1Jpper limits of air humidity for preventing warm respiratory discomfort

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1 ELSEVIER Energy and Buildings 28 ( 1998) Jpper limits of air humidity for preventing warm respiratory discomfort J@m Toftum *, Anette S. Jgrgensen, P.O. Fanger Laboratory of Indoor Environment and Energy, Department of Energy Engineering, Technical University of Denmurk, Building 402, Lyngby DK-2800, Denmark Received 3 March 1997; accepted 9 March 1997 Abstract The effect of humidity and temperature of inhaled air on perceived acceptability of the air was studied. Thirty-eight subjects evaluated air at 14 combinations of temperature and humidity to verify that insufficient evaporative and convective cooling of the mucous membranes in tha upper respiratory tract is a cause of local warm discomfort and of a perception of poor air quality. Unpolluted, conditioned air was led from a climate chamber to a box where the subjects one by one evaluated the air. The inhaled air was rated warmer, more stuffy and less acceptable with increasing air humidity and temperature. A model was developed that predicts the percentage of persons dissatisfied due to insufficient respiratory cooling as a function of the actual evaporative and convective cooling of the respiratory tract. Together with a previously proposed mode1 for predicting discomfort due to high skin humidity, the respiratory model may be used to specify upper limits for humidity in the indoor environment Elsevier Science S.A. All rights reserved. Ke words: Respiratory cooling; Humidity limit; Local discomfort; Air quality; Inhaled air - 1. Introduction Humidity may be a cause of discomfort due to inhalation of humid and warm air or to an uncomfortably high level of sk,!n humidity. In the present study, human subject experiments were conducted to investigate the effect of humidity an d temperature of inhaled air and of skin humidity on percelved discomfort. This paper presents the results of the experiments on humidity and temperature of inhaled air on discomfort, whereas discomfort caused by skin humidity will be discussed in a separate paper [ 81. In humans, the respiratory tract acts as an air-conditioning system that regulates the humidity content and temperature of the inspired air on its way to the lungs. The site of the conditioning process is determined by the pulmonary ventilation rate and the temperature and water content of the inhaled air [ 11. With air at temperatures and humidities commonly found indoors and at low pulmonary ventilation rates, thr, main part of the conditioning takes place in the upper respiratory tract. In most indoor environments, inhalation of air will cause a cooling of the mucous membranes in the upper respiratory tract, which contributes both to the perception of the thermal environment and to the perceived air quality. Cooling takes place through a combination of con- -- * lorresponding author. Tel.: f ; fax: ; e-nlail: jt@et.dtu.dk vective and evaporative heat loss. A low temperature of the inhaled air results in greater convective cooling and low humidity causes greater evaporative cooling. At high air temperatures and humidities, the respiratory cooling will decrease with the result that the air may be perceived as stuffy and uncomfortable. Berglund and Cain [2] considered the possibility of supplying cool and dry air to a space to alleviate the perception of poor air quality, without even removing contaminants in the air. In their study, Berglund and Cain showed that air freshness and acceptability perceptions decreased with increasing temperature and humidity. In particular, relative humidities above 65% were associated with unacceptable air quality judgments. These findings were confirmed by Fang et al. [ 31, who showed that air polluted by different building materials and unpolluted background air were found less acceptable when temperature and humidity were increased. In addition, the impact of temperature and humidity on perceived air quality dominated the influence of the pollution sources at high enthalpies of the inhaled air. In standards prescribing acceptable thermal indoor environments, upper limits for humidity have been specified that were unrelated to thermal comfort or related solely to discomfort caused by too high levels of skin humidity [ In a separate paper by the authors, a model was introduced that predicts discomfort due to high skin humidity in the comfort range of temperatures [ 81. This skin humidity model can be /98/$ Elsevier Science S.A. All rights reserved. PIrSO (97)

2 16 J. Tqfium et al. /Energy and Buildings 28 (19Yll) used to determine discomfort due to high skin humidity at many combinations of environmental parameters and clothing characteristics for sedentary persons. In the comfort zone, the model allows high air humidities without causing substantial discomfort from humid skin. The upper limit for indoor air humidity may be determined rather by discomfort due to inhalation of humid or warm air. The purpose of the present study was to verify that insufficient respiratory cooling is a cause of local thermal discomfort and to develop a model predicting the percentage of dissatisfied persons as a function of respiratory cooling. Respiratory cooling is anticipated to be a function of temperature and humidity of the inhaled air. 2. Experimental method The impact of temperature and humidity on perception of inhaled air was studied at I4 combinations of air temperature and relative humidity as shown in Table I. For each condition, human subjects assessed the air with respect to thermal sensation, freshness and acceptability. 2. I. Facilities Air from a climate chamber designed for air quality studies [9] was presented to the subjects in a small box placed outside the chamber. By controlling the temperature and humidity of the air in the chamber, the desired condition in the box could be provided. By means of a glass tube system containing a fan, the air was led from the climate chamber to the box, where it was assessed. The air moved upwards in the box and the air motion resembled a piston flow. To promote a homogeneous air velocity profile, a perforated plate was mounted across the air inlet in the bottom of the box. The assessments were made in the center of the box where the air velocity (without subject) was m/s. The box was made of stainless steel with dimensions 0.95 X0.34X0.29 m3. Air temperature and humidity in the box and inside the climate chamber were monitored continuously by temperature and humidity transmitters. The transmitter measures temperature by means of a Pt- 100 sensor in the range - 20 C to 80 C with an accuracy of +0.2 C and relative humidity in the range 0% to 100% with an accuracy of rt 2% rh in the O- 90% rh range and + 3% rh in the % rh range. Air temperature and humidity were carefully controlled before and during each series of assessments. Between assessments, the subjects rested in the same room as that in which the box was located. Air temperature, air velocity and humidity in the waiting room were monitored continuously during the experimental period Subjects Thirty-eight subjects, 19 females and 19 males, most of them university students, were paid to take part in the exper- Table 1 Combinations of temperatures and relative humidities of the air assessed by the subjects Air temperature ( C) Relative Partial vapor humidity pressure (S) (Pa) Enthalpy (kj kg- ) iments. All subjects were volunteers who gave their informed consent prior to the experiments. Anthropometric data for the subjects are shown in Table 2. To maintain a low skin humidity and a thermal sensation near neutral, the subjects were dressed in a thin, one-layer experimental clothing ensemble made of cotton. The clothing insulation value of the ensemble was not measured, but was estimated to approximately 0.5 clo. The cotton fabric had a very low resistance to diffusion of water vapor ( N 2 mm still air equivalent). In addition, the subjects wore socks and highly moisture permeable cotton shoes Experimental procedure The experiments were conducted in four repeated sessions, in the mornings and afternoons on two consecutive days. Between eight and twelve subjects participated in each session. On arrival, the subjects put on the experimental clothing ensemble and were then seated in the waiting room outside the climate chamber. Before the first assessment of the air in the box, the subjects evaluated their general thermal sensation and perception of skin humidity, and the air in the waiting room was judged as regards thermal sensation, freshness and acceptability. The subjects then in random order approached the box, positioned the head inside the box and evaluated the inhaled air after three to four inhalations according to the Table 2 Anthropometric data for the subjects Sex Age Height Weight Dubois area (Y) Cm) (kg) (m-3 Females 23.0f * f Males 22.s* * * f0.14 Females and 22.8 f kO tO.16 males

3 J. To&m et al. /Energy and Buildings 28 (1998) questionnaire shown in Fig. 1. Fig. 2 shows a subject prior to ard during evaluation of the air in the box. After a subject had finished an assessment and completed a questionnaire, he. or she was seated and another subject began an assessment. When all subjects had evaluated air in one condition, the air temperature and humidity in the climate chamber were changed to new levels. After 10 to 1.5 min, when a stable condition inside the box had been reached, the assessment procedure was repeated. Due to limitations in the climate chamber air-handling system, the conditions in the box could not be presented to the subjects in random order without unacceptably long waiting times. Instead, the order of conditions shown from top to bottom in Table 1 was used. The subjects made five to six assessments of the air in the box per hour and were asked to evaluate the conditions in the waiting room every half hour during the experimental period Calculations Heat and moisture transport in the upper respiratory tract are complex processes. The cooling of the mucous membranes during inhalation is proportional to the temperature and water vapor gradients from the surface of the respiratory tract to the inhaled air. On its way to the lungs, the inspired air passes the nostrils, the nasal 3 passage and the nasopharynx How do you thermally rate me air m the box? Hot Fresh? WarIll Sightly NeUfral Slightly COOI Cold warm cwl UC you lee1 the a,, KI the box to ix located at the back of the nasal passage. The cooling of the mucous membranes at the surface of the nasal passage is presumed to be decisive for the thermal perception of the inhaled air. The surface temperature of the nasal passage is therefore important for an empirical correlation of respiratory cooling and subjective thermal perception of the air. Air entering the alveoli has a temperature near that of the body (37 C) ] lo]. With nasal breathing, experiments have shown that the temperature of inspired air at 23 C increased to 32.4 C in the nasopharynx [ 111. With forced breathing (high activity), the temperature of ambient air at 27 C rose from 28.3 C within the nostrils to 32 C in the nasopharynx. Thus, for use in the present study, an average surface temperature of the nasal passage of 30 C was anticipated for sedentary subjects in comfortable environments. Inside the nasal passage, the temperature of the inhaled air changes along the path to the nasopharynx, simultaneously affecting the heat transfer between the inner surface of the nose and the air. Nevertheless, for the purpose of the present study this temperature change was disregarded, and the dry respiratory cooling thus approximated by C,,,=K,(30-t,) At the mucosal surface, the condition for water vapor concentration is assumed to be saturation, which is a function of Neither lresh nor stuffy? I HOW do you rate the mermal acceptabilay of the air in the box: Acceptable Just acceptame I Just unacceptable Unacceptable Fig. 1, Questionnaire concerning thermal sensation, freshness and acceptability of the inhaled air. Fig. 2. A subject prior and during inhalation of conditioned air in the assessment box outside the climate chamber.

4 18 J. Tojtum et ul. /Energy and Buildings 28 (19YXj IS-23 the surface temperature [ IO]. The mucosal lining consists of 95% water and 5% glycoprotein [ II]. The saturated vapor pressure at the surface of the mucosal lining therefore deviates slightly from saturated vapor pressure of water in air at 3O C, which is 4250 Pa. This difference is presumed to be of only little importance, and therefore the latent respiratory cooling was evaluated by E,,,=K,(42.5-O.Olp,) The constants K, and K2 are functions of average convective and evaporative heat transfer coefficients, respectively, and specific properties of air and water vapor. t, is the temperature and pa the water vapor pressure of the surrounding air. Adding C,,, and E,,, provides an expression for the total respiratory cooling. Based on these assumptions, the thermal perception of the inhaled air may be linked to the respiratory cooling by the expression Y_f[K,(30-r,)+K,(42.5-O.Olp,)] in which Y in this study designates subjective thermal sensation, freshness or acceptability of the inhaled air (Fig. 1) Data processing Multiple linear regression analysis was used to correlate the driving potentials for convective and evaporative heat losses with subjective perception of the inhaled air. As dependent variables, the average for all subjects (N= 38) votes of thermal sensation, freshness and acceptability were used at each assessed combination of air temperature and humidity. The acceptability scale shown in Fig. 1 may be used to record both a pseudo-continuous acceptability response and a binary dissatisfaction response. A mark put on the upper half of the scale designate satisfaction and on the lower half dissatisfaction. From assessments recorded with this scale, logistic regression analysis was used to correlate the driving potentials for convective and evaporative heat losses with the percentage of dissatisfied due to decreased respiratory cooling. 3. Results At all exposures, the temperature and the relative humidity in the box deviated less than 0.2 C and 3% rh, respectively, from the planned levels shown in Table 1. The discrepancy in experimental exposures between subjects and sessions was thus negligible. The experimental condition at 29 C, 25% rh was evaluated by 21 subjects only, as the climate chamber was unable to condition the air to the desired level of humidity in two of the four sessions. Multiple linear regression with the driving potential for convective ( 30 - t,) and evaporative ( II),,) heat transfer as explanatory variables was conducted with thermal mean vote, freshness vote and acceptability vote as dependent variables. The applied regression model was V=a+h(30-t,)+c(42.5-O.Olp,) The results of the regression analysis are shown in Table 3. The coefficients describing the sensible and latent heat transfer between the surface of the nasal passage and the inhaled air (K, and K2) are contained in the regression coefficients b and c. The regression equations link the convective and evaporative heat transfer processes to subjective thermal perception of the inhaled air. For fully suppressed respiratory cooling (t;, = 30 C and pa = 4250 Pa) the empirical equations predict votes very near the end points of the applied scales (warm, stuffy, unacceptable). Fig. 3 shows the mean thermal sensation vote as a function of the potential for evaporative cooling at the four applied levels of convective cooling. At increasing convective and evaporative cooling, the subjects sensed the air as cooler. In addition, the inhaled air was perceived increasingly stuffy at decreased evaporative and convective cooling (Fig. 4). Accordingly, the acceptability of the air was higher the cooler and more fresh the air was rated, i.e. at low air temperatures and humidities (Fig. 5). In Table 4, the percentage of dissatisfied among the 38 subjects due to decreased respiratory cooling is shown at the air temperatures and water vapor pressures to which they were exposed in the box. During all experimental sessions, the air temperature in the waiting room rose from approximately C at the beginning of a session to C at the end (average for all sessions). The relative humidity varied in a rather narrow interval between 45% rh and 50% rh. It was attempted to maintain a dry skin and an overall thermal sensation for the body around neutral. Fig. 6 shows thermal sensation and sensation of skin humidity, averaged for all 38 subjects, as a function of time. Thermal sensation varied between neutral and slightly warm, whereas the sensation of skin humidity was very near normal throughout the experimental period. When the subjects evaluated the air in the box, they may, consciously or unconsciously, have compared this with the air in the waiting room. Therefore, some bias from the conditions in the waiting room on the assessments of the air in Table 3 Regression coefficients determined by multiple linear regression with ther- IIIdl mean vote, freshness vote and acceptability vote as dependent variables Y Thermal mean vote Freshness mean vote Acceptability mean vote n I.06 Thermal vote: - 3 = very cold, 3 = hot. Freshness vote: 0 = fresh, 26 = stuffy. Acceptability vote: I = acceptable, - 1 =unacceptable. b R

5 J. Toftum et al. /Energy and Buildings 28 (199X) SI. warm 3 > s t Neutral 0 3!i E Cool -,! SI. cool O.Olp. Fig. 3. Mean thermal sensation of inhaled air (N= 38) as a function of the vapor pressure difference ( O.Olp,) and temperature difference (30 - t,) be:ween the surface of the nasal passage and the air. The dotted lines show predictions made by the regression equation given in Table O.Olp Fi;;. 4. Mean freshness vote of inhaled air (N= 38) as a function of the vapor pressure difference (42.5 -O.Olp,) and temperature difference (30 - r,) between the surface of the nasal passage and the air. The dotted lines show predictions made by the regression equation given in Table 3. Acceptable 1 30-k ("C) b & 0.5 fi c.. + A.-b ,:..,,..-a..... : d t q 7 +4 Al! Unacceptable ~. Fig. 5. Mean acceptability vote of inhaled air (N= 38) as a function of the vapor pressure difference ( O.Olp,) and temperature difference (30 - t,) be! ween the surface of the nasal passage and the air. The dotted lines show predictions made by the regression equation given in Table 3. the box was expected, although in what way was unknown. Fig. 7 shows thermal sensation, freshness and acceptability of the air in the waiting room as time progressed. Initially, the thermal sensation of the air was very near neutral, but increased to between neutral and slightly warm. During the course of experiments, the air in the waiting room was per-

6 20 J. Toftum et al. /Energy and Buildings 28 (1998) Table 4 Air temperature, water vapor pressure and the percentage of subjects dissatisfied with the thermal perception of the inhaled air at this condition Air Water vapor Percentage of temperature pressure dissatisfied ( C) (@a) (%I ceived as neither stuffy nor fresh. Also, the air was rated rather close to acceptable (0X3-0.7). Altogether, the subjects assessed the air in the waiting room as being satisfactory. 4. Discussion Both temperature and humidity of the inhaled air had an impact on the human perception of respiratory thermal sensation, freshness and acceptability. The subjects perceived the inhaled air as cooler, less stuffy and more acceptable at low temperatures and humidities. The results of the present study substantiated the hypothesis that the sensation of air humidity is linked to the cooling of the respiratory tractduring inhalation. Overall thermal neutrality may be attained at numerous combinations of the thermal environmental parameters, even at high air humidities. But local thermal discomfort may still be caused by for example cold or warm floors or by too high vertical temperature gradients. Requirements have therefore been specified for the surface temperature of floors and for maximum acceptable vertical temperature gradients in order to avoid these types of local thermal discomfort [4,6]. Likewise, requirements for the temperature and humidity of the air need to be specified in order to avoid local thermal discomfort due to insufficient cooling of the mucous membranes in the upper respiratory tract. Thus, the experimental data presented in Table 4 were used to correlate the driving potentials for convective and evaporative heat trans- SI. warm 1.0 -_-.-- -~ 5 SI. humid -*Skin humidity Neutral SI. dry Time (min) Fig. 6. Thermal sensation of body and perceived humidity of skin. Votes cast every half-hour during the experimental sessions so 120 Time (min) Fig. 7. Mean thermal sensation, freshness and acceptability of the air in the waiting room, Votes cast every half-hour during the experimental sessions.

7 J. Toftum et al. /Energy and Buildings 28 (1998) fe.r with the percentage of dissatisfied due to decreased respiratory cooling (PD) using logistic regression analysis. The analysis resulted in a model for predicting the percentage of d; ssatisfied due to decreased respiratory cooling: 100 PD= l+exp[ (30-r,)+o.l4(42.5-o.olpj] % Observed and predicted percentages of dissatisfied are compared in Fig. 8. The sample correlation coefficient between observed and predicted percentages of dissatisfied was The model is based on assessments of air at temperatures in the range C and water vapor pressures ranging from 1000 Pa to 3000 Pa. Fig. 9 shows combinations OF air temperature and humidity which cause lo%, 20% and 30% dissatisfied due to decreased respiratory cooling as pred,cted by the model. The figure specifies rather stringent requirements for the temperature and humidity content of the inhaled air, even for unpolluted air. The model was developed on the assumptions that the average surface temperature of the nasal passage was 30 C and that the condition at the surface was saturation. The temperature of the nose depends to some extent on the environmental temperature and on the degree of vasoconstriction, which for persons feeling cold is typically high on the face. Nevertheless, the paranasal sinuses (mucosal-lined air spaces surrounding the nasal passages) may serve as insulators helping to maintain a relatively constant temperature within the nose [ lo]. The proposed model is valid for sedentary, thermally neutral persons having a low skin humidity and for first impressions of air acceptability. On respiration, the mucous membranes in the upper respiratory tract alternately are cooled by inhalation of the surrounding air and heated during exhalation by the warm and humid air coming from the alveoli. The period of the respiratory cycle is around four seconds (sedentary activity) implying that a steady-state condition at the mucosal surface will never occur. Adaptation to insuffi- % Dissetisfled (observed) % Fig. 8. Comparison of observed and predicted percentages of dissatisfied due to decreased respiratory cooling under all experimental conditions studied. The sample correlation between observed and predicted percentage of dissatisfied was Air temperature ( C) Fi ;. 9. Combinations of air temperature and humidity corresponding to IO%, 20% and 30% dissatisfied due to decreased respiratory cooling calculated from the logistic regression model.

8 22.I. Toftum el ~1. /Energy and Buildings 28 (1998) g :: Q Enthalpyofair (kj/kg) Fig. 10. Comparison of relationships between acceptability of inhaled air and enthalpy observed in the present study and by Fang et al cient respiratory cooling is therefore unlikely to occur. Also, no change in respiratory cooling over time is expected insofar as the nose can maintain the mucosal lining and the temperature of its inner surface. A 1 C change in air temperature had the same effect on acceptability and freshness as 12 1 and 130 Pa change in vapor pressure, respectively. For thermal sensation, a 1 C change in air temperature corresponded to 23 1 Pa change in vapor pressure, i.e. the relative influence of air temperature on thermal sensation was almost twice that on freshness and acceptability. Consequently, the relative influence of humidity on freshness and acceptability was higher than on thermal sensation. Dry ambient air at a high temperature may therefore be perceived as warm without being stuffy and unacceptable. Humid ambient air at high temperature, on the other hand, will be perceived as being warm, stuffy and unacceptable. Berglund and Cain [ 21 found that, on average, a 1 F change in air temperature had the same effect on staleness (freshness) as a 6 F change in dew point temperature. In the present study, a 1 C change in air temperature on average had the same effect on freshness as 1.1 C change in dew point temperature. In this study, the observed relative weight of humidity on the freshness vote was thus markedly higher than that found by Berglund and Cain. It should be borne in mind, however, that the subjects in this study made an assessment of the air after 10 to 1.5 s of exposure, whereas Berglund and Cain in their analysis used steady-state values after 60 minutes of exposure. Thus, the effect of humidity on perceived freshness seems to be most pronounced in first impressions of the inhaled air. The air supplied to the assessment box had a low level of background pollution due to the construction of the climate chamber (non-polluting stainless steel), and the materials used in the glass duct system. In addition, no significant sensory emission sources were present in the climate chamber. Therefore the votes of stuffiness should be caused by the temperature and humidity of the inhaled air itself. This is also reflected in the air being perceived as highly fresh at low temperatures and humidities. Fang et al. [ 31 showed that the acceptability of air containing pollutants emitted from different building materials was influenced significantly by temperature and humidity. At high temperature and humidity levels, the acceptability of the air was mainly determined by these thermal factors, whereas the pollution level dominated at low temperatures and humidities. Fig. 10 compares the acceptability votes concerning the inhaled air cast in the present study and obtained by Fang et al. [ 31 as a function of the enthalpy of the air. In the present study, the subjects assessed the air in a box as shown in Fig. 2, whereas the subjects in [ 31 were presented to the air in cones covering only the nose. In spite of the difference in the methods used, the relationships between acceptability and enthalpy in the two studies are almost the same. In the present study, assessments of the air were made with nose breathing. With mouth breathing, a large part of the air by-passes the nasal passages, and therefore the perception of the inhaled air originates from cooling of the mucous membranes in the mouth. Most adults prefer breathing through the nose, but nasal obstruction may result in mouth breathing [ 121. Also, with increased ventilation demands, e.g. on physical activity, mouth breathing will constitute a larger part of the pulmonary ventilation. Berglund and Cain [ 21 found only a slight effect of activity on perceived freshness-stuffiness and acceptability of the inhaled air at activity levels ranging from 1 to 3 met. This indicates that at activity levels typical for indoor environments (not sports arenas), the acceptability of the inhaled air will depend only slightly on the pulmonary ventilation rate. Nevertheless, Berglund and Cain found that the air was perceived as more humid with increasing pulmonary ventilation, but this did not affect the acceptability. The present study confirms that both humidity and temperature of the air in indoor environments have an important impact on the discomfort perceived by humans. At high air humidity or air temperature, the cooling of the mucous membranes in the upper respiratory tract on inhalation is decreased, causing the air to be perceived as stuffy and unac-

9 J. Toftum et al. /Energy and Buildings 28 (1998) ccptable. Together with a previously proposed model for predicting discomfort due to high skin humidity [ 81, the present results add requirements to the air humidity in buildings, based on thermal comfort for the occupants. For a maximum accepted percentage of dissatisfied, the two models can specify upper limits far humidity for the idaarenviranment. A comparison of the two models showed that the respiratory model prescribes the most restrictive limits, excluding arange ofenvironments accepted by the skin humidity model. Combined, the two models provide an extension of the existing d&p &i&a 6.~ tire &err& irrtienvironrnenc. Erruirunmerits where people feel thermally neutral may be established al many combinations of activity, clothing, air temperature, mean radiant temperature, velocity and air humidity. The two models proposed may exclude some of these conditions n*hich cause an uncomfortable skin humidity or an unaccq?aibk idtztkd wk. 5. Conclusions Air humidity and temperature have a significant impact on PL?ii%iEd fl+ar!iv; w.tf~?.fci??~, *?ii%&lr?*> -ad -ib-ix!?%biii rl -& inhaled air. The effect is assumed to be related to warm discomfort in the respiratory tract caused by an insufficient evaporative and c anvective cooling of the mucous membranes. A model has been elaborated that predicts the percentage ot-persons dissatisfied due to insufficjent respiratory cooling at a Function of temperature and humidity of the inhaled air. Ii applies for thermally neutral persons at sedentary activity levels. The respiratory mode1 provides more stringent requirements for air humidity than a previously developed skin humidity model. Acknowledgements Financial support for this study from Vera and Carl Johan Michaelsens Fund is gratefully acknowledged. References III [61 [71 [81 E.R. McFadden et al., Thermal mapping of the airways in humans, J. Appl. Phys., 58 ( 1985) 564. L.G. Berglund and W. Cain, Perceived air quality and the thermal envirnnmeti, Proc.. LQ 119,,Sanllieg~~,& 19R19.w. 9X-99. L. Fang, G. Clausen and P.O. Fanger, The impact of temperature and humidity on perception and emission of indoor air pollutants, hoc. Indoor Air 96, Vol. 4, 1996, Nagoya, Japan, pp ASHRAE , Thermal environmental conditions for human occupancy, American Society of Heating, Refrigerating and Air Conditioning Engineers, 1791 Tullie Circle, Atlanta, USA, ASHRAE 55a-1995, Addendum to Thermal environmental conditions for human occupancy, American Society of Heating, Refrigerating and Air Conditioning Engineers, Atlanta, GA, IS , Moderate thermal environments-determination of the PMV and PPD indices and specification of the conditions for thermal comfort, International Organization for Standardization, Geneva. Switzerland, prenv , Ventilation for buildings, Design criteria for the indoor environment, Draft CEN (European Committe for Stdndardization) TC 156 (pre-standard), Toftum, A.S. Jorgensen and P.O. Faqter. U,quer limits for indoor air humidity to avoid uncomfortably humid skin, Energy Build., 28 (1998) l Albrechtsen, Twin climatic chambers to study sick and healthy buildings, Proc. Healthy Buildings 88, Vol. 3, 1988, pp D.F. Proctor and D.L. Swift, Temperature and water vapor adjustment, in J.D. Brain, D.F. Proctor and L.M. Reid (eds.), Respiratory Defense Mechanisms, Part I, Marcel Dekker, New York, J..4nderson, G..!mxJq& D.F. Pxmu, cited by D.E Pmxur, md D.L. Swift, Temperature and water vapor adjustment, in J.D. Brain, D.F. Proctor, L.M. Reid (eds.), Respiratory defense mechanisms, Part I, Marcel Dekker, New York, D.L. Swift and D.F. Proctor, Access of air to the respiratory tract, in J.D. Brain, D.F. Proctor, L.M. Reid (eds.), Respiratory Defense Mechanisms, Part I, Marcel Dekker, New York, 1977.