PERCEIVED AIR QUALITY, THERMAL COMFORT, AND SBS SYMPTOMS AT LOW AIR TEMPERATURE AND INCREASED RADIANT TEMPERATURE J Toftum *, G Reimann, P Foldbjerg, G Clausen and PO Fanger International Centre for Indoor Environment and Energy, Technical University of Denmark ABSTRACT This study investigated if low air temperature, which is known to improve the perception of air quality, also can reduce the intensity of some SBS symptoms. In a low-polluting office, human subjects were exposed to air at two temperatures 23 o C and 18 o C both with and without a pollution source present at the low temperature. To maintain overall thermal neutrality, the low air temperature was partly compensated for by individually controlled radiant heating, and partly by allowing subjects to modify clothing insulation. A reduction of the air temperature from 23 C to 18 C suggested an improvement of the perceived air quality, while no systematic effect on symptom intensity was observed. The overall indoor environment was evaluated equally acceptable at both temperatures due to local thermal discomfort at the low air temperature. INDEX TERMS Low air temperature, radiant heating, thermal comfort, perceived air quality, SBS symptoms INTRODUCTION The perception of indoor air quality depends not only on the chemical composition of the air, but also on its psychrometric properties: cool and dry air is perceived as less stuffy and more acceptable than warm and moist air (Berglund and Cain 1989, Fang et al. 1998, Toftum et al. 1998). It has been suggested that perceived air quality may influence the intensity of some SBS symptoms (Wyon 1998, Wargocki et al. 1999). Therefore, the consequences of low air temperature may not only be improved perceived air quality, but also a reduction of the intensity of SBS symptoms. Fang et al. (1999) showed that a decrease in the outdoor air supply rate from 1 L/s per person to 3.5 L/s per person could be compensated for by decreasing indoor air enthalpy from 45 kj/kg to 35 kj/kg, so as to avoid deteriorating perceived air quality. Thus, indoor air of low enthalpy could permit lower ventilation rates and in some climatic regions contribute substantially to energy conservation in buildings. To avoid compromising thermal comfort, low air temperature needs to be compensated for, e.g. by means of radiant or contact heating, preferably under individual control to accommodate inter-individual differences. The objective of this study was to determine whether indoor air at low temperature, which is known to improve perceived air quality, can also reduce SBS symptom intensity. METHODS Human subjects in a state of thermal neutrality were exposed to air at two temperatures 23 C and 18 C. To maintain overall thermal neutrality, the reduced air temperature was partly compensated for by radiant heating, controlled individually according to subjects expressed thermal sensation, and partly by allowing subjects to modify clothing insulation. At 18 o C * Contact author email: jt@mek.dtu.dk 267
experiments were conducted with and without an additional pollution source present in the office. Thermal sensation and comfort, perceived air quality and the intensity of a range of typical SBS symptoms were compared between experimental conditions. METHODS Subjects were exposed to the three experimental conditions shown in Table 1. Table 1. Experimental conditions. Condition 1 Condition 2 Condition 3 (reference) Air temperature ( C) 23 18 18 Operative temp. ( C) 22 2.5 2.5 Clothing insul. (clo) 1.1 1.4 1.4 Radiant heating not used used used Pollution source absent absent present Experiments were conducted in an ordinary, low-polluting office space (6 x 6 m 2 ) located at the Technical University of Denmark. The office was equipped with a ventilation and airconditioning system hidden for the subjects behind a screen (Figure 1). Under all experimental conditions, the ventilation rate was kept constant at 6 h -1 corresponding to 45 L/(s person) and the humidity ratio was.42 kg/kg. The duration of all experimental sessions was three hours. Four workstations were installed in the occupied zone of the office, each consisting of a desk, a chair and a PCmonitor (PC s were located outside the office). As shown in Figure 1, the workstations were equipped with electrical radiant heating panels at each side and behind the subject's chair. An additional panel to prevent cold legs was mounted under the desk. The surface temperature of the radiant panels, and thus the operative temperature, was controlled individually for each workstation, based on the subject's thermal sensation vote. The surface temperature of the three vertical panels at each workstation was jointly controlled, whereas the horizontal panel was Figure 1. Experimental set-up in the office. controlled separately. Subjects were not aware that the surface temperature of the heating panels was adjusted. The pollution source was 8 m 2 of 2-years-old tufted bouclé carpet with 1% polyamid fibres and latex backing, which had been removed from a building with a history of indoor environment complaints. The carpet was cut into 2 x.2 m 2 pieces that were attached back-toback and hung on a mobile stainless steel rack. Mixing fans located at different positions ensured full mixing of the air in the office. 268
A total of 28 subjects (14 females and 14 males) participated in the experiments as paid volunteers. Each subject was supposed to participate in the three experimental sessions and a prior training session, but only 23 subjects participated in the experiment with the pollution source present in the office. At each workstation, thermistors were used to measure the air temperature in the breathing zone of the subjects and the surface temperature of the radiant panels. Air velocity was measured once under each experimental condition. Air humidity and temperature were measured at the centre of the office and used as input to the control system. The ventilation rate was measured prior to or after each experiment by means of tracer gas. During the 18 min exposure, subjects were activated by simulated office work and were regularly asked to assess the air quality, overall and local thermal sensation and comfort. After 9 min of exposure, the subjects evaluated the intensity of symptoms and perceptions, including nose, throat, and eye dryness, headache, etc. on visual analogue scales (Kildesø et al. 1999). After entering the office, the subjects assessed the air quality and then began the office tasks (multiplication, proof reading, addition, and text typing), which were interrupted every 15-35 min to fill in a questionnaire. To maintain an average activity level of around 1.2 met, subjects performed a short stepping exercise after 4 min and again after 13 min. At the beginning of the 18 C sessions, subjects were asked to add a fleece jacket and leg warmers to their clothing ensemble. They were instructed to keep these garments on for at least 3 min and subsequently to adjust their clothing during the remaining period to attain and maintain a neutral thermal sensation. RESULTS Physical measurements performed under each of the three experimental conditions are summarized in Table 2. Table 2. Results of physical measurements (average ± std.). Parameter 23 C Source absent Experimental condition 18 C Source absent 18 C Source present Air temp. in breathing zone ( C) 23.1 ±.4 18. ±.5 18. ±.5 Operative temp. ( C) 22.4 ±.4 2.9 ±.6 2.6 ±.8 Enthalpy (kj/kg) 36 ± 1 29 ± 1 28 ± 1 Ventilation rate (h -1 ) 5.8 ±.5 5.6 ±.3 5.8 ±.3 Under all experimental conditions, the physical parameters were close to the desired values. At two of the workstations, the air temperature in the breathing zone on average was.2-.3 C lower than the desired value and at the other two workstations around.3 C higher. However, this temperature difference was too modest to reveal any systematic impact on subject responses. Figure 2 shows that it was possible after 6 min to attain and maintain an average thermal sensation for the body as a whole around neutral. The thermal acceptability decreased significantly when the air temperature was reduced from 23 C to 18 C (p <.4), despite 269
thermal sensation being almost the same at both temperatures. Local thermal sensation votes showed that particularly hands, face and feet were perceived as colder at 18 C than at 23 C. In addition, draught, caused by too high air velocities at 18 C, resulted in local thermal discomfort being more pronounced at the low temperature (p <.8). Both these factors contributed to a decrease of thermal acceptability. Thermal sensation (7-pt) 3 2 1-1 -2-3 23 C, source absent 18 C, source absent 18 C, source present 3 6 9 12 15 18 Time (min) Clearly 1 acceptable Thermal acceptability 23 C, source absent 18 C, source absent 18 C, source present Clearly -1 unacceptable 3 6 9 12 15 18 Time (min) Figure 2. Average overall thermal sensation votes and average thermal acceptability votes. Figure 3 shows the acceptability of the air just after entry to the office, as an average of six assessments made during the experiment, and when re-entering after refreshing the olfactory senses outside the experimental office after 18 min. The figure indicates that with no pollution source present in the office, air at 18 C was perceived as being more acceptable than air at 23 C. With the pollution source present, the perceived air quality at 18 C was approximately the same as at 23 o C without the source, except when subjects re-entered the office. At 18ºC, the overall indoor environment was evaluated as being significantly better without the source in the office than when the source was present (p <.3) (Figure 4). Without the source present, air temperature had no influence on the evaluation of the overall indoor environment. 1 1 Acceptability.5 -.5-1 23 C, source absent 18 C, source absent 18 C, source present Entry Average Re-entry Acceptability of overall environment.5 -.5-1 23 C, source absent 18 C, source absent 18 C, source present Figure 3. Perceived air quality at first entry, as an average of six assessments during the experiment and at re-entry after refreshing olfactory senses (average ± std.). T exposure is shown in Figure 5. Generally, sympt symptoms varied systematically between condi Figure 4. Acceptability of the overall indoor environment (average ± std.). he average intensity of selected symptoms as expressed by subjects after 9 minutes of om intensity was modest, and none of the tions. The figure indicates that the highest 27
symptom intensity was observed when subjects were exposed to air at 18 C and when the pollution source was present in the office. During the experiments, subjects were activated by different typical office tasks, which also provided a measure of their mental performance. No significant differences between conditions in the speed with which the subjects completed the tasks or in the error ratio could be detected. 23 C, source absent 18 C, source absent 18 C, source present Sleepy Difficult to concentrate Tired Awake Easy to concentrate Rested Dizzy Difficult to think Strong headache Not dizzy Easy to think No headache 25 5 75 1 125 Figure 5. Average symptom intensity (error bars show standard deviations). DISCUSSION The results of the current study suggested an improvement of the perceived air quality when the air temperature was reduced from 23 C to 18 C corresponding to the results of several earlier studies (Fang et al. 1998, 1999; Toftum et al. 1998). However, a reduction of the air temperature had no systematic effect on symptom intensity. Due to the relatively high heat load generated by the radiant heating panels, a ventilation rate of 45 L/(s pers) was needed to sustain the temperature in the low-polluting office. This ventilation rate was much higher than the minimum recommended by current standards, such as e.g. ASHRAE 62 (1996) and CR 1752 (1998). Thus, with no pollution source in the office, the comfort and health effects of reducing the air temperature were studied by further improvement of an already acceptable perceived air quality, and it is possible that more pronounced effects would appear if the reference condition was of a poorer quality. Under all experimental conditions, the total exposure time was 18 min and symptom intensity was evaluated after 9 minutes. This may have been too short for symptoms to develop, particularly in an environment that on average was perceived as acceptable by the subjects. Yet, subjects' assessments indicated that the introduction of a pollution source resulted in slightly higher symptom intensity at 18 C. While overall thermal neutrality was attained both at 23ºC and at 18ºC, thermal acceptability differed between temperatures due to local thermal discomfort caused by unwanted cooling of some body parts, including hands, feet and face. Without the pollution source present in the office, overall acceptability of the indoor environment was nearly the same at 23ºC and at 18ºC, indicating that the expected positive effects of improved perceived air quality may have been moderated by decreased thermal acceptability when subjects assessed the overall indoor environment. Introducing a pollution source significantly reduced the acceptability of the overall indoor environment. 271
CONCLUSIONS A reduction of the air temperature from 23 C to 18 C suggested an improvement of the perceived air quality, while no systematic effect on symptom intensity was observed after 9 min of exposure. The overall indoor environment was evaluated equally acceptable at both temperatures due to local thermal discomfort at the low air temperature. The combination of low air temperature and local heating of the body was expected to promote improved perception of the indoor environment and to result in fewer health symptoms. However, at low air temperature, local thermal discomfort problems need to be addressed in order to optimize simultaneously thermal comfort, perceived air quality and symptom intensity. ACKNOWLEDGEMENT This study was supported by the Danish Technical Research Council (STVF) as part of the research programme of the International Centre for Indoor Environment and Energy established at the Technical University of Denmark for the period 1998-27. REFERENCES Berglund, L.G., Cain, W., 1989: Perceived air quality and the thermal environment. Proc. IAQ 89, San Diego, pp. 93-99. BSR/ASHRAE Standard 62 1996: Ventilation for acceptable air quality. American Society for Heating, Ventilating, and Air-Conditioning Engineers, Inc. CEN CR 1752 1998: Ventilation for buildings Design criteria for the indoor environment. European Committee for Standardization. Fang, L., Clausen, G., Fanger, P.O., 1998: Impact of temperature and humidity on the perception of indoor air quality. Indoor Air, 8(2), 8-9. Fang, L., Wargocki, P., Witterseh, T., Clausen, G., Fanger, P.O., 1999: Field study on the impact of temperature, humidity and ventilation on perceived air quality. Proc. of Indoor Air '99, Edinburgh, Vol. 2, pp. 17-112. Kildesø, J., Wyon, D., Skov, T., Schneider, T., 1999: Visual analogue scales for detecting changes in symptoms of the sick building syndrome in an intervention study. Scandinavian Journal of Work, Environment & Health, 25, 361-367. Pejtersen, J., Brohus, H., Hyldgaard, C.E., Bach Nielsen, J., Valbjørn, O., Hauschildt, P., Kjærgaard, S.K., Wolkoff, P., 21: Effect of renovating an office building on occupants' comfort and health. Indoor Air, 11, 1-25. Toftum, J., Jørgensen, A.S., Fanger, P.O., 1998: Upper limit for air humidity for preventing warm respiratory discomfort. Energy and Buildings, 28( 3), 15-23. Wargocki, P., Wyon, D.P., Baik, Y.K., Clausen, G., Fanger, P.O., 1999: Perceived air quality, Sick Building Syndrome (SBS) symptoms and productivity in an office with two different pollution loads. Indoor Air, 9, 165-179. Wyon, D.P., 1998: Documented indoor environmental effects on productivity. Presented at Danvac conference "Green Buildings", Copenhagen, 5 November. 272