OCCUPANT EVALUATION OF 7-HOUR EXPOSURES IN A SIMULATED AIRCRAFT CABIN PART 1: OPTIMUM BALANCE BETWEEN FRESH AIR SUPPLY AND HUMIDITY

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1 OCCUPANT EVALUATION OF 7-HOUR EXPOSURES IN A SIMULATED AIRCRAFT CABIN PART 1: OPTIMUM BALANCE BETWEEN FRESH AIR SUPPLY AND HUMIDITY P Strøm-Tejsen, DP Wyon, L Lagercrantz and L Fang International Centre for Indoor Environment and Energy, ( Dept. of Mechanical Engineering, Technical University of Denmark, DK-2800, Denmark. ABSTRACT Low humidity in the aircraft cabin environment has been identified as a possible cause of symptoms experienced during long flights. A mock-up of a 21-seat section of an aircraft cabin with realistic pollution sources was built inside a climate chamber, capable of providing fresh outside air at very low humidity. Experiments simulating 7-hour transatlantic flights were carried out at four outside air supply rates - 1.4, 3.3, 4.7 and 9.4 L/s per person (3, 7, 10 and 20 cfm/p) - yielding average humidity levels of 28%, 16%, 11% and 7% RH, respectively. Four groups of subjects were exposed to the four conditions. The subjects completed questionnaires to provide subjective assessments of symptoms commonly experienced during flights. Increasing humidity to 28% RH by reducing outside air supply rate did not reduce the intensity of the symptoms typical of the aircraft cabin environment, and intensified headache, dizziness and claustrophobia. INDEX TERMS Aircraft cabin air quality, Low humidity, Passenger comfort, Sick building syndrome (SBS) symptoms, Ventilation requirements INTRODUCTION In a modern aircraft, the air provided to passengers and crew is outside air brought in through the engine compressors mixed with air taken from the cabin that has been filtered and recirculated. The environmental control system (ECS) compresses the low-pressure outside air and supplies it to the cabin to maintain a comfortable temperature and pressure level at cruising altitude. At a typical altitude of 11,000 m, the outside air temperature is usually about -55 C and the air contains very little moisture. The main sources of humidity in the cabin are exhaled air and evaporated skin moisture, and the level of humidity is consequently low, typically 14-19% RH. These values are below the levels previously considered comfortable for human occupancy, and the quality of air and low levels of humidity have been a cause of concern for many years (NAS 1986, NAS 2002). In principle, cabin air could be humidified to higher levels using humidification equipment, but this would conflict with safety margins due to condensation and corrosion. From the review by Nagda and Hodgson (2001) it appears that low humidity is related to irritation of eyes, and dryness of skin and mucous membranes. Studies have indicated that passengers and cabin crew find the air in the aircraft cabin to be too dry and experience symptoms such as dry, itchy, or irritated eyes, dry or stuffy noses, and skin dryness. The intensity of symptoms seems to increase with duration of exposure. They conclude that a modest increase in RH, e.g. from an average of 14-19% to about 22-24%, might have beneficial effects. Recent climate chamber exposures to very low humidity up to 5 hours indicate that the effects on subjective symptoms, though significant, are very small (Wyon et al., 2002). Reducing the outside air supply rate would increase humidity, but also the concentration of contaminants. The optimal makeup of fresh outside and filtered recirculated air has not been studied systematically. It was the objective of the present investigation to address this issue. FACILITIES AND METHODS Aircraft cabin, environmental control system and instrumentation The aircraft cabin used for the investigation was a mock-up of a three-row (21-seat) section of a conventional Boeing 767 aircraft. The section was built inside a climate chamber, with an access corridor to an adjacent climate Corresponding author pst@mek.dtu.dk 40

2 chamber that was used for administering physiological tests, and a toilet facility. The cabin section was built of aluminium components and incorporated overhead ducting and supply plenums taken from a Boeing 767 cabin air system and authentic window panels. It was furnished with used components from aircraft cabins e.g. 21 cabin seats and carpets, which contributed to a pollution source strength similar to that of a real aircraft cabin. Background noise at a level of 72 db(a) was established by playing recorded aircraft cabin noise through loudspeakers mounted above the cabin roof. The climate chamber is served by two ventilation systems. The principal system could provide air with a moisture content as low as 0.05 g/kg, or <0.3% RH at 23.3 C, i.e. comparable to outside air at cruising altitude. The secondary system cooled the air surrounding the cabin, reducing the inside surface temperature of the un-insulated window panels to establish the correct temperature difference between cabin air and the radiant temperature of the cabin sidewalls. This ensured a correct modelling of thermal conditions and airflow pattern inside the cabin. The cabin ventilation system should model the performance of an aircraft ECS except for pressurization. The ducting comprises: Outside air supply system: The principal ventilation system of the climate chamber simulates the cooled pressurized outside air. The air is brought to the authentic overhead ducting after mixing with recirculated air. Exhaust and recirculation system: Air exits through ducts located along the floor on both cabin sides and is exhausted to the outside or the adjacent chamber, a proportion being recirculated through a HEPA filter and mixed with the fresh outside air. The HEPA filter used for the experiments had 18 months of previous airline usage. Two thirds of the filter area was blocked to ensure realistic air velocities through the filter. During experimental sessions, occupants were allowed to leave the cabin to visit the toilet or for physiological tests in the adjacent chamber. The air quality in these areas should consequently be similar to the air in the cabin so that the subjects perception of air quality and environmental factors, despite these short interruptions, would remain unchanged. Therefore, the adjacent chamber and the toilet facility were ventilated only with exhaust air from the cabin, at the rate of approx. 24 L/s in all test conditions. The cabin and ventilation system were equipped with sensors and instrumentation for control and recording at 4-6 minutes intervals of: (1) Rate of total airflow to the cabin, outside airflow, and flow to the adjacent chamber, (2) Relative humidity and temperature of supply air to the cabin and inside the cabin, (3) Surface temperature of window panels, (4) Pressure inside the cabin, and (5) CO 2 concentrations of outside air, cabin air, and adjacent chamber. Subjects and experimental plan A total of 68 subjects were recruited for the investigation. They were divided into four groups of subjects. Each group included 2 young females who played the role of flight attendants with an activity level of approx. 2.0 met, the remainder acting as sedentary passengers (1.0 met), evenly divided between young (aged 18-30) and elderly (55-70), male and female. Clock Relative Event time time 08:45-0:15 Assembly outside cabin 09:00 0:00 Take off 09:05 0:05 Instruction and exercise 09:10 0:10 First questionnaire 09:45 0:45 In-flight movie 11:30 2:30 Lunch served 12:15 3:15 Second questionnaire 12:25 3:25 First physiological test 13:15 4:15 In-flight movie 15:00 6:00 Third Questionnaire 15:15 6:15 Second physiological test 16:00 7:00 Touch down 16:01 7:01 Subjects disembark Figure 1. Schedule for flight sessions In a repeated measures design, each group was exposed to four different conditions, always on the same day of the week, a total of 16 simulated flights. In the four conditions, outside air supply to the cabin was set at four levels: 1.4, 41

3 3.3, 4.7 and 9.4 L/s per person, whereas the total air supply (outside and recirculated air) was maintained at a constant rate of 200 L/s. Other physical quantities held constant were the temperatures of cabin exhaust air (23.3 C) and window panels (18.3 C), typical of temperatures in a Boeing 767 aircraft cabin at cruising altitude. The 7-hour sessions, comparable in duration to a transatlantic flight, were scheduled according to Figure 1. Intake of alcohol and smoking were not allowed. Passengers could sleep, read, talk or work during the time available, and they were allowed to exchange seats. Subjective assessments Three times during each session, after 10 minutes, 3¼ hours and 6 hours of exposure, subjects were asked to complete a questionnaire, marking visual-analogue (VA) scales to indicate their assessments of Air Quality, Environment, SBS Symptoms, Thermal Comfort and Noise. The VA-scales and a summary of the questionnaire and its 29 VA-scales, similar to those used previously by Wargocki et al. (1999), are shown in Figure 2 and Table 1. Acceptability scale: Clearly acceptable Grading scale: No odour / irritation Slight odour / irritation Scale for Thermal Sensation: Hot Warm Just acceptable Just unacceptable Moderate odour / irritation Strong odour / irritation Very strong odour / irritation Overpowering odour / irritation Slightly warm Neutral Slightly cool Cool Clearly unacceptable Cold Bipolar scale: Figure 2. Visual Analogue scales Table 1. VA-scales used in questionnaires Variable Type of scale Low - High value Variable Type of Low - High value Air Quality SBS Symptoms scale Indoor Air Acceptability Clearly unaccept. - Clearly Nasal Bipolar Nose clear - Nose Odour Intensity Grading No odour - Overpowering Nasal Dryness Bipolar Nose runny - Nose dry Eye Irritation Grading No irritation - Mouth Bipolar Mouth not dry - Mouth Nasal Irritation Grading No irritation - Lip Dryness Bipolar Lips not dry - Lips dry Throat Irritation Grading No irritation - Skin Dryness Bipolar Skin not dry - Skin dry Environment Eye Dryness Bipolar Eyes not dry - Eyes dry Air Bipolar Too humid - Too dry Eyes SmartingBipolar Eyes not smarting - Eyes Freshness of Air Bipolar Air stuffy - Air fresh Eyes Aching Bipolar Eyes not aching - Eyes Illumination Bipolar Too dark - Too bright Headache Bipolar No headache - Severe Noise Bipolar Too quiet - Too noisy Thirst Bipolar Not at all thirsty - Very Thermal Comfort and Noise Dizziness Bipolar Not dizzy - Dizzy Thermal Thermal Cold - Hot Fatigue Bipolar Rested - Tired Thermal Acceptability Clearly unaccept. - Clearly Mental State Bipolar Bored - Interested Air Movement Acceptability Clearly unaccept. - Clearly Sleepiness Bipolar Alert - Sleepy Noise Level Acceptability Clearly unaccept. - Clearly Mental Bipolar Relaxed - Uptight Claustrophobi Bipolar Not a problem - Objective physiological measurements Twice during each flight, after approx. 3½ and 6¼ hours of exposure, subjects participated in four tests: Visual Acuity, Nasal Peak Flow at inspiration, Skin Dryness and Mucous Ferning. Visual Acuity was assessed using a self-administered test on a computer and one of the 15-inch flat (TFT) computer screens in the cabin. The three last tests were administered in the adjacent climate chamber that was maintained with the same air quality as in the cabin. 42

4 RESULTS Physical measurements The data recorded from the instrumentation of the cabin and ventilation system were normally distributed and therefore tabulated as means and standard deviations (SD). Table 2 shows a summary of the quantities held constant during the 4 conditions (Column 2-6), and of the resulting RH and CO 2 -levels (Column 7-9). The daily averaging period was from the time when steady conditions had been reached for RH and CO 2 until the end of the simulated flight. Conditi on [L/s per Table 2. System Data averaged over conditions and daily periods from 9:45 to 15:45 Flow of Flow of Cabin air Window RH, CO 2, outside outside temp. panel cabin air cabin air [L/s air* [ C] temp. [%] exhaust Total air supply to cabin CO 2, outside air [ppm] Avera SD Avera SD Avera SD Avera SD Avera SD Avera SD Avera SD Avera SD *Present requirement of fresh air per occupant 2 according to the US Federal Aviation Regulation 25 (FAR 25) is 0.25 kg/min (0.55 pounds/min). Subjective assessments Data from subjective assessments and objective physiological tests cannot be assumed to be normally distributed, and were therefore analysed using the non-parametric Friedman two-way analysis of variance by ranks, followed by the Wilcoxon matched-pairs signed-ranks test for those cases in which the Friedman P-value was <0.05. Subjects with missing data were omitted from the analysis. Table 3 shows the significant results that were obtained. Table 3. VA-scale assessments with significant differences between conditions Period of exposure VA-scale Friedman Wilcoxon matched-pairs signed-ranks test Test All 1.4 vs 1.4 vs 1.4 vs 3.3 vs 3.3 vs 4.7 vs 1 st 10 min Throat Irritation h nd 3¼ Air i Noise i h 0.009h Eye Dryness i h Claustrophobia h 0.008h 0.002h rd 6 hours Noise h i Headache h Dizziness h 0.007h 0.002h Claustrophobia h 0.001h 0.009h 0.011h Significant differences from the Friedman test (p<0.05) and Wilcoxon test (p<0.05) are shown in bold. h means that scale value of first condition is higher than second condition. Objective physiological measurements Skin Humidity [%] First Skin Dryness Test 45 P=0.000 P< P< P<0.006 Wet Skin Humidity [%] Second Skin Dryness Test P=0.000 P=0.000 P< Dry Outside air supply [L/s per person] Outside air supply [L/s per person] 75% % Med Med ian 25% ian % Figure 3. Measurements of Skin Dryness on the inside of the right middle finger 43

5 No statistically significant differences between conditions were observed for Visual Acuity, Nasal Peak Flow or Mucous Ferning. The Skin Dryness was significantly different both times. Figure 3 shows the quartile values of the group means of the Corneometer data for a total of 57 subjects. The P-values are from the Wilcoxon tests between pairs of conditions. DISCUSSION Physical measurements The system data summarized in Table 2 show that the intended cabin conditions were accurately maintained. Total air supply to cabin, Flow of outside air, Cabin temperature and Window panel temperature were all controlled at the specified levels within narrow limits. Humidity and CO 2, caused by the passengers and their activities, varied during each of the simulated flights. Thus, the RH and CO 2 -values summarized in Table 2 are not precise figures but representative averages. The average RH was 7% at 9.4 L/s per person and 28% at 1.4 L/s. Subjective assessments The perceptions of Air Quality as indicated on the five VA-scales (Indoor Air Quality, Odour Intensity, Eye Irritation, Nasal Irritation, and Throat Irritation) were all favourable with only small differences between conditions. Only Throat Irritation after 10 minutes of exposure, showed a significant difference between conditions, but the symptom intensity was very low in all conditions, and no statistically significant differences were found after 3¼ hours and 6 hours of exposure. Three of the four Environmental factors (Air Humidity/Dryness, Freshness of Air, Illumination, and Noise) were all assessed neutrally and have seemingly not been the cause of any particular concern. The assessments of Air Humidity/Dryness differed significantly between the 1.4 L/s per person and the 9.4 L/s per person conditions, but only after 3¼ hours exposure and not after 6 hours. The judgments of noise were less favourable with ratings halfway between Neutral and Too noisy. The background noise level of 72 db(a) was considered a slight nuisance. Table 3 shows noise, after both 3¼ and 6 hours of exposure, to be significantly worse in the 3.3 L/s per person conditions, quite possibly by chance. SBS-symptoms such as: Nasal Occlusion, Nasal Dryness, Skin Dryness, Eye Dryness and Eyes Smarting, are presumably all associated with the low humidity of the cabin. It was thus of particular interest to examine whether any of these symptoms were improved due to the increased humidity in the 1.4 L/s per person condition, but only Eye Dryness, after 3¼ hours of exposure, was reduced. Analysis of other SBS symptoms showed significant differences for Claustrophobia, Headache and Dizziness (Table 3). However, it is important to consider the level of the observed effects. After 6 hours of exposure Headache intensity was quite low for all conditions, although still very considerably reduced by providing more outdoor air. The intensity of Dizziness was also low, but providing more outdoor air halved it. The intensity of Claustrophobia was very low and was only slightly reduced by providing more outdoor air. Assessments of the remaining SBS symptoms: Mouth Dryness, Lip Dryness, Eyes Aching, Thirst, Fatigue, Mental State, Sleepiness and Mental Tension, did not show significant differences between conditions in any of the three questionnaires. The four assessments of Thermal Comfort and Noise (Thermal Sensation, Thermal Environment, Air Movement, and Noise Level) were not expected to be influenced by humidity or fresh air, and there were no significant differences between conditions. Cabin temperature was apparently maintained at an acceptable level. Thermal Sensation was assessed between Neutral and Slightly warm, and Thermal Environment and Air Movement were rated between Just acceptable and Clearly acceptable. Objective physiological measurements The Skin Dryness tests showed a reduction in skin moisture level in the less humid conditions. A comparison between values of skin humidity between the first and the second session showed only slight differences, and it appears that skin dryness attained a stable level before the first measurement. The results are in agreement with Wyon et al. (2002) that showed skin moisture to be lower at 15% than at 35% RH. CONCLUSIONS 44

6 Based on the present investigation it can be concluded that increasing the humidity in an aircraft cabin by reducing the amount of outside air would not reduce the intensity of the symptoms that are typically associated with the cabin environment. No significant differences of symptoms were found between the four conditions, except that reduced outside air flow intensified other complaints commonly associated with air travel, i.e. Headache, Dizziness and Claustrophobia. The reported intensity of these SBS symptoms was low. ACKNOWLEDGEMENT The present work was supported financially by the Boeing Aircraft Company and by the Danish Technical Research Council as part of the research programme of the International Centre for Indoor Environment and Energy at the Technical University of Denmark. REFERENCES Nagda NL. and Hodgson M Low Relative Humidity and Aircraft Cabin Air Quality, Indoor Air, 11: NAS The Airliner Cabin Environment: Air Quality and Safety. Washington DC: National Academy of Sciences/National Research Council, National Academy Press. NAS The Airliner Cabin Environment and the Health of Passengers and Crew. Washington DC: National Academy of Sciences/National Research Council, National Academy Press. Wargocki P., Wyon DP., Baik YK., Clausen G. and Fanger PO Perceived Air Quality, Sick Building Syndrome (SBS) Symptoms and Productivity in an Office with Two Different Pollution Loads, Indoor Air, 9: Wyon DP., Fang L., Meyer HW., Sundell J., Weirsøe CG., Sederberg-Olsen N., Tsutsumi H., Agner T. and Fanger PO Limiting Criteria for Human Exposure to Low Humidity Indoors, Proceedings of the 9th International Conference on Indoor Air Quality and Climate - Indoor Air 2002, Santa Cruz: Indoor Air 2002, Vol 4, pp