Thermal responses from repeated exposures to severe cold with intermittent warmer temperatures

Models and Indicators to Assess Thermal Sensation Under Steady-state and Transient Conditions

The assessment of thermal sensation is the first stage of many studies aimed at addressing thermal comfort and at establishing the related criteria used in indoor and outdoor environments. The study of thermal sensation requires suitable modelling of the human body, taking into account the factors that affect the physiological and psychological reactions that occur under different environmental conditions. These aspects are becoming more and more relevant in the present context in which thermal sensation and thermal comfort are represented as objectives or constraints in a wider range of problems referring to the living environment. This paper first considers the models of the human body used in steady-state and transient conditions. Starting from the conceptual formulations of the heat balance equations, this paper follows the evolution occurred during the years to refine the models. This evolution is also marked by the availability of increasingly higher computational capability that enabled the researchers developing transient models with a growing level of detail and accuracy, and by the validation of the models through experimental studies that exploit advanced technologies. The paper then provides an overview of the indicators used to characterise the local and overall thermal sensation, indicating the relations with local and overall thermal comfort.

Brief Rewarming Blunts Hypothermia-Induced Alterations in Sensation, Motor Drive and Cognition

Background: It is well known that cold exposure experienced during occupational or recreational activities may adversely affect motor, cognitive performance, and health. Most research has used prolonged passive external rewarming modalities and focused on the direct effects on the kinetics of physiological and psychological responses in hypothermic subjects. However, the brief whole body rewarming effects on physiological and psychological responses in parallel with functional consequences on cognitive and neurophysiological functions have not been investigated. This study explores these effects in 12 healthy young men.

Methods: Subjects (20 ± 1 years) participated in 4 randomized trials, which were designed to compare the effects of whole-body brief (5-min) rewarming in 37°C water with rewarming for the same duration in 24°C (air) thermoneutral environment in mildly hypothermic subjects. After each rewarming, indicators of neuromuscular function (reflexes, central activation ratio, electromyography of exercising muscle, and contractile properties of calf muscles) and cognitive function (attention, simple motor speed, and information processing speed) were assessed.

Results: Compared to rewarming in thermoneutral environment, after brief rewarming in 37°C water, significantly lower metabolic heat production (MHP) (206 ± 33.4 vs. 121.9 ± 24.3 W·m², P < 0.01), heart rate (76 ± 16 vs. 60 ± 12 b·min⁻¹, P < 0.01), cold strain (6.4 ± 3.1 vs. 5.3 ± 2.7, P < 0.01), improved thermal comfort and induced cessation of shivering were found. Electrically induced maximum torque amplitudes increased (P100, 102.8 ± 21.3 vs. 109.2 ± 17.5 Nm and PTT100, 83.1 ± 17.1 vs. 92.7 ± 16.0 Nm, P < 0.05), contraction half-relaxation time decreased (599.0 ± 53.8 vs. 589.0 ± 56.3 ms, P < 0.05), and M_max-wave latency shortened (17.5 ± 2.2 vs. 15.6 ± 2.0 ms, P < 0.05) after 37°C water rewarming. Unlike rewarming in thermoneutral environment, 37°C water rewarming blunted the hypothermia-induced alterations in neural drive transmission (4.3 ± 0.5 vs. 3.4 ± 0.8 mV H-reflex and 4.9 ± 0.2 vs. 4.4 ± 0.4 mV V-wave, P < 0.05), which increased central fatigue during a 2-min maximum load (P < 0.05). Furthermore, only in brief warm water rewarming cerebral alterations were restored to the control level and it was indicated by shortened reaction times (P < 0.05).

Conclusions: Brief rewarming in warm water rather than the same duration rewarming in thermoneutral environment blunted the hypothermia-induced alterations for sensation, motor drive, and cognition, despite the fact that rectal and deep muscle temperature remained lowered.

Thermal conditions in freezing chambers and prediction of the thermophysiological responses of workers

The present work is dedicated to the assessment of the cold thermal strain of human beings working within freezing chambers. To obtain the present results, both field measurements and a numerical procedure based on a modified version of the Stolwijk thermoregulation model were used. Eighteen freezing chambers were considered. A wide range of physical parameters of the cold stores, the workers clothing insulation, and the working and recovering periods were observed. The combination of these environmental and individual parameters lead to different levels of thermal stress, which were grouped under three categories. Some good practices were observed in the field evaluations, namely situations with appropriate level of clothing protection and limited duration of exposure to cold avoiding unacceptable level of hypothermia. However, the clothing ensembles normally used by the workers do not provide the minimum required insulation, which suggests the possibility of the whole body cooling for levels higher than admissible. The numerical predictions corroborate the main conclusions of the field survey. The results obtained with both methodologies clearly show that, for the low temperature of the freezing chambers, the clothing insulation is insufficient, the exposure periods are too long, and the recovering periods are inadequate. Thus, high levels of physiological strain can indeed be reached by human beings under such working environments.

The Use of Footwear Insulation Values Measured on a Thermal Foot Model

The use of physiological data from human tests in modelling should consider background data, such as activity, environmental factors and clothing insulation on the whole body. The present paper focuses on local thermal comfort of feet with special attention on the effects of physical changes of footwear thermal properties. An alternative test method is available for footwear thermal testing besides the standard method. The possibility to use insulation values acquired on a thermal foot model in practice is shown here. The paper describes the correlation between cold and pain sensations, and foot skin temperatures of the subjects and relates these to insulation measured on a thermal foot model. Recommendations are made for footwear choice according to environmental temperature.

Acute cold exposure and cognitive function: evidence for sustained impairment

Several industries experience periods of cold exposure and rewarming throughout the workday but mental performance under these conditions is unknown. A better understanding of cognition during the rewarming phase after cold exposure may help reduce accidents and improve performance. Ten young men (wearing ~0.1 clo) underwent three consecutive mornings trials where they were exposed to cold air (10°C) and then subsequently rewarmed (25°C air). A computerised test battery was administered during each stage of the protocol to determine working memory, choice reaction time, executive function and maze navigation. Rectal and skin temperature, oxygen consumption and thermal sensation were also measured throughout and showed a typical response. Relative to baseline performance, working memory, choice reaction time and executive function declined during exposure to 10°C, and these impairments persisted 60 min into the recovery period (i.e. once physiological parameters had returned to baseline). Further work is needed to develop countermeasures to this predicament.

PRACTITIONER SUMMARY

This study showed that working memory, choice reaction time and executive function declined during exposure to 10°C air, and these impairments persisted 60 min into the rewarming period (i.e. once measurable physiological parameters had returned to normal). Individuals may be at risk for injury after removal from a cold environment.

Protection of feet in cold exposure

The paper summarizes the research on cold protection of feet. There exist several conflicting requirements for the choice of the best suited footwear for cold exposure. These conflicts are related to various environmental factors, protection needs and user comfort issues. In order to reduce such conflicts and simplify the choice of proper footwear the paper suggests dividing the cold into specific ranges that are related to properties and state of water and its possibility to penetrate into, evaporate from or condensate in footwear. The thermo-physiological background and reactions in foot are briefly explained, and main problems and risks related to cold injuries, mechanical injuries and slipping discussed. Footwear thermal insulation is the most important factor for protection against cold. The issues related to measuring the insulation and the practical use of measured values are described, but also the effect of socks, and footwear size. Other means for reducing heat losses, such as PCM and electrical heating are touched. The most important variable that affects footwear thermal insulation and foot comfort is moisture in footwear. In combination with motion they may reduce insulation and thus protection against cold by 45%. The paper includes recommendations for better foot comfort in cold.

Work in Artificial Cold Environments

The physiological characteristics of work in cold stores, as a typical artificial cold environment, are reviewed mainly from our various field and experimental studies. There are about 4,000 cold stores in Japan, and 85% of them are kept at temperatures below -20 degrees C. Although the duration of cold exposure per stay in a cold store was very short, forklift workers entered the cold stores very frequently. Cold stress and the decrease in workers' performance were the same as for continuous exposure to cold. Since the peripheral skin temperature of subjects at night is higher than that in the afternoon, they are less likely to feel cold or pain sensation at night. However, there was a marked decrease in rectal temperature and in manual performance. There is an increased risk of both hypothermia and accidents for those who work at night. The cold store workers, however, had adapted to cold through daily repeated cold exposures.