Hanns-Christian Gunga, Alexander Stahn and Ines Langer:
Global warming, growing incidence of extreme climatic conditions, and rising population densities in metropolitan areas lead to considerable increases in risk for human health. Thus, there is increasing interest in exploring the frequency of extreme temperature situations in urban agglomerations and its consequences on health, with the ultimate aim to develop multi-factorial preventive strategies for reducing climate-related increases in morbidity and mortality. Increased rates of morbidity and mortality during heat waves can be found especially in risk-groups such as children in their first year of life, bedfast patients, drug addicts, alcoholics, and diabetics, immobile and homeless persons. Preventive measures demand an awareness of the problems, not only in the population, but also in many responsible institutions, and especially in the group of medical professionals and social services. A change in lifestyle is required in the group of people mentioned above including dietary measures, sufficient fluid intake and regulation of room climate. To gain a better understanding of heat waves on human health we suggest developing a non-invasive technology for collecting significant physiologic information on the impact of heat stress on people in metropolitan areas.
The idea of the present research was therefore to refine a non-invasive technology for monitoring heat stress in the field. Ultimately, such technology could be used to build up a data base which will support the understanding of the relationship between extreme climate conditions and health risks. Moreover, this data could be used to develop a computer-model as an early warning system for predicting urban population- and region-specific human health risks of future extreme climate conditions situations and provide architectural assistance and support in future city developments to optimize the protection of the population from climate hazards.
In recent years, heat waves were found to commence earlier in the year and to last longer, locally accompanied by heavy rain and varying levels of humidity. These temperatures exceed the human comfort interval, and must be looked upon as extreme strain, since the body’s heat-defensive system is constantly active. Full recovery during sleep is almost impossible under these circumstances, causing significant increases in morbidity and mortality. In urban agglomerations, these extreme climates such as heat waves strike a population that is growing older, has impaired levels of fitness, and suffers increasingly from regarding diseases such as diabetes, cardiovascular diseases and dementia. Furthermore, the lifestyle of remarkable fractions of the population with daily excessive television consumption of several hours is further jeopardizing the body’s defensive mechanisms. Cardiopulmonary exercise and training of the thermoregulatory mechanisms supporting the adaptation to extreme temperatures are missing. Another severe factor is the exponentially increasing level of obesity and adiposity in the population. Furthermore, due to cardiovascular diseases, patients are forced to take medications that constrain the body’s defensive mechanisms against heat. Furthermore, individuals of the aging population living alone, currently about 30% to 40% of the population in German cities are also considered a risk group. These people are frequently older persons who are not considering themselves as at risk, and consequently do not take any precautions. They are frequently unable to maintain ap- propriate behavior to extreme climates. This consideration is of utmost importance, as the behavior of hu- mans determines 90% of their thermoregulation. The body’s own defensive mechanisms are of high importance though, but they cannot compensate the substantial consequences owed to deficits in behavior. While the link the statistical link between heat stress and health risks has been well documented, a better understanding between climate conditions and health risks is clearly needed.
Present technologies for human core body temperature and heat stress monitoring such as inserting a thermo sensor in the esophagus, nasopharynx, rectum, or tympanum/auditory meatus are apparently neither clinically acceptable nor practicable for exploring thermoregulatory responses as a function of climate conditions. Recently, we therefore presented a new non-invasive method called Double Sensor located at the forehead, combining a skin surface temperature sensor with a heat flux sensor, to achieve this goal under various environmental conditions. Based on this experience the purpose of the present study was to explore the potential of the Double Sensor technology for monitoring core body temperature, heat stress and circadian rhythm in humans outside the laboratory during daily life. Specifically, the aim was to assess the feasibility of the approach to simultaneously monitor core body temperature (Double Sensor technology), heart rate (clas- sical ECG as well as chest strap transmitter) and environmental conditions (ambient temperature, humidity, and atmospheric pressure) for 36 hours using a mobile monitoring system.
To investigate human populations-specific thermoregulatory responses as function of climate conditions in the field, a mobile technology for non-invasive and continuous core temperature measurements and heat stress was refined and tested in young healthy subjects.
The mobile technology assessed continuous core body temperature using heat flux measurements (Double Sensor) at the head, heart rate and environmental temperature, humidity and air pressure.
Data were collected using a portable physiological monitoring system (HealthLab, Koralewski Industrie- Elektronik, Hambühren) in 11 young, healthy subjects (4 women and 7 men, aged 19 to 27) during a standardized activity protocol for 24 to 36 hours. The miniaturized system consists of a central processor for data storage and communication with a PC via Bluetooth (50 g, 46 x 85 x 16 mm) and two amplifiers (50 g, 46 x 85 x 16 mm), one for detecting the physiological data (heart rate via telemetry and heat flux via cable), and one for detecting the environmental conditions. The entire hardware setup is battery-operated and can be comfortably worn by people for extended time periods. Subjects were equipped and monitored with the technology for 24 h to 36 h.
Initial results showed that heart rate data obtained by three-lead electrocardiography was prone to artifacts and poor subject discomfort. This approach was therefore replaced by a chest strap transmitter, allowing a wireless data recording and storage. This setup improved both data quality for heart rate as well as subject compliance. While all data sets confirmed the feasibility of the technique, it was also found that sudden changes of environmental conditions such as wind speed or large ambient temperature drops can substantially impact heat flux and therefore also jeopardize Double Sensor temperature recordings.
It was concluded that future research is necessary to reduce the impact of varying environmental conditions outside the clinical and laboratory setting on Double Sensor temperature recordings. It is suggested that the full potential of the technique might be revealed by refining the external shielding and protection of the Double Sensor from environmental effects and eventually integrate the data into present telecommunication systems. First pilot tests have been performed using an additional foam ring that is imply placed onto the sensor and preliminary results seem to be promising, indicating smaller effects of ambient cold conditions on heat flux, and hence, core body temperature recordings. However, these results have to be tested in larger and longer studies under various environmental conditions and different activities.
Once the feasibility of the technology is confirmed, it could be used to provide the basis for the development of a prediction system for determining the impact of heat stress on humans. This would promote the genera- tion of human heat stress data in representative samples in diverse urban areas during different environmental conditions and provide the basis to link research on climate change and health to other research disci- plines such as meteorology, informatics, architecture, and engineering. These collaborative efforts could be used to develop computer-models for predicting urban population- and region-specific human health risks of future extreme climate conditions, provide continuous support regarding preventive strategies for reducing climate-related morbidity and mortality, develop individual mobile health risk monitoring systems, and assist global and specific urban planning and architectural development.
In addition, this technology could also be useful in promoting the development of human models of thermo physiology. A promising approach might be to combine this technology with infrared thermography and whole-body scanning facilities (stereo photogrammetry), which could yield new data regarding the location of a ‘single’ core temperature in the body and their heat distributions.
Finally, irrespective of the effect of changing ambient temperatures on discrete core body temperature measurements, Double Sensor temperature profiles reflected the circadian rhythm of the subjects very well. This confirms our previous findings in laboratory and clinical settings showing that in spite of devia- tions of up to about ±1 ◦C compared to rectal temperature measurements the Double Sensor technology seems to be a valid, non-invasive approach for monitoring circadian rhythm.
Due to the important of role of the circadian rhythm on sleep and physical and mental performance, it is suggested that in addition to classical non-invasive measures for circadian rhythm, such as dim light onset of melatonin in saliva samples, this approach could contribute to the understanding of circadian rhythm in humans in the field, i.e. under conditions outside typical clinical and laboratory settings.