The Research Of Thermal Comfort For Human In Spaceflight

Airtight cabin thermal environment of spacecraft not only affects the comfort andwell being of astronauts, but also influences work efficiency and the circulation ofmaterial energy flow of the cabin. It is an inevitable environmental factor. The thermalambient evaluation for crew module has been one of key links for engineering design andmaterial and enery circulation design of spacefcraft in long term spaceflight mission.Spaceflight and its bed rest analog impair thermoregulatory responses, includingincreased body temperature and decreased effectiveness of cutaneous vasodilation. Inaddition, the disappeared natural convection due to reduced gravity and low air flow in thecabin of space vehicles may limit heat loss capacity of body，which may further disturbethermal balance and thermal comfort of astronauts. These changes in turn may have negative influence on physical and psychological health, daily life, routine work, andvarious spaceflight activities such as extravehicular activity （EVA）, landinging, and egress.Therefore, it is important to maintain constant body temperature and thermal comfort ofastronauts in microgravity, especially in long-term spaceflight missioin. At present, the airflow pattern of forced ventilation through the use of fans or blowers in manned spacecraftis constant air flow （CAF）.A research report of Mir Space Station pointed out that microgravity impairs naturalconvection and disturbs maintance of body temperature. The thermal environmentalevaluation of28long-term investigations during stayed on the space station showed thatpoorly controlled thermal environment is common in the crew module. Moreover,increased body temperature occurred at various degrees in all crew members during the23th investigation mission （maximum body temperature was37.5°C） due to high cabintemperature. True or simulated natural air flow （SNAF） has been found to increase coldsensation and improve acceptability significantly compared to CAF. Therefore, it isnecessary to explore the effects of SNAF on heat loss and maintenance of coretemperature under microgravity environment.The prerequisite for maintainance and adjustment of thermal comfort duringspaceflight is objectively evaluating the human comfort status of the thermal environment.The thermal comfort is a psychological perception of people based on physiologicalthermoregulation mechanisms when human is exposed to a combination of variousenvironmental factors. With the automatic thermoregulation activity, changes take place insubjective thermal reaction.So far, the relationship between subjective response andphysiological variables was not yet fully established. Thermal comfort evaluation forastronauts during spaceflight has been implemented mainly by investigating thermalsensation and comfort degree by subjective questionnaire survey. The evaluation resultsare not accurate and timely enough, due to space psychological factors, individual hidingfactor, communication between space and ground. Hence, it is important to establish thecombination of subjective and objective evalulation to improve ambient thermalenvironment of astronauts through accurate grasp of the individual comfort and health status using questionnaire and physiological information monitoring resources duringspaceflight activities including EVA. We need explore the effective correspondencebetween subjective thermal response and sensitive thermal physiological variables, andthen establish combining physiological and subjective indicators.Under the same thermal environmental conditions, people’s thermal comfort are notconsistent due to individual factors such as gender, living area, habits and lifestyle, basalenergy metabolism rate, etc. Energy metabolic rate （EMR） is the body’s heat productionper unit time. EMR not only is important individual parameter of the thermal comfortevaluation model and the heat balance equation, but also is a considerable engineeringdesign reference of thermal environmental control system of manned spacecraft. Inconsequence, it is necessary to accurately understand the individual EMR of the astronautsfor thermal environmental assessment and thermal comfort studies in spaceflight.Thus, the purpose of this thesis is following:1) to determine whether SNAF canpreserve core temperature and therml comfort more effectively than CAF at a similarmean air velocity a during head down bed-rest （HDBR）;2) to explore the relationshipbetween subjective thermal response and physiological index under airtight climatechamber, and to recommend sensitive physiological indicators which can reflect thermalcomfort during spaceflight;3) to elevate the accuracy of EMR estimation undermicrogravity and EVA condition by establishing DLW method during HDBR and byimpoving the Heart rate （HR） prediction method, respectively.The following methods will be used in our experiments:First of all, we adopted30d-6°HDBR to simulate the physiological influences ofweightlessness. SNAF was used to countermeasure the thermoregulation dysfunction.During3days before HDBR （pre-HDBR） and the29th day of HDBR （HDBR29）, sevenhealthy males were exposed to three air flow patterns at23°C while in a supine posture: astill air environment as the control state （CON, mean air velocity <0.05m/s,50minutes）, SNAF （=0.2m/s,30minutes） and CAF （=0.2m/s,30minutes）. Thermalsensation vote （TSV）, rectal temperature （Tre）, skin temperature （Tsk）, and cutaneousvascular conductance （CVC） were measured during the sessions to analyze the effects ofSNAF on maintenance of constant core temperature and thermal comfort after HDBR. And then, we focused on the the relationship between subjective thermal responseand physiological index in an airtight environment. A total of15young men exposed tosix ambient temperatures in airtight climate chamber for60min, namely34°C,31°C,28°C,26°C,23°C and20°C. The physiological variables including Tsk, Tre, EMR and heartrate variability （HRV） were measured during test. The subjective response variablesincluding （TSV） and thermal comfort vote （TCV） were also measured. Linear correlationanalysis was used to determine the correlative degree between subjective response andphysiological index.Finally, we improved the accuracy of EMR assessment during spaceflight byestablishing DLW method during HDBR and by impoving the HR prediction method.1)21healthy males underwent30days of-6°HDBR. They were randomly assigned to threegroups according DLW taken method:15d measured period using routine dosage method（group A）,20d measured period using additional taken method （group B） and using1.5times routine dosage method （group C）. We analyzed the accuracy of the EMR calculatedby the three methods. Then, three subjects （2males and1female） underwent13dayswork and rest according real spaceflight task in airtight combinant experimental cabin.Oxygen consumption method and DLW method were used to estimate the mean EMR ofthe subjects, especially. The estimated results were also compared to further determin theefficacy and accuracy of DLW method;2)The EMR and Heart rate （HR） data of ten youngmales were collected during exercise by modes of treadmill, upper limb cycle ergometerand lower limb cycle ergometer, respectively. The individual linear regression betweenHR and EMR in various modes of exercise and in whole modes of exercise wereestablished. Then, the accuracy and error of two kinds of statistical analysis methods werecompared by the replication experiments.The major findings of the present study were as follows:1. Effects of simulated natural air movement on thermoregulatory response duringHDBR1) The subjects’ resting axillary temperature （Taxil） significantly increased with thetime extension of HDBR and reached a significant difference after day15of HDBR （P < 0.05）, and the Taxilincreased by0.37°C on the20th day. No significant difference wasfound on HR, oxygen consumption at rest, systemic blood pressure （SBP） and diastolicblood pressure （DBP） during29days of HDBR （P>0.05）. After HDBR, subjects’averageweight was decreased about1.7kg （P <0.01）; The water intake decreased significantly onthe first day of HDBR （P <0.01）, although the urine output did not changed.2) Treincreased by an average of0.18°C after29days of HDBR （P <0.05）.Compared to Trein CON pre-HDBR, the value in CAF was still increased significantly onHDBR29. However, no difference was found between Trein SNAF on HDBR29and thatin CON pre-HDBR （P>0.05）. This suggests core temperature under SNAF closest to thatof the control pre-HDBR.3) Turbulence intensity （Tu） in SNAF was higher than that in CAF. The greater Tredrop under SNAF may be the main reason that Tu affects the convective heat transferbetween the subjects and the environment.4) No significant difference was found for mean Tsk（） in CON betweenpre-HDBR and on HDBR29. Compared to and CVC values in CAF or CON, thevalues were significantly lower in SNAF on HDBR29（P <0.05）.5) There was no significant difference for TSV in CON between pre-HDBR and onHDBR29. Moreover, there were no statistical differences in TSV between CON, SNAF,and CAF pre-HDBR. Compared to TSV in CON and CAF, the value in SNAF on HDBR29decreased significantly （P <0.05）. All TSV under the three air flow patterns werewithin a range of “slightly cool” to “neutral”.2. The physiological research for thermal comfort response in airtight environment1) The relationship between ambient temperature and TSV shows an increasingtrend （R2=0.99）. The neutral air temperature which subjects felt thermal neutral insedentary activity was25.78°C. The TCV value shows inverted “U” curves with the riseof environmental temperature, and the cureve peak corresponded to the ambienttemperature was around26°C;2) The environmental temperature seems to have significant effects on Tskand HRV,and to have little effect on Tre, forehead-foot temperature gradient and EMR. Compared with the skin temperature of trunk, the value of extremities had the significant changewhen ambient temperature varies from20°C to34°C. The multiple comparisons indicatethat Tskof forearm and upper limb were more easily influenced by ambient temperature;3) With the environmental temperature rising, the High frequency （HFnorm） power ofHRV would decrease, the Low frequency （LFnorm） power and LF/HF ratio of HRV wouldincrease, and with the ambient temperature rose.The minimum human body EMR was inthermal neutrality ambient;4) Correlation analysis shows that Tsk, HRV, EMR and Trewas related to TSV （P <0.05）. Furthermore, the multiple comparisons indicate that the and Tskof upper limband forearm were most sensitive to thermal sensation （r>0.82）. The that subjects feltcomfort was33.4°C during sedentary activity.3. The research of estimate methods for energy metabolism in thermal comfort condition1) All of the estimated EMR variables, including the isotope abundance values, totalbody water values, isotope elimination coefficients and correlation coefficients of18O and2H, were consistent with the quality control required range in three taken methods （groupA, group B and group C）. The mean EMR of the subjects was between （433.3±79.2） kJ/h^{512.5±29.2} kJ/h used by DLW method during HDBR.2) The subjects’ mean EMR value that evaluated by oxygen consumption method inairtight cabin was417.36kJ/h during13days of whole simulated spaceflight period. Itwas close to the results used by DLW method （408.89kJ/h）, and the activity level of thesubjects was between rest and relaxed activity.3) There were significant differences in HR-EMR relationship among separatemodes of exercise. At same exercise loads, the HR value was lowest in treadmill, and washighest in upper limb cycle ergometer. And for EMR estimates, compared with the wayregardless of the exercise modes, the relationship between HR and EMR was closer（r=0.97） and the estimated error was lower （P <0.05） by using the statistics method ofseparate modes exercise sample.This study got following conclusions:1) Microgravity simulated by HDBR can result in higher Treat rest, which appeared to be related to decreased heat dissipation due to altered Skin blood flow （SkBF）.2) SNAF might more effective than CAF at preserving core temperature in athermoneutral ambient environment during HDBR; thermal sensation was close toneutrality under SNAF. The mechanism by which simulated natural air flow preservescore temperature may be related to the increase of convective heat loss due to turbulenceintensity.3) The upper limb Tskforearm Tskand among the physiological variables werethe closest related to subjective thermal response in airtight chamber, and can be proposedas indicator to evaluate the thermal comfort of astronaut in spaceflight.4) For the first time in China, to our knowledge, we established the DLW methodwhich can correctly estimate the human EMR during simulated microgravity. Moreover,routine dosage method can be used to EMR evaluation of15days of spaceflight.Comparing with additional taken method,1.5times routine dosage method would beconvenient for EMR evaluation of20days of spaceflight.5) The estimation of EMR is more accurate if modes of exercise are used in HRprediction instead of only individual factor, which would decrease the statistical error andincrease accuracy of HR-EMR evaluation for astronaut during EVA or exercise duringspaceflight.