People survive longer in severe cold if they wear clothing and outerwear that are specially designed for cold-weather conditions. Knowing the amount of a time a person can survive for a set of environmental conditions is critical for developing new fabrics and clothing that extend survival time.
To improve the design of advanced, cold-weather fabrics, researchers at the Department of Mechanical Engineering at the Indian Institute of Technology in New Delhi undertook a series of experiments to evaluate how three key factors—ambient temperature, fabric insulation, and metabolic heat (heat generated by a person as a function of the physical work performed)—impact survival time. Their research is described in a paper published by the Journal of Heat and Mass Transfer in July 2023.
Testing ambient temperature, fabric insulation, and metabolic heat
Previous studies have shown that if metabolic heat generation is lower than the heat loss to the environment, the human body core temperature starts to drop. If the body core temperature decreases below 27 °C, the chances of death become higher. “Therefore, the time taken to decrease body core temperature from 37 °C to 27 °C could be considered the survival time of a human,” stated mechanical engineers Vikrant Dupade, B. Premachandran, R.S. Rengasamy, and Prabal Talukdar in their research paper.
Extending survival time is the main goal when exploring new ways to design and test clothing for extreme cold-weather conditions. To better understand the influence of ambient temperature, fabric insulation, and metabolic heat on the loss of body heat, the research team designed an experiment to test the thermal effectiveness of fabric combinations in cold-weather conditions.
A guarded hot plate instrument was placed inside a cold chamber capable of maintaining the ambient temperature between 210 K and 310 K (-63 °C and 36 °C). A piece of layered artificial skin placed on the hot plate with same thermal diffusivity as human skin represented the human body core.
Fabric combinations that were placed on top of the artificial skin consisted of several nonwoven layers underneath an outer layer made of breathable material. The insulating layers consisted of knitted fabric (fabric insulation ¼ 0.53 clo [clothes thermal insulation]) and high loft nonwovens. Different thicknesses were used to vary the thermal insulation. The fabric ensemble was overlain by a windproof fabric. “This top layer shields the fabric ensemble from air, rain, and snow while transferring perspiration, in the form of vapor, to the atmosphere,” the research team wrote.
The team then performed experiments on a set of layers of extreme cold weather clothing using the guarded hot plate instrument inside the climatic chamber. Survival time was measured at three different subzero temperatures, three different fabric insulation combinations, and three metabolic heats:
- Ambient temperatures of 213 K (-60 °C), 233 K (-40 °C), and 253 K (-20 °C)
- Fabric insulations of 1.2 clo, 1.88 clo, and 2.56 clo (clothes thermal insulation)
- Metabolic activities of sleeping, sitting, and standing
"In the first set of experiments, the nonwoven with 4-mm thickness and a thermal insulation of 0.67 clo was placed over the knitted fabric with thermal insulation of 0.53 clo,” Dupade wrote. “This process was repeated for the other two fabric assemblies that had two and three layers of nonwoven, which had total insulation of 1.88 clo and 2.56 clo, respectively.”
Each set of experiments was performed three times. The transient temperature profiles at different locations of the fabric assembly were measured continuously. Further, a one-dimensional heat transfer model was developed to carry out the numerical simulation.
Results and conclusions
The team compared both the experimental and numerically simulated transient temperature profiles inside the fabric assembly for all possible combinations of ambient temperature, fabric insulation, and metabolic heat. They found that for ambient temperature, survival time increased by 204 percent for an increase in ambient temperature of 9.4 percent and by 744 percent for an increase in ambient temperature of 18.8 percent. Increasing fabric insulation by 56 percent increased survival time by 190 percent, while increasing insulation by 113 percent improved survival time by 512 percent.
Parametric analysis showed that the ambient temperature is the most influential parameter affecting the survival time, followed by fabric insulation. Metabolic heat did not have a significant effect on survival time.
The survival time for humans exposed to extreme cold-weather conditions increases significantly with an increase in ambient temperature and fabric thermal insulation. “Prior knowledge of the survival time for people exposed to extremely cold climatic conditions will be helpful in designing better cold weather clothing,” concluded the team.
Mark Crawford is a technology writer in Corrales, N.M.
