The Effects of Electrostatic Particle Filtration and Supply-Air Filter ...

© 2008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE’s prior written permission.

VOLUME 14, NUMBER 3

HVAC&R RESEARCH

MAY 2008

The Effects of Electrostatic Particle Filtration and Supply-Air Filter Condition in Classrooms on the Performance of Schoolwork by Children (RP-1257) Pawel Wargocki, PhD

David P. Wyon, PhD Member ASHRAE

Kasper Lynge-Jensen

Carl-Gustaf Bornehag, PhD

Received May 15, 2007; accepted December 20, 2007 This paper is based on findings resulting from ASHRAE Research Project RP-1257.

Two independent field intervention experiments involving a total of about 190 pupils were carried out in winter and early spring of 2005 in five pairs of mechanically ventilated classrooms that received 100% outdoor air. Each pair of classrooms was located in a different school. Electrostatic air cleaners were installed in classrooms and either operated or disabled to modify particle concentrations while the performance of schoolwork was measured. In one school, the used supply-air filters in a ventilation system without recirculation were also replaced with new ones to modify classroom air quality, while the filters in use in other schools were not changed. The conditions were established for one week at a time in a blind crossover design with repeated measures on ten-to-twelve-year-old children. Pupils performed six exercises exemplifying different aspects of schoolwork as part of normal lessons and indicated their environmental perceptions and the intensity of any symptoms. A sensory panel of adults judged the air quality in the classrooms soon after the pupils left. Operating the electrostatic air cleaners considerably reduced the concentration of particles in the classrooms. The effect was greater the lower the outdoor air supply rate. There were no consistent effects of this reduction on the performance of schoolwork, on the children’s perception of the classroom environment, on symptom intensity, or on air quality as perceived by the sensory panel. This suggests there are no short-term (acute) effects of particle removal outside the pollen season. When new filters were installed, the effects were inconsistent, although this is believed to be due to sequential and unbalanced presentation of filter conditions and to the fact that the used filters retained very little dust.

INTRODUCTION Many studies have reported high concentrations of particles in classrooms (EFA 2001; Dijken et al. 2005; Simoni et al. 2006), but until recently none demonstrated that removing particles in classrooms improves the performance of schoolwork. The only study that directly tested this hypothesis was a recent field experiment in Sweden in which electrostatic air cleaners were operated or disabled in two pair of classrooms (Mattsson and Hygge 2005). The study focused on the possible benefits of air cleaners for children with allergies or hypersensitivity and was, therefore, performed during the pollen season. The air cleaners reduced the concentraPawel Wargocki is an associate professor, David P. Wyon is a visiting professor, and Kasper Lynge-Jensen is a doctoral student at the International Centre for Indoor Environment and Energy, Department of Mechanical Engineering, Technical University of Denmark, Lyngby, Denmark. Carl-Gustaf Bornehag is an associate professor associated with Karlstad University, Sweden; Swedish National Testing and Research Institute; and the International Centre of Indoor Environment and Energy, Technical University of Denmark.

327

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tion of airborne particles and tended to reduce the amount of cat pollen, although the effect was not statistically significant (Mattsson et al. 2004). When the air cleaners were operated, children “stating themselves to be sensitive to airborne particulate contaminants” experienced a significantly greater reduction in eye and airway irritation, and these pupils scored about 25% higher on one of the five performance tests (finding synonyms); however, multiple testing (i.e. chance) could be a possible reason for this isolated result. It has always been assumed that respirable particles in indoor air must have some negative effects on health and that this may have negative consequences for task performance. The origin of this assumption is that in the absence of additional indoor sources, such as combustion, cooking, or smoking, indoor particles are, to a very large extent, the same particles found in outdoor (ambient) air (Fromme et al. 2005) where, according to reliable epidemiological evidence, they do have negative effects on the health of older people with pre-existing medical problems, on asthmatics of all ages (NRC 2004; Dominici et al. 2006; Hartog et al. 2003; Peters et al. 1997) and on children (Ward and Ayres 2004; Moshammer et al. 2006). In spite of this evidence, a recent major literature review of 1725 relevant publications on the health effects of particles in indoor air (EUROPART) concluded that there was “inadequate scientific evidence that airborne indoor particulate mass or number concentrations can be used as generally applicable risk indicators of health effects in nonindustrial buildings” (Schneider et al. 2003). The concentration of particles indoors can be reduced by installing electrostatic deposition air cleaners, particulate collection devices that remove particles from a flowing gas (such as air) using the force of an induced electrostatic charge. Electrostatic air cleaners are able to remove small airborne particles very effectively (Mattsson et al. 2004; Croxford et al. 2000; Skulberg et al. 2005), including allergenic particles (Hacker and Sparrow 2005; Francis et al. 2003; van der Heide et al. 1999), whose concentrations in school classrooms are often as high as they are in the homes of pet owners (Almqvist et el. 1999; Berge et al. 1998). However, operation of electrostatic air cleaners has not reliably been shown to result in a benefit for occupants. Installation of free-standing electrostatic air cleaners with disposable deposition plates in a London office reduced the concentrations of small particle fractions ≤2µm (PM2) more than they reduced those of larger fractions (PM10), but they did not reduce surface dust (Croxford et al. 2000) and showed no effect on symptoms reported by occupants of the office (Wyon et al. 2000). In a similar intervention experiment in Norway offices, operation of electrostatic air cleaners reduced dust concentration but again had no significant effect on reported symptoms (Skulberg et al. 2005). Rosén and Richardson (1999) operated ionizers (with no deposition plates) in two daycare centers in Sweden during a three-year study and reported that their intervention reduced the number of small particles by 78% and the number of larger particles by 45% but that absenteeism due to illness was significantly reduced in only one of the two institutions. There were large changes in absenteeism over time in the control condition, which indicated that this apparent effect of the intervention could have been due to external factors. The present experiments were designed to determine whether reducing the concentration of airborne particles in school classrooms improves the performance of schoolchildren on homework and whether the condition of the bag filter in the ventilation system affects this. The experiments are part of a larger study investigating the effects of improving classroom conditions on the schoolwork performance (Wargocki and Wyon 2007a, 2007b).

METHODS Experimental Design The study was designed as a series of field experiments in existing classrooms occupied by children performing their normal schoolwork. These were crossover experiments in pairs of classrooms in which electrostatic air cleaners were operated or disabled in the same week in

VOLUME 14, NUMBER 3, MAY 2008

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each adjacent classroom. The operation mode of the electrostatic air cleaners was switched between the classrooms the following week (crossover design) (see Table 1). The main advantage of this design is that any external factors that affect performance, environmental perceptions, and symptoms in a given week equally affect the results obtained under both conditions established during that week, thus avoiding bias. To eliminate any random bias due to individual differences in symptom intensity or the ability to perform schoolwork, the experiments were run as repeated-measures designs. Experiment 1EF, performed in January 2005, was a 2 × 2 design in which each operation mode of the air cleaners was reimposed after exchanging used and new supply-air particle filters. Experiment 2EF was performed in March and April of 2005, and the supply-air filters had been in use for some months. Both experiments were performed outside the pollen season to avoid this additional source of individual difference in sensitivity to particles. During the experiments, teachers and pupils were allowed to open and close the windows and doors as they would normally according to their habits. No changes were made in the schedule of normal school activities so as to maintain the teaching environment and routines as normal as possible. The interventions were approved by parents, teachers, the school board, the responsible local authority, and the ethics review board. Children were not asked for their consent so that they would remain blind to experimental conditions.

Schools, Classrooms, and Ventilation Experiments were carried out in five elementary public schools, all run by the local authorities, for children aged 6–16 years (see Table 2). The school buildings were made of bricks and the classrooms had floor coverings and typical school furniture; outdoor clothing used by pupils was left outside the classrooms, and smoking was not allowed. All classrooms were ventilated with mechanical ventilation systems that supplied 100% outdoor air filtered and preheated in central air-handling units (AHUs) with or without heat recovery; no cooling or humidification was provided (see Table 2). The AHUs were operated intermittently (the system was on 9–10 hours per day and off during weekends and school holidays). Each pair of classrooms in a given experiment was supplied with outdoor air from the same AHU.

Pupils About 190 ten-to-twelve-year-old children participated in the experiments; the children were generally healthy, but no prior medical examination was carried out and no information was obtained on how many children in the study were allergic.

Interventions Electrostatic air cleaners (designed according to the principle described in detail by Török and Loreth [1993]) were installed in cabinets mounted against the walls in each classroom. They were almost inaudible— the noise level produced by the fans was about 30–35 dB(A). Contrary to what is reported in the literature for other electrostatic air cleaners (Niu et al. 2001), they did not produce ozone; this was verified in laboratory tests performed by the experimenters prior to installation of air cleaners in the classrooms. In experiment 1EF, two air cleaners were installed in each classroom, while three air cleaners were operated in each classroom in experiment 2EF. The airflow through each cleaner was 800 m3/h (470.9 cfm); i.e., the total airflow through the air cleaners corresponded to 8–12 ach. Given the efficiency of the air cleaners, which according to the specifications of the manufacturer was 99%, the clean-air delivery rate was similar to the flow through the units. In the reference condition, the concentration of particles was not reduced: cabinets with fans but with no deposition panels or ionizers were installed and operated in experiment 1EF, while in experiment 2EF unmodified electrostatic air cleaner cabinets continued to operate with the corona effect disabled. The fans mounted on the cabinets were, thus,

Experiment 2EF

1950s (1997) 1977 (1997) 1967 (1997) 1951–59 (1996) 1948 (1998)

Rurala Ruralb Urbanc Urbanc Ruralb

1-1

2-1

2-2

2-3

2-4

School Location

Construction (Renovation) Year

57

53

60

50

65

Area, m2

in a small coastal town 20–30 km north of Copenhagen, Denmark. in a village outside Lund, Sweden. cLocated in Lund, a university town in southern Sweden. dThere were about the same number of pupils in each of the two classes.

bLocated

Experiment 2EF, Spring Classroom 1 Classroom 2 Used filters Electrostatic air cleaners Placebo unitsb Used filters Placebo unitsb Electrostatic air cleaners

164

167

169

136

187.5

Volume, m3

PVC

PVC

Linoleum

PVC

Linoleum

Floor

Classroom

30

40

40

40

40

Number of Pupilsd

EU7, used (20 months)




EU7, used (12 months) or new

Filter

Rotary wheel

Rotary wheel

No heat recovery

Rotary wheel

Cross flow plate

Heat Recovery System

Ventilation System

Table 2. Characteristics of Classrooms and Ventilation Systems in the Schools Selected for the Present Experiments

Experiment 1EF

aLocated

Experiment 1EF, Winter Classroom 1 Classroom 2 Used filters Electrostatic air cleaners Placebo unitsa New filters Electrostatic air cleaners Placebo unitsa New filters Placebo unitsa Electrostatic air cleaners Used filters Placebo unitsa Electrostatic air cleaners

without electrostatic air cleaner and top-mounted fans in operation. with electrostatic air cleaner, corona charge disabled and top-mounted fans in operation.

Experiment

bCabinets

aCabinets

4

3

2

1

Week

Table 1. The Partially Balanced Design of Experiments 1EF and 2EF

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always in operation in the placebo condition. In experiment 2EF, six identical air cleaners were used in the rural schools and six other identical units were used in the urban schools. The same deposition panels were used for the duration of the experiment. In experiment 1EF, the supply-air filters (class EU7 bag filters) were either new or used at the outset of each experiment; they were the same filters used in another experiment conducted parallel with experiment 1EF in two other classrooms served by the same AHU (Wargocki and Wyon 2007a). The filters had been in use for twelve months.

Physical Measurements Spot measurements of airborne particle density were made using a dust monitor for 20 minutes at the end of each week after the children had left the classroom; the size ranges assessed were >0.75, >1, >2, >3.5, >5, >7.5, >10, and >15 µm. An ultrafine particle counter measuring in the size range 0.02–1 μm was also used. PM2.5 and PM10 were not measured, as the intention was to examine whether there was an effect on the concentration of particles across all sizes rather than to compare the measured concentrations with the requirements in standards. No measurements of allergens were made. The rate of dust sedimentation onto horizontal surfaces was measured by placing clean glass plates each week at a (child-proof) height of 2.2 m (7.2 ft). A surface-dust meter was used to assess the percentage of the surface covered by dust at the end of the week. This was accomplished by using forensic gelatin tape to lift the dust and then inserting the tape into an instrument that measured the amount of light scattered by a laser beam. Temperature, relative humidity (RH), and carbon dioxide were measured continuously and supplemented with spot measurements of air velocity, noise, and operative temperatures in the classrooms in experiment 1EF. The continuous measurements were used to calculate weekly averages during the periods when children were present in the classrooms, excluding even short breaks between teaching periods. The measurements of CO2 made when pupils were present in the classrooms were used to estimate the effective ventilation rates, as in previous experiments (Wargocki and Wyon 2007a, 2007b). The instruments were calibrated before use. Weather data for the whole period were registered.

Measurements of Performance Each week, in appropriate lessons, the children’s usual teachers administered parallel versions of language-based and numerical performance tasks representing different aspects of schoolwork from reading to mathematics. Due to lack of cooperation from a teacher in one classroom in school 2-2 in experiment 2EF, the tasks were presented to pupils by experimenters in the presence of a teacher. The presentation of tasks was distributed fairly evenly over the whole week, and the teachers were asked to apply the same task always on the same weekday. No more than one task was performed during one lesson, and generally no more than two tasks were performed per day. The tasks were selected to be a natural part of an ordinary school day and included multiplication, subtraction, number comparison, logical thinking, reading and comprehension, and acoustic proof-reading, as described in detail by Wargocki and Wyon (2007a, 2007b). Number comparison and acoustic proof-reading tasks were omitted in experiment 2EF. The teachers taught the children how to perform the tasks by working through examples with the class to make sure the children understood each task. In the case where experimenters presented the tasks, the instructions were played from a prerecorded CD in the children’s own language. The duration of the tasks was short enough to ensure that most children could not complete them in the time available. Up to ten minutes were allocated for each task. If any of the pupils completed the tasks before the allocated time, all other pupils were immediately told by the teacher to stop and the actual time that had elapsed was noted. Different versions of each task were prepared and task versions were confounded with occasions, i.e.,

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HVAC&R RESEARCH

version 1 was always encountered before version 2, and so on. The tasks were presented to children in their own language (Danish or Swedish). Performance was measured in terms of speed (how quickly each pupil worked per unit of time) and errors (the percentage of errors committed). In the case of acoustic proof-reading, only the errors and false positives were recorded because the speed of performance was imposed by the rate at which the text was dictated. The children's performance was first analyzed using a complete design analysis, i.e., analyzing for each exercise only the results obtained from pupils who had taken that test in all conditions, and then repeating the analysis using all available data, i.e., including the performance of pupils who had not performed the exercise in all conditions (incomplete design). If performance differed significantly between occasions—disregarding the interventions and due, perhaps, to learning, increased familiarity with the exercise, fatigue, or differences between test versions—the analyses were repeated after adjusting the results for this effect. The adjustment was made by multiplying the individual performance of each pupil on a given task in a week by a coefficient (C) calculated as C = A/B, the ratio of the average performance of the pupil’s class in the first week the task was introduced (A) to the average performance of the pupil’s class on the task that week (B). The performance of each individual task was then normalized by dividing by average performance of that task throughout each experiment, disregarding conditions.

Measurements of Perceptions and Symptoms The children marked visual-analogue (VA) scales each week during the last lesson each Friday to indicate the intensity of various symptoms and their perceptions of the environment (Figure 1). The items on VA scales included the following: the perception of classroom temperature; air movement; air dryness; air freshness; illuminance and noise; and symptoms of nose congestion, nose throat, lips and skin dryness, hunger, fatigue, and headache. They also indicated whether they had slept poorly or too little the preceding night and whether they felt like working on the day the VA-scale was marked. The VA-scales were administered by the teachers, who, by going through examples, also taught the children how to use them. In the case of one classroom in school 2-2 in Experiment 2EF, the VA-scales were presented by the experimenters who (as in their presentation of the performance tasks) played the instructions from a prerecorded CD. As in the analysis of performance, the results obtained on the VA-scales were analyzed using both complete and incomplete design analyses.

Measurements of Perceived Air Quality Sensory panels of 35–39 adults were recruited to assess the air quality in the classrooms, as described in Appendix B of ASHRAE Standard 62.1-2007, Ventilation for Acceptable Indoor Air Quality (2007). The subjects were mainly students aged 18–35 years old, with an approximately equal mix of females and males in each panel. They were blind to conditions. Different panels

Figure 1. Examples of the visual analogue scales used to obtain estimates of symptom intensity and perceptions of the environment. Each scale was 100 mm (3.9 in.) long.


333

were used in experiments 1EF and 2EF. The measurements were made once a week in the afternoon about 1–1.5 h after the pupils had left the classrooms, but while the ventilation system was still operating, to avoid any disturbance of normal school activities. The panel entered the classrooms every 2–3 min in groups of two-to-three at a time and assessed the air quality immediately upon entering; the doors were closed during these assessments and the order of assessments was balanced. Between assessments, the panel members stayed in a well-ventilated hall, near the classrooms, or outdoors. The panel was transported between schools in a bus. Panel members assessed air quality using four scales: a continuous acceptability scale (Wargocki 2004), an odor intensity scale (Yaglou et al. 1936), and two horizontal VA-scales that described the freshness and dryness of classroom air.

Statistical Analysis Shapiro-Wilk’s test was used to determine whether residuals were normally distributed and, if necessary, the data were log-transformed. When the normality assumption was fulfilled, repeated measures 2 × 2 analysis of variance was performed for pupils for whom data were available in all four conditions of experiment 1EF. A general linear model with Type V sum of squares was also used for all available data, even for pupils for whom no data were available in some conditions. Friedman two-way nonparametric analysis of variance was used when the normality assumption was not met, and for the markings on VA scales. Wilcoxon’s matched-pairs signed-ranks test was used to analyze the main effects of running electrostatic air cleaners and filter condition when the assumption of normality was not valid (Wargocki and Wyon 2007a, 2007b). In experiment 2EF, the Wilcoxon test was always used to test for a difference between conditions, first separately for each school and then separately for urban schools and rural schools after pooling the data from the two schools in each location category, and finally after pooling the data from all schools. To examine whether average performance across all tasks exhibited any significant trend, pair-wise comparisons with the Wilcoxon test were made using group averages calculated for the performance data of all pupils separately for each task in either of the two conditions: electrostatic air cleaners operating/disabled, or filters new/used. The P-level for rejection of the null hypothesis was set to 0.05 (2-tail).

RESULTS Physical Measurements The classroom conditions measured during experiments are shown in Tables 3 and 4. They show that temperature, RH, CO2, and the estimated effective ventilation rates were not affected by the interventions, while operating electrostatic air cleaners considerably reduced the concentration of airborne particles. This effect was greater for classrooms with lower outdoor air supply rates. Operation of electrostatic air cleaners also reduced the amount of settled dust; the effect was more pronounced in experiment 1EF in which classrooms were ventilated with the lowest outdoor air supply rate (Table 3) and less consistent in experiment 2EF (Table 4). Exchanging a used filter with a new one in experiment 1EF had no consistent effect on the concentration of particles or on the amount of settled dust (Table 3). The air velocity in experiment 1EF was below 0.14 m/s (27.6 fpm), and the noise level with pupils absent was ~36–44 dB(A); neither of these quantities differed between the experimental conditions.

Measurements of Performance Figure 2 summarizes the results of complete design analyses that examine the effects of operating electrostatic air cleaners on how pupils performed the tasks presented to them during the experiments. Due to teacher error in experiment 1EF, the subtraction, multiplication, and

39±3 1335±148 270±48 23+1 3.1±0.6

CO2, mean±sd (ppm)

Estimated effective ventilation rate (m3/h)a

Average number of pupils+teacher (per class)

Estimated effective ventilation rate (L/s per person)a

5305 2873 1155 404 148 51 23 0 4.0±2.4

>0.75 μm [counts/1000 cm3] b

>1.0 μm [counts/1000 cm3] b





>10.0 μm [counts/1000 cm3] b

>15.0 μm [counts/1000 cm3] b

Settled dust [% covering of a glass plate]

aAverage production rate of CO = 16.8±1.9 L/h per person (eleven-year-old children+teacher). 2 bThe results from the first two weeks due to malfunction of the measuring instrument.

5575

>0.02 μm (ultrafines) [counts/cm3]

Particles

Spot Measurements

18.9±1.3

RH, mean ±sd (%)

2.0±0.5

0

4

10

27

68

177

440

842

957

3.4±0.7

23+1

290±52

1454±288

38±2

19.6±1.0

Electrostatic Air Cleaners

Used Filters Placebo Units

Temperature, mean ±sd (oC)

Continuous Measurements

Parameter

5.3±1.6

0

21

52

167

475

1296

2868

4770

3646

3.6±0.8

24+1

317±74

1353±119

44±3

19.0±1.1

3.2±1.3

0

5

12

35

82

191

472

894

2331

3.3±0.7

23+1

279±59

1412±251

44±3

19.2±1.1


New Filters Placebo Units

Classroom

School 1-1 (Rural)

Experiment 1EF, Winter

NA

0

1

5

28

112

900

2799

5364

NA

NA

NA

NA

400

87±8

4.1±2.2

Outdoor

Table 3. Conditions in Classrooms in Experiment 1EF as Determined by Continuous Measurements When Classrooms Were Occupied by Children (Mean±sd) or in Empty Classrooms without Children, as Determined by Spot Measurements at the End of Each Intervention; the Effective Ventilation Rates Were Estimated from Measurements of CO2

334 HVAC&R RESEARCH

854±144b 491±134b 21+1b 6.5±1.8b

CO2, mean±sd (ppm)




4812 2059 1378 739 320 124 48 22 NA 1.0±0.1

>0.75 mm [counts/1000 cm3]

>1.0 mm [counts/1000 cm3]





>10.0 mm [counts/1000 cm3]

>15.0 mm [counts/1000 cm3]


Particles

>0.02 mm (ultrafines) [counts/cm3]

1.0±0.1

NA

16

33

73

303

299

515

757

1741

6.6±0.5b

21+1b

501±40b

779±36b

30±3

20.2±0.4

NA

NA

14

28

57

100

178

373

787

3810

NA

NA

NA

400

63±17

8.0±2.2

Outdoor

2.9±0.6

NA

37

71

154

317

615

1049

1547

1659

9.7±5.3

21+1

766±422

733±229

28±3

21.7±0.3

School 2-2 (Urban)

1.5±0.5

NA

11

23

49

93

171

292

467

745

12.7±1.0b

21+1

1010±82b

625±60b

28±3

21.5±0.5


Classroom Placebo Units

Used Filters

NA

NA

7

15

32

164

122

271

615

2774

NA

NA

NA

400

63±17

8.0±2.2

Outdoor

production rate of CO2: School 2-1: 16.6±1.3 L/h per person (eleven-year-old children + teacher), School 2-2: 17.2±1.8 L/h per person (eleven- and twelve-year-old children + teacher), School 2-3 and 2-4: 17 L/h per person (ten- and eleven-year-old children + teacher). bMeasurements from one class only.

aAverage

34±6

Spot Measurements

20.3±0.2

RH, mean ±sd (%)


Classroom Placebo Units

Temperature, mean ±sd (°C)


Parameter

School 2-1 (Rural)

Experiment 2EF, Spring

Table 4. Conditions in Classrooms in Experiment 2EF as Determined by Continuous Measurements When Classrooms Were Occupied by Children (Mean±sd) or in Empty Classrooms without Children, as Determined by Spot Measurements at the End of Each Intervention; the Effective Ventilation Rates Were Estimated from Measurements of CO2

VOLUME 14, NUMBER 3, MAY 2008 335

682±42 1026±239 21+1 13.3±2.7

CO2, mean±sd (ppm)




1166 683 334 140 61 27 13 NA 1.2±0.4




>5.0 mm [counts/1000 cm3] cm3]

>10.0 mm [counts/1000 cm3]

>15.0 mm [counts/1000 cm3]


>7.5 mm [counts/1000

1720

>0.75 mm [counts/1000 cm3]

Particles

>0.02 mm (ultrafines) [counts/cm3]

Classroom

1.6±0.0

NA

18

60

74

162

338

621

976

1055

12.8±2.3

21+1

1016±175

664±38

26±2

22.4±0.5


NA

NA

15

33

76

136

224

421

795

3350

NA

NA

NA

400

63±17

8.0±2.2

Outdoor

Used Filters

Placebo Units

1.4±0.2

NA

22

39

77

155

297

509

767

1290

19.2±1.7

17+1

1244±113

539±39

26±3

21.3±0.4

School 2-4 (Rural)

1.1±0.3

NA

11

24

58

113

202

319

451

725

20.0±3.8

18+1

1368±262

550±49

27±3

21.4±0.4


Classroom

NA

NA

9

18

39

77

160

324

676

4250

NA

NA

NA

400

63±17

8.0±2.2

Outdoor

production rate of CO2: School 2-1: 16.6±1.3 L/h per person (eleven-year-old children + teacher), School 2-2: 17.2±1.8 L/h per person (eleven- and twelve-year-old children + teacher), School 2-3 and 2-4: 17 L/h per person (ten- and eleven-year-old children + teacher). bMeasurements from one class only.

aAverage

27±3

Spot Measurements

22.4±0.5

RH, mean ±sd (%)

Placebo Units

Temperature, mean ±sd (°C)


Parameter

School 2-3 (Urban)

Experiment 2EF, Spring

Table 4. Conditions in Classrooms in Experiment 2EF as Determined by Continuous Measurements When Classrooms Were Occupied by Children (Mean±sd) or in Empty Classrooms without Children, as Determined by Spot Measurements at the End of Each Intervention; the Effective Ventilation Rates Were Estimated from Measurements of CO2 (continued)

336 HVAC&R RESEARCH


337

number checking tests were not administered in week four in one class, the logical reasoning was not administered in week one in one class, and acoustic proof-reading was not administered in weeks one and two in one class. Repeated measures analysis in a complete 2 × 2 design was therefore not possible for these tasks. The analyses of the difference between two or three other conditions, excluding the ones for which data were missing for one class, were based on an incomplete design; as a measure of performance, adjusted (least square) means were used. The differences between single conditions were not analyzed because in the present design they are confounded with any differences between weeks caused by external factors or a gradual change in performance in the course of the experiment. The analyses of the effects on performance show that there were no consistent effects of operating air cleaners on the speed at which different tasks were performed. In experiment 1EF, operating air cleaners apparently improved speed in the reading and comprehension task (P