools in La Rochelle, France[END_REF][START_REF] Nasreddine | Portable novel microdevice for BTEX real-time monitoring: Assessment during a field campaign in a low consumption energy junior high school classroom[END_REF][START_REF] Trocquet | Near Real-Time Monitoring of Formaldehyde in a Low-Energy School Building[END_REF][START_REF] Verriele | The MERMAID study: indoor and outdoor average pollutant concentrations in 10 low-energy school buildings in France[END_REF], in European countries [START_REF] De Gennaro | Indoor and outdoor monitoring of volatile organic compounds in school buildings: indicators based on health risk assessment to single out critical issues[END_REF][START_REF] Madureira | Indoor air quality in Portuguese schools: levels and sources of pollutants[END_REF][START_REF] Pegas | Indoor air quality in elementary schools of Lisbon in spring[END_REF][START_REF] Smedje | Asthma among secondary schoolchildren in relation to the school environment[END_REF][START_REF] Synnefa | An Experimental Investigation of the Indoor Air Quality in Fifteen School Buildings in Athens, Greece[END_REF][START_REF] Villanueva | Aldehyde Measurements in Indoor and Outdoor Environments in Central-Southern Spain[END_REF] and worldwide [START_REF] Adgate | Outdoor, indoor, and personal exposure to VOCs in children[END_REF][START_REF] Chan | Estimated effect of ventilation and filtration on chronic health risks in U.S. offices, schools, and retail stores[END_REF][START_REF] Godwin | Indoor air quality in Michigan schools[END_REF][START_REF] Mi | Current asthma and respiratory symptoms among pupils in Shanghai, China: influence of building ventilation, nitrogen dioxide, ozone, and formaldehyde in classrooms[END_REF][START_REF] Mishra | Volatile Organic Compounds: Characteristics, distribution and sources in urban schools[END_REF][START_REF] Shendell | Air concentrations of VOCs in portable and traditional classrooms: results of a pilot study in Los Angeles County[END_REF][START_REF] Zhong | Volatile Organic Compounds (VOCs) in Conventional and High-performance School Buildings in the U[END_REF].

The studies reporting airborne aldehydes concentrations performed in French and European schools are summarized in Tables 1 and2, respectively. Most of studies performed in schools are focused on VOCs, TVOC, formaldehyde and other pollutants such as NO2, CO2, CO, PM2.5 and PM10. A large part of them have been logically only dedicated to the molecules identified as the most dangerous, namely formaldehyde and benzene [START_REF] Izard | Air intérieur dans les écoles et crèches : Air intérieur dans les écoles et crèchesBilan des mesures 2009 -2011[END_REF][START_REF] Kirchner | Qualité de l'air et confort dans les écoles en France : premiers résultats de la campagne nationale[END_REF][START_REF] Michelot | Monitoring indoor air quality in French schools and day-care centres[END_REF] which are known to be carcinogenic (IARC, 2006). However, other aldehydes can have negative health effects as well. For example, [START_REF] Breizh | Poor air quality in classrooms related to asthma and rhinitis in primary schoolchildren of the French 6 Cities Study[END_REF] reported, from a study performed in 401 classrooms of 108 primary schools in 6 major French cities, that higher indoor concentrations of acrolein were statistically associated with increased past year asthma and past year allergic asthma, but not with increased rhinoconjunctivitis [START_REF] Breizh | Poor air quality in classrooms related to asthma and rhinitis in primary schoolchildren of the French 6 Cities Study[END_REF]. Several effects were observed in humans after exposure to acetaldehyde vapours such as eyes irritation, skin, and upper and lower airways, and even bronchoconstriction in asthmatics. In addition, acetaldehyde has been classified as possibly carcinogenic to humans (Group 2B) by the International Agency for Research on Cancer (IARC) since 1999. Consequently, the French Agency for Food, Environmental and Occupational Health & Safety established in 2014 a guideline value for acetaldehyde of 160 µg.m -3 for an exposure duration of one year or more to protect from the long-term effects (ANSES, 2014).

In this work, active sampling was employed to monitor aldehydes concentrations in three different schools in the city of La Rochelle. This field campaign was conducted in the framework of Impact'Air project which aimed at monitoring French indoor regulated air pollutants (formaldehyde, benzene and carbon dioxide), identifying the main emission sources of air pollutants (building materials vs furniture for example) and evaluating the impact of various ventilation practices on indoor air quality. [START_REF] Lara-Ibeas | BTEX near real-time monitoring in two primary schools in La Rochelle, France[END_REF] already reported and discussed the BTEX and CO2 values measured during this field campaign [START_REF] Lara-Ibeas | BTEX near real-time monitoring in two primary schools in La Rochelle, France[END_REF] while the data related to the real-time formaldehyde monitoring were submitted elsewhere (Trocquet et al., 2020a[START_REF] Trocquet | Continuous real-time monitoring of formaldehyde over 5 weeks under various ventilation scenarios in two French primary schools[END_REF].

We report thus C2-C7 aldehydes concentrations in two elementary schools and one kindergarten over 5 consecutive weeks and 1 week, respectively. To our knowledge, this is the first study which reports aldehydes monitoring by means of active sampling on DNPH tubes over a period greater than one month. Such long-term continuous measurements allow to obtain temporal variations in aldehydes concentrations according to different conditions (occupation/inoccupation) and different ventilation practices.

Methodology

Site description

The present work was performed in a medium French town of around 75,000 inhabitants called La Rochelle and located in the west of France near the sea. Two public primary schools and one kindergarten from now noted schools 1, 2 and 3 (S1-S3) were chosen considering as representative schools of the city. They were all built in the same decade, i.e., from 1965 to 1975, and are in residential suburban areas at 2-5 km from the city center (See Figure SI.1). S1 and S3 are located at about 150 m and 300 m from the highway, respectively.

In each school, the analytical instruments were installed inside an adjoining room of the selected classroom to minimize noise and limit the sampling tube length. In all the classrooms, the floors were tiled, and the walls painted.

The volumes of the three studied classes studied were similar: 187 m 3 (S1), 198 m 3 (S2) and 180 m 3 (S3). In Schools 1 and 2, the investigated classrooms were located on the first floor and had 4 westfacing windows and 2 or 3 doors, most of these doors being connected to the corridor. The classroom of S3 was on the ground floor, had 4 large south-east facing bay windows and was equipped with 3 doors to facilitate the movement of young children towards the corridor, the playground, and the library. During school hours, the classrooms were occupied by 25 students of 9-11 years in S1, 16 students aged between 7 and 9 years in S2 and 23 students of 3-6 years in S3.

All the investigated classrooms were naturally ventilated and equipped with heating radiators to regulate the temperature at about 20°C during the day and 16°C during the night from Monday to Friday.

Sampling and analytical instruments

The ISO 16000-3 reference method related to off-line aldehydes monitoring in air, is based on active sampling using 2,4-Dinitrophenylhydrazine (DNPH) tubes (Waters, Sep-Pak) followed by hydrazones quantification by high performance liquid chromatography equipped with a UV detector (ASTM International, 2009). This method allows the quantification of C1-C8 aldehydes present in ambient air. In this study, air was sampled with three automatic sampling devices, namely MP4V1, MP4V2 and MP4V3, composed of a mass flow controller (Bronkhorst, Nijverheidsstraat, The Netherlands), a gas pump (830 KNDC, KNF) and 4 ways, each one equipped with two solenoid valves (VDW23-6G-3, SMC). These autosamplers operated at 0-2 L min -1 . Once the samplings were completed, DNPH tubes were stored at 4 °C and then eluted with 2-3 mL of acetonitrile (Sigma Aldrich, 99.8%). An amount of 20 µL of the resulting hydrazones solution was then injected and quantified by HPLC/UV using an external calibration. A nonpolar C18 column and a wavelength of 360 nm were employed for hydrazones separation and detection, respectively. More technical details are available in previous publications [START_REF] Marchand | Concentrations and determinants of gaseous aldehydes in 162 homes in Strasbourg (France)[END_REF][START_REF] Trocquet | Near Real-Time Monitoring of Formaldehyde in a Low-Energy School Building[END_REF].

The calibration plots were linear for the hydrazone concentration ranges used in this study, i.e., between 0.5 and 10 mg L -1 , the correlation coefficients being higher than 0.99. With a sampling air volume of 200 L and a signal/noise ratio of  3, the aldehyde detection limits in the atmosphere were estimated to be (in units of µg m -3 ): 0.14 (AA) ; 0.12 (PA); 0.14 (BA) and 0.27 (HA). The limits of detection increases with the retention time of the aldehyde, the peak being significantly narrower for acetaldehyde than for hexanal. For each series of 2,4-DNPH tubes, two or more blank tubes were used to precisely quantify the residual aldehyde concentrations. When reducing the air sample volume to 120 L, the detection limits of aldehydes becomes (in units of µg m -3 ): 0.23 (AA), 0.20 (PA), 0.23 (BA) and 0.45 (HA).

Concretely, MP4V1 and MP4V2 autosamplers were installed in School 2 except for week 4 when MP4V1 was moved to School 3 while the MP4V3 autosampler was only used in School 1 for the whole campaign. In previous studies, [START_REF] Trocquet | Near Real-Time Monitoring of Formaldehyde in a Low-Energy School Building[END_REF] and [START_REF] Nasreddine | Portable novel microdevice for BTEX real-time monitoring: Assessment during a field campaign in a low consumption energy junior high school classroom[END_REF] have shown that the VOC concentrations were almost stable during the night while their concentrations exhibited significant variations during the day due to the ventilation [START_REF] Nasreddine | Portable novel microdevice for BTEX real-time monitoring: Assessment during a field campaign in a low consumption energy junior high school classroom[END_REF][START_REF] Trocquet | Near Real-Time Monitoring of Formaldehyde in a Low-Energy School Building[END_REF] and the presence of occupants. Therefore, it was chosen to sample on a long time at a low flow rate, i.e., 0.35 L min -1 for 600 min, and on a short time at a high flow rate, i.e., 35-60 min at 2 L min -1 , for night and day, respectively. The resulting air samples volumes varied usually between 120 and 210 L with only two air samples of 70L.

During the whole campaign, Temperature, Relative Humidity and CO2 were measured using Q-Trak TM IAQ monitor (TSI Incorporated, St. Paul, Minnesota, USA) in all schools. In addition, LUM'Air (Cairpol) was used to alert when threshold levels of CO2 were exceeded.

Sampling conditions and ventilation scenarios

The field campaign was conducted in S1 and S2 during winter for 5 consecutive weeks from 15 th of February 2016 to 20 th of March 2016. S3 was investigated only for the week 4.

All instruments excepting CO2 probes were installed in a room adjacent to the investigated classroom. A 6 mm PTFE sampling line was used to connect each instrument to the classroom. The sampling lines were 4-6 meters long and they were located at 50 to 100 cm under the classroom ceiling.

The different ventilation and occupation scenarios assessed in this field campaign have been listed in the Table S1. During the first two weeks, there was no ventilation since the windows and doors were always closed and the classrooms remained unoccupied corresponding to a period of school holidays. Before the field campaign starts, furniture was moved out from the classrooms. Then, IAQ monitoring was conducted in the empty classrooms for the first week to determine the contribution of building materials to aldehydes' levels. At the beginning of the second week, the furniture was moved back to the classroom and IAQ measurements continued for the whole week to assess the concentration of aldehydes potentially emitted by the furniture. It should be noted that the ramp of the stairs leading to the hallway was repainted at the beginning of the second week, which distorted a part of the results which is discussed further in this manuscript. School activities in the investigated classrooms started on Monday of the third week. Therefore, for the weeks 3 to 5, classes were occupied by students and their teachers. Each week, different ventilation instructions were followed. In S1, during the third week (from 1 st to 6 th March 2016), no specific instructions were given to the teachers. Instead, they followed its subjective criteria for ventilation. This week represents the ventilation practices that are performed in the school usually during the school year. During the fourth week (from 7 th to 13 rd of March 2016), ventilation was made according to the CO2 level reached in the room, inducing the lighting of LED on the CO2 detector: orange and red LED became ON when CO2 concentration was higher than 1000 and 1700 ppm, respectively. For the fifth week (from 14 th to 20 th of March 2016) ventilation instructions given by the French Indoor Air Quality Observatory were followed. According to these latter instructions, windows were opened an average of 30 minutes in the following cases: before and after the classes, every two hours of class and after all the cleaning events. All classroom doors remained open during the night.

In S2 the ventilation scenarios previously explained were also evaluated but in a different order because of the availability of a single LUM'AIR CO2 detector. Consequently, ventilation practices following the recommendations given by the French Observatory of Indoor Air Quality were carried out during the fourth week (from 7 th to 13 rd of March 2016) whereas the occupants used the LED indicator on the CO2 detector during the fifth week (from 14 th to 20 th of March 2016), in opposite to S1.

In S3, measurements were conducted only over the fourth week, i.e. from 7 th to 13 rd of March 2016, during the presence of students and when no specific ventilation instructions were followed, so that teachers and children opened doors/windows as usually.

Statistical analysis

Statistical analyses were performed with SigmaPlot, version 11.0. The statistical tests performed were variances analysis (Student's t-test).

For each aldehyde, its set of concentrations obtained during a full week was compared with that measured during a reference week. First, the second week was considered as reference to evaluate 1) the addition of furniture between weeks 1 and 2; 2) the impact of ventilation and the influence of the students' presence on indoor air quality between weeks 3-5 and 2. Then, the third week was considered as reference (usual ventilation) to evaluate the two last ventilation scenarios (weeks 4 and 5). Finally, the fourth week was chosen as reference one to evaluate the efficiency of the two last scenarios between them. The statistical results are given using the conventional classification depending on the p value: * p < 0.05; ** p < 0.01; *** p < 0.001. p2 corresponds to the reference week 2 while p3 and p4 are related to the third and fourth weeks chosen as reference.

Results and discussion

This paper reports the values of all aldehydes' concentrations measured in the investigated sites, except those of formaldehyde whose time tracking is the subject of a specific publication [START_REF] Trocquet | Continuous real-time monitoring of formaldehyde over 5 weeks under various ventilation scenarios in two French primary schools[END_REF]. The results obtained during this field campaign are shown in Figures 12345and Tables 345. The number of weekly measurements on DNPH tubes varied between 6 and 19 depending on the week and the school.

The temperature inside the studied room varied between 14.9 and 24.4°C for S1, 17.4 and 23.5°C for S2 and 15.5 and 22.8°C for S3. The relative humidity ranged from 41.8 to 69.6% in S1, from 35.3 to 67.3% in S2 and from 31.8 to 53.8% in S3 (see Table 6).

Results: aldehydes monitoring in schools

The main aldehydes detected were formaldehyde (FA), acetaldehyde (AA), propionaldehyde (PA), benzaldehyde (BA) and hexanal (HA). However, some traces of butyraldehyde, valeraldehyde, tolualdehyde and either 2-butanone or methacrolein (coeluted peak) were sometimes detected, their quantity being below the quantification limits. The results are discussed below school by school.

School 1

The concentration-time profiles of aldehydes are shown in Figures 1a and1b for the 5 weeks and the two first weeks, respectively. Three of the 4 aldehydes follow the same behavior, their concentrations increase slightly during week 2 and to a lesser extent during week 5. Conversely, the concentration of hexanal changed very differently: its level is relatively low until the middle of the second week and then increases sharply before decreasing exponentially. At the end of week 5, the concentration of hexanal is equal to about 7.2 µg m -3 , which is still significantly higher than that of 4 µg m -3 recorded during the first 10 days. This increase observed in the middle of the second week is due to the unplanned repainting of the ramp of the stairs leading to the floor where the classroom was located. This observation was previously reported for xylenes concentrations that suddenly increased up to 59.5 and 41.4 ppb for m/pxylenes and o-xylene respectively [START_REF] Lara-Ibeas | BTEX near real-time monitoring in two primary schools in La Rochelle, France[END_REF]. Indeed, aldehydes and more particularly hexanal are often present in paints [START_REF] Fortmann | Characterization of emissions of volatile organic compounds from interior alkyd paint[END_REF][START_REF] Fortmann | Characterization of emissions of volatile organic compounds from interior alkyd paint[END_REF].

The mean and extreme values measured in S1 for each aldehyde and for each week are presented in Table 3. These data are displayed on a box plot in Figure 2. Their concentrations varied in the ranges 3.22-20.22 µg m -3 , 0.00-5.22 µg m -3 , 1.05-2.95 µg m -3 and 2.90-51.51 µg m -3 for acetaldehyde, propionaldehyde, benzaldehyde and hexanal, respectively.

During the first week where the classroom was empty, the average aldehydes' concentrations were the following (in units of µg m -3 ): 13.74 (AA); 1.40 (PA); 2.15 (BA) and 3.92 (HA). Only two measurements can be then representative of the aldehydes' concentrations in presence of furniture since they were conducted before the paint event. However, 3 of 4 aldehydes (AA, BA and HA) exhibit levels significantly higher (p2 < 0.001) than those recorded in week 1: 16.96 µg m -3 (AA); 2.84 µg m -3 (BA); 6.14 µg m -3 (HA). This trend tends to confirm the increase of aldehydes concentrations observed for school 2 after the furniture was placed in the classroom (see below). All aldehydes concentrations increased excepting for propionaldehyde, which was surprisingly found to be statistically lower than the one recorded in week 1 (p2 < 0.001).

After the painting event, we can distinguish two types of behaviors depending on the aldehyde as illustrated in Figure 2. For hexanal and propionaldehyde, the concentrations measured during the week 2 increase significantly once the paint was applied (p2 < 0.001) while no significant change was observed for acetaldehyde (p = 0.133). A small decrease was even statistically observed for BA (p = 0.003). This observation demonstrates that the used paint emitted both hexanal and propionaldehyde and not acetaldehyde and benzaldehyde. Consequently, the mean hexanal concentration obtained in the second part of week 2 was 40.13 ± 6.65 (n = 8), which is 6.5 times higher than that of 6.14 ± 0.06 (n = 2) before the painting application. Propionaldehyde concentration increased also from 0.41 ± 0.06 µg m -3 (n = 2) to 3.97 ± 0.68 (n = 8).

For the next three weeks corresponding to the different ventilation scenarios, hexanal and propionaldehyde mean concentrations were always much higher (p2 < 0.001) than those found before the paint event in week 2. However, a strong continuous exponential decay was observed for both aldehydes between the paint application and the end of the field campaign (see Figures 1 and2). Hexanal exhibited mean concentrations in the range 8.50 -20.66 µg m -3 for weeks 3-5, much higher than that of 6.14 ± 0.06 µg m -3 (n = 2) determined in week 2 before the paint application. Once again, these high concentrations of hexanal are explained by the paint emissions which remain high from 1 to 3 weeks after application.

Similarly to the observations performed in School 2 for all aldehydes and discussed below, the mean acetaldehyde levels of 9.43-13.52 µg m -3 recorded for weeks 3-5 in School 1 were statistically lower (p2 < 0.001) than that of 16.96 ± 0.50 µg m -3 (n = 2) determined during the second week in presence of furniture and before the paint event. The same trend was highlighted for benzaldehyde which exhibited lower values of 1.89-2.29 µg m -3 in weeks 3-5 (p2 < 0.001) compared to week 2 before the painting (2.84 ± 0.11 µg m -3 (n = 2)). These results suggest that aldehydes are mainly emitted by internal sources since their concentrations were statistically higher when no ventilation was conducted in the classroom. Furthermore, in most cases, these concentrations decreased when any type of ventilation was applied highlighting the relevance of the ventilation practices during school hours to decrease children's exposure to indoor air pollutants.

Among the three last weeks, the week 5 showed the highest mean acetaldehyde concentration of 13.52 ± 1.22 (n = 17) as confirmed by the Student's t-tests performed considering weeks 3 or 4 as the reference week where p3 and p4 values were lower than 0.001. Such observation was not done for benzaldehyde.

In contrast to the results obtained in S2 presented below, the higher mean aldehydes' levels were recorded the week 3 when no specific ventilation instructions were given to the teachers. This trend agrees with the CO2 levels, the highest mean concentration being measured during the third week as well (1296 ppm). These results highlight how the lack of a common guide for ventilation practices in schools may result in different levels of exposure for children even in schools with similar features.

In S1, the proposed ventilation practices seem to be more efficient reducing the aldehydes' concentration in the classroom as well as decreasing the CO2 levels. Among them, the lowest mean aldehydes' concentration was recorded the week 4 when the CO2 indicator was used coinciding with the lowest mean CO2 levels measured (820 ppm).

School 2

The concentration-time profiles of aldehydes are displayed in Figures 3a and3b for the 5 weeks and the two first weeks, respectively. Except for week 4, the samples were taken in duplicate. These two simultaneous measurements were performed over the same duration and using the same sampling gas flow rate so that the volumes sampled were identical. As evidenced by the figures 1a and 1b, the data obtained from the samples taken simultaneously using two different samplers are in very good agreement. All aldehydes exhibit the same trend over all 5 weeks, higher aldehydes concentrations being observed for all of them in week 2 and at the start of week 5 (Sunday 13 and Monday 14 March 2016). Their concentrations varied for the whole period in the ranges 3.47-21.64 µg m -3 , 0.62-2.63 µg m -3 , 1.21-3.47 µg m -3 and 3.24-8.05 µg m -3 for acetaldehyde, propionaldehyde, benzaldehyde and hexanal, respectively.

The minimum and maximum values as well as the mean values obtained in school 2 for each aldehyde and each week are presented in Table 4. These same data are shown on a conventional boxplot in Figure 4. During the first week where the classroom was empty, the weekly average aldehydes' concentrations were the following (in units of µg m -3 ): 11.17 (AA); 1.07 (PA); 2.32 (BA) and 3.99 (HA). When the furniture was added into the classroom, these average concentrations increased slightly up to the following values (in units of µg m -3 ): 13.11 (AA); 1.37 (PA); 2.72 (BA) and 5.40 (HA). The observed increase of 17.2-35.3% depending on the aldehyde seems to indicate a possible contribution of furniture to the total aldehydes' emission in this classroom. This trend glimpsed in Figure 3 is effectively confirmed in Figure 4 and by the Student's t-tests which demonstrated that values measured for weeks 1 and 2 are statistically different for all aldehydes (p2 <0.001). During these two first weeks, the CO2 levels recorded were lower compared with the three last weeks due to the absence of occupants. The higher maximum CO2 levels measured during the second week corresponds to the period in which the furniture was moved back into the classroom.

For the three next weeks, i.e., when the classrooms were occupied and different ventilation practices were applied, the mean concentrations found for all the aldehydes were always lower than the values measured during week 2, considered as the reference week in the absence of occupants. An exception was nevertheless recorded: in week 5, the propionaldehyde concentration of 1.58 ± 0.18 µg m -3 was slightly higher than that of 1.37 ± 0.17 µg m -3 found in week 2. The lower mean concentration observed during the last three weeks suggest that any type of ventilation results in a decrease of aldehydes concentration. This observation reinforces the idea that furniture is probably one of the main aldehydes' emission sources in the classroom and, therefore, aldehyde concentrations were higher when the furniture was present in the absence of ventilation.

According to Figure 2, the concentrations recorded in weeks 3-5 obviously show much greater variability than during the first two weeks when the class was closed. These variabilities were induced by the large differences in concentrations recorded between situations where either occupants opened the windows to renew air (school time) or the classrooms were closed for a long duration (night and week-end). For these 3 weeks where ventilation was conducted, the concentrations were statistically lower (p2 < 0.001) than the reference ones recorded in week 2 for all aldehydes except during the week 4 for acetaldehyde (p2 = 0.093).

In order to evaluate the efficiency of the different ventilation practices to improve air quality, a Student's t-test was performed. Week 3, where usual ventilation practices were conducted, was taken as a reference week for carrying out the statistical test. BA concentrations recorded in weeks 4 and 5 were found to be statistically lower than those measured in week 3. In contrast, the concentrations of AA and PA found in week 5 were significantly higher (p3 < 0.001) while those determined in week 4 were not different (p3 = 0.221 and p3 = 0.727 for AA and PA respectively). It is interesting to note that the concentrations of all aldehydes during week 5 were statistically higher than those measured for week 4 (p4 <0.001) suggesting that ventilation according specific instructions based on short and frequent window opening may be slightly more efficient than that linked to CO2 indicator and containment alert. However, the mean CO2 value measured during week 4 (756 ppm) was higher than the levels measured during week 5 (682 ppm) showing that ventilation was probably carried out more often when the CO2 indicator was used. Surprisingly, among the three different ventilation scenarios, the lowest mean values for all the aldehydes excepting benzaldehyde were recorded during the third week whereas in this week, the highest mean CO2 levels were measured (1221 ppm).

School 3

In School 3, measurements were only performed during the week 4 where the occupants followed the usual ventilation instructions given by the manager of the educational institution. The concentrations are shown in Figure 5a. Since the ventilation and occupations conditions were similar, these values can be thus compared to those recorded during week 3 in Schools 1 and 2. In School 3, the aldehydes average concentrations of 5.34 (AA), 0.83 (PA), 0.90 (BA) and 3.28 (HA) µg m -3 are significantly lower than the values measured in School 2 (9.51 (AA), 1.22 (PA), 2.31 (BA) and 5.14 (HA) µg m -3 ) and in School 1 (11.74 (AA), 1.90 (PA), 2.29 (BA) and 20.66 (HA) µg m -3 ). In addition, their concentrations varied slightly during this week as illustrated in Figure 5b where time variability is limited for all aldehydes. The same observation is confirmed by the results presented in Table 5 where the quoted standard deviations are moderate.

Comparison with literature data

As already mentioned, the studies related to indoor aldehydes measurements and performed in French and European Schools are gathered in Tables 1 and2, respectively. This discussion is focused on aldehydes excluding formaldehyde. Among the French studies, only three of them reported the concentrations of other aldehydes such as acetaldehyde, propanal, benzaldehyde, hexanal [START_REF] Canha | Assessment of ventilation and indoor air pollutants in nursery and elementary schools in France[END_REF][START_REF] Trocquet | Near Real-Time Monitoring of Formaldehyde in a Low-Energy School Building[END_REF][START_REF] Verriele | The MERMAID study: indoor and outdoor average pollutant concentrations in 10 low-energy school buildings in France[END_REF]. The same DNPH derivation method followed by analysis by HPLC/UV was used for aldehydes quantification in these three studies. Two of them [START_REF] Canha | Assessment of ventilation and indoor air pollutants in nursery and elementary schools in France[END_REF][START_REF] Verriele | The MERMAID study: indoor and outdoor average pollutant concentrations in 10 low-energy school buildings in France[END_REF] conducted passive sampling using Radiello tubes over a period typically of 4.5 days from Monday to Friday (1 measurement per site), while [START_REF] Trocquet | Near Real-Time Monitoring of Formaldehyde in a Low-Energy School Building[END_REF] monitored the concentration of aldehydes over 10 consecutive days by means of active sampling using autosamplers equipped with a pump and a mass flow controller (34 indoor and 16 outdoor measurements on 1 site) [START_REF] Trocquet | Near Real-Time Monitoring of Formaldehyde in a Low-Energy School Building[END_REF].

In general, and more particularly in the above-mentioned studies, only one single or few measurements were performed on the same site. Most of the time, the air samples are taken by means of a passive sampler. This passive sampling strategy makes possible to determine an average value of the pollutants' concentrations typically over 4.5 days or 1 week but does not allow to determine the variation of concentration during the sampling duration. However, it is an effective technique to assess pollutant's concentration in numerous sites at a reasonable cost [START_REF] Canha | Assessment of ventilation and indoor air pollutants in nursery and elementary schools in France[END_REF][START_REF] Kirchner | Qualité de l'air et confort dans les écoles en France : premiers résultats de la campagne nationale[END_REF][START_REF] Verriele | The MERMAID study: indoor and outdoor average pollutant concentrations in 10 low-energy school buildings in France[END_REF]. In active sampling, a pump is used to flow the ambient air through the sorbent cartridges. Since larger air volumes are in contact with the sorbent, the sampling time can be reduced to some tens of minutes or hours resulting in a better resolution of the concentration-time profiles.

Most of the studies reported in Tables 1 and2 used passive sampling over a period of several days [START_REF] Breizh | Poor air quality in classrooms related to asthma and rhinitis in primary schoolchildren of the French 6 Cities Study[END_REF][START_REF] Breizh | Poor air quality in classrooms related to asthma and rhinitis in primary schoolchildren of the French 6 Cities Study[END_REF][START_REF] Canha | Assessment of ventilation and indoor air pollutants in nursery and elementary schools in France[END_REF][START_REF] Izard | Air intérieur dans les écoles et crèches : Air intérieur dans les écoles et crèchesBilan des mesures 2009 -2011[END_REF][START_REF] Kirchner | Qualité de l'air et confort dans les écoles en France : premiers résultats de la campagne nationale[END_REF][START_REF] Madureira | Indoor air quality in Portuguese schools: levels and sources of pollutants[END_REF][START_REF] Michelot | Monitoring indoor air quality in French schools and day-care centres[END_REF][START_REF] Pegas | Indoor air quality in elementary schools of Lisbon in spring[END_REF][START_REF] Smedje | Asthma among secondary schoolchildren in relation to the school environment[END_REF][START_REF] Verriele | The MERMAID study: indoor and outdoor average pollutant concentrations in 10 low-energy school buildings in France[END_REF][START_REF] Villanueva | Aldehyde Measurements in Indoor and Outdoor Environments in Central-Southern Spain[END_REF]. Only 3 studies, including this one, reported measurements made with active sampling [START_REF] Synnefa | An Experimental Investigation of the Indoor Air Quality in Fifteen School Buildings in Athens, Greece[END_REF][START_REF] Trocquet | Near Real-Time Monitoring of Formaldehyde in a Low-Energy School Building[END_REF]. This work is the first to report propionaldehyde concentration in indoor air school so that non comparison can be done.

In this work, the weekly average concentration of acetaldehyde varied in the ranges 9.51-13.11 µg m -3 in School 2, 9.43-17.55 µg m -3 in School 1, and was equal to 5.34 ± 1.35 µg m -3 during the week 4 in School 3 (see Tables 345). The values found in S1 and S2 are slightly higher than some of them reported in the literature. Indeed, [START_REF] Breizh | Poor air quality in classrooms related to asthma and rhinitis in primary schoolchildren of the French 6 Cities Study[END_REF] reported acetaldehyde median concentrations of about 9 µg m -3 , [START_REF] Breizh | Poor air quality in classrooms related to asthma and rhinitis in primary schoolchildren of the French 6 Cities Study[END_REF] in 401 classrooms of 108 primary schools in 6 major French cities. Similarly, [START_REF] Villanueva | Aldehyde Measurements in Indoor and Outdoor Environments in Central-Southern Spain[END_REF] and [START_REF] Madureira | Indoor air quality in Portuguese schools: levels and sources of pollutants[END_REF] reported acetaldehyde mean concentration of 8.5 ± 5.4 µg m -3 in an urban and an industrial area of central-southern Spain [START_REF] Villanueva | Aldehyde Measurements in Indoor and Outdoor Environments in Central-Southern Spain[END_REF] and of 9.31 ± 7.82 in Porto in Portugal [START_REF] Madureira | Indoor air quality in Portuguese schools: levels and sources of pollutants[END_REF]. The difference is even more pronounced with the other literature data. Indeed, acetaldehyde was present in 100% of the studied classrooms by [START_REF] Kirchner | Qualité de l'air et confort dans les écoles en France : premiers résultats de la campagne nationale[END_REF], its median value was equal to only 5.1 µg m -3 [START_REF] Kirchner | Qualité de l'air et confort dans les écoles en France : premiers résultats de la campagne nationale[END_REF] which is significantly lower than the average values recorded in the present study. A similar value of 5.5 ± 1.9 µg m -3 was also reported from measurements performed in winter (10 schools) and summer 2010 (7 schools) by [START_REF] Canha | Assessment of ventilation and indoor air pollutants in nursery and elementary schools in France[END_REF] in the South of France [START_REF] Canha | Assessment of ventilation and indoor air pollutants in nursery and elementary schools in France[END_REF]. [START_REF] Verriele | The MERMAID study: indoor and outdoor average pollutant concentrations in 10 low-energy school buildings in France[END_REF] reported average acetaldehyde level of 6.0 µg m -3 in a low-consumption teaching building equipped with programmable double-flow ventilation [START_REF] Verriele | The MERMAID study: indoor and outdoor average pollutant concentrations in 10 low-energy school buildings in France[END_REF]. One year later in the same building, the concentration was found to be even significantly lower, i.e. in the range 2-5 µg m -3 [START_REF] Trocquet | Near Real-Time Monitoring of Formaldehyde in a Low-Energy School Building[END_REF]. It is interesting to note that all these last values are consistent with the average acetaldehyde concentration of 5.34 ± 1.35 µg m -3 determined in School 3. This show that Schools S1 and S2 have higher indoor airborne levels of acetaldehyde than those recorded on average in France and Europe.

As summarized in Tables 345, the weekly average concentrations of benzaldehyde were in the range 1.93-2.72 µg m -3 and 1.89-2.84 µg m -3 in S1 and S2, respectively. As the trend observed for acetaldehyde, the average value of 0.90 ± 0.11 µg m -3 found during the week 4 in S3 was significantly lower than those measured in both Schools 1 and 2, confirming that this kindergarten was less polluted.

For hexanal, the weekly average concentration varied in the ranges 3.99-5.40 µg m -3 in School 2, 3.92-40.13 µg m -3 in School 1, and was equal to 3.28 ± 0.82 µg m -3 (n = 9) during the week 4 in School 3 (see Tables 345). Excluding the huge concentrations generated after the paint application, the hexanal levels measured in this work are lower than most of the literature data: 7.0 ± 4.7 µg m -3 (South of France, winter, [START_REF] Canha | Assessment of ventilation and indoor air pollutants in nursery and elementary schools in France[END_REF])); 17.0 ± 7.0 µg m -3 (South of France, summer, [START_REF] Canha | Assessment of ventilation and indoor air pollutants in nursery and elementary schools in France[END_REF])); 9.6 µg m -3 (North of France, [START_REF] Verriele | The MERMAID study: indoor and outdoor average pollutant concentrations in 10 low-energy school buildings in France[END_REF]). Only [START_REF] Trocquet | Near Real-Time Monitoring of Formaldehyde in a Low-Energy School Building[END_REF] reported very low hexanal concentrations below 1 µg m -3 in a very a low-consumption building equipped with programmable double-flow ventilation.

Conclusions

This study reports the first temporal indoor aldehyde's measurements simultaneously in two elementary schools. During this field campaign, different scenarios were investigated to determine the main emission sources and the efficiency of diverse ventilation practices. This type of experimental approach monopolizes a lot of materials and energy, which permits to study a limited number of schools unlike the use of passive samplers and limits the experimental measurements to indoors.

In the absence of occupation and with closed classroom windows and doors to limit air exchanges, very similar average concentrations of 11.17-13.74 µg m -3 (AA), 1.07-1.40 µg m -3 (PA), 2.15-2.32 µg m -3 (BA) and 3.92-3.99 µg m -3 (HA) were measured in both schools, whose presence can be attributed to emissions from building materials.

It has also been shown that adding furniture to classrooms leads to a small but significant increase in the concentrations of acetaldehyde (13.1-16.96 µg m -3 ), benzaldehyde (2.72-2.84 µg m -3 ) and hexanal (5.40-6.14 µg m -3 ). These observations were supported by Student's t-tests exhibiting p2 values lower than 0.001. Therefore, furniture can be considered as an emission source of these above-mentioned aldehydes, although to a lesser extent than building materials. Conversely, the concentration of propionaldehyde increased in School 2 (p2 < 0.001) but decreased in school 1 (p2 < 0.001) when furniture was added.

Once occupied by the pupils, two different behaviors were observed in S1 and S2. In S2, the concentrations of all aldehydes were found to be statistically higher (p2 < 0.001) than those measured during the reference week 2 showing that teaching activities related to manual work (painting, drawing, etc.) could contribute to aldehydes emissions inside the classroom. Indeed, it is unlikely that these aldehydes come from outside and enter the classroom during window openings during these three weeks as it was shown by [START_REF] Liu | Estimating contributions of indoor and outdoor sources to indoor carbonyl concentrations in three urban areas of the United States[END_REF] in three urban areas of the United States where indoor concentrations were almost always higher for aldehydes such as acetaldehyde, benzaldehyde and hexanal [START_REF] Liu | Estimating contributions of indoor and outdoor sources to indoor carbonyl concentrations in three urban areas of the United States[END_REF].

In S1, the paint application appeared not to emit two of the 4 aldehydes, namely acetaldehyde and benzaldehyde. Conversely, very high concentrations of hexanal up to 51.51 µg m -3 were measured during week 2 with an average difference of 33.99 µg m -3 between before and after the paint application. Propionaldehyde exhibited a behavior like hexanal but in a smaller proportion. Hexanal and propionaldehyde showed exponential decreases with time but their weekly levels stayed significantly higher (p2 < 0.001) than those found at the beginning of week 2. This fact makes any interpretation impossible on the effect of ventilation for these two aldehydes in this school. Conversely, the concentrations of the two aldehydes not emitted by the paint decreased as expected, showing a positive impact of ventilation on indoor air quality.

Occupation of schools by children can therefore either have positive effects on air quality regardless of the chosen ventilation scenario or have negative effects on air quality if the pollutant emissions linked to teaching activities have more impact than that linked to the ventilation itself. However, this ventilation avoids a significant deterioration of indoor air quality because the increase of pollutants' concentrations remained generally limited. This study therefore does not allow us to favor a ventilation scenario among the three investigated ones. Besides, this study shows that building maintenance such as repainting can greatly contribute to the degradation of indoor air quality. Also, it would be desirable for this renovation and maintenance work, which is certainly necessary, to be carried out on the one hand with products (paints, stains, glues) with low emissivity and on the other hand at the start of the summer holidays with adequate and continuous ventilation until the start of the academic year to permit the full pollutants removal.

Finally, such a study over 5 weeks allows to assess the relative impact of emissions from furniture and building materials but also the effects of ventilation on the concentrations of aldehydes.

Acknowledgments

This work has been conducted in the framework of IMPACT'AIR project financially supported by ADEME (Agence De l'Environnement et de la Maîtrise de l'Energie), the city of La Rochelle and the Ligue contre le cancer.