Many of the human body’s responses to cold mirror those initiated by exposure to heat. Others are unique physiological mechanisms engaged to pursue diametrically-opposed objectives. The risks associated with cold stress are very different from those of heat stress, requiring unique forms of strain for proper and effective management.
This installment parallels Part 2 of the “Thermal Work Environments” series, providing an overview of thermoregulatory functions activated by cold stress. The heat balance equation is also revisited, discussing each term in the context of cold environments. These two installments are “companion pieces;” each can stand alone, but are most helpful when reviewed in conjunction.
The hypothalamus (see Exhibit 1) is responsible for regulation of core body temperature. Heat-retention functions, such as vasoconstriction and shivering, are managed by the posterior hypothalamus.
Vasoconstriction reduces blood flow to the outer regions of the body; heat is retained in the body’s core. As core temperature drops, the heart rate decreases to further limit heat loss.
Shivering is an involuntary response, engaged to offset heat loss. A person can postpone the onset of shivering through force of will, but doing so could exacerbate an already-dangerous situation. The onset of shivering should be treated as a warning; a warm refuge should be sought.
The objective of homeothermy requires viewing the body’s heat balance from different perspectives. Heat stress requires a focus on the management (i.e. minimization) of heat gain. Cold stress, in contrast, requires a focus on heat loss which adds a layer of risk not present during heat stress.
When experiencing heat stress, the temperature of a person’s extremities cannot be driven higher than his/her core temperature by thermoregulatory means. Internal, involuntary functions are activated to transfer heat from the core to the extremities and the surroundings; heat transfer ceases when temperatures equalize.
In contrast, cold stress can result in large temperature differentials between the core and the extremities. As the hypothalamus activates thermoregulatory functions to maintain core temperature, the extremities are actively neglected. The human body will sacrifice its appendages for the survival of the being.
Therefore, thermoregulation and heat balance, in the context of cold environments, must be considered in two stages: (1) retention of full physical and cognitive function, and (2) survival of the being, i.e. maintenance of core temperature at all costs. These two objectives have very different thermal requirements.
The second stage represents emergency situations, where circumstances are beyond control. This presentation focuses on the first stage, as it is more-frequently applicable to occupational scenarios. Successful management of heat balance in the first stage precludes the emergency responses of the second.
The form of heat balance equation used in this series is
S = M + W + C + R + K + E + Resp,
where S is heat storage rate, M is metabolic rate, W is work rate, C is convective heat transfer (convection), R is radiative heat transfer (radiation), K is conductive heat transfer (conduction), E is evaporative cooling (evaporation), and Resp is heat transfer due to respiration. Each value is positive (+) when the body gains thermal energy (“heat gain”) and negative (-) when thermal energy is dissipated to the surrounding environment (“heat loss”). Each term can be expressed in any unit of energy or, if time is accounted for, power, but consistency must be maintained. The following discussion provides some detail on each component of the heat balance equation in the context of cold environments, contrasting with that of hot environments where it improves clarity.
As discussed in Part 2, the hypothetical “perfect” equilibrium is attained at S = 0. In a cold environment, two equilibria are possible, corresponding to the stages referenced above. Here, S = 0 is the target average to maintain full function. A person can suffer a loss of function when S < 0 for an extended period, trends downward, or becomes exceptionally low.
The average “normal” core temperature is 98.6° F (37° C). Ideally, the drop in core temperature will be limited to 97° F (36° C), the temperature below which most people suffer some loss of physiological function. Increasing core temperature significantly above normal can also be hazardous if sweating is induced (discussed further below). Oral temperatures in the 97 –99° F (36 – 37° C) range are safe for most people in most situations.
The rate at which the body generates heat, M, varies with activity. Precise values are difficult to obtain; representative estimates, such as those shown in Exhibit 2, are usually used instead. The value of M when a person is at rest under normal conditions is called the basal metabolic rate (BMR).
A person’s size and weight, growth stage, diet, fitness, and drug use can affect his/her metabolic rate. A young, growing, physically-fit person has a higher BMR than a comparably-sized older adult. A lower BMR must be compensated for by other means to maintain heat balance in a cold environment.
When the combination of BMR and activity are insufficient, shivering may be induced, adding as much as 400W to a person’s metabolic rate. Shivering is an involuntary, uncoordinated activation of skeletal muscles with the sole purpose of heat production. It reduces further heat loss, but cannot generate sufficient heat to replace that already lost (i.e. raise body temperature).
The work rate (W) represents the portion of energy consumed in the performance of work that is not converted to heat. Many formulations of the heat balance equation exclude this negative (heat-reducing) term, deeming it safe to ignore, as it is usually less than 10% of M. However, in critical situations, the work rate may gain significance.
By definition, the air temperature in a cold environment is below safe body temperature. Therefore, the convection (C) component is negative. In a cold environment with significant air movement, convection causes the greatest heat loss. Convective heat loss is combatted with insulating layers of clothing. The most effective combination of clothing layers depends on several personal, environmental, and behavioral variables. Choices must be made among breathable, waterproof, and zippered garments, boots, hats, gloves, and other options to match their performance with the environmental conditions and the work to be performed. Any exposed skin increases convective heat loss.
The radiation (R) term will also be negative in most cases. In open air, in direct sunlight, for example, significant radiation may be received. However, the insulating layers protecting against other forms of heat loss will likely prevent an appreciable heat gain. In the vast majority of situations, it is those insulating layers that prevent radiative heat loss. In the absence of insulation (i.e. clothing) and appreciable air movement, radiation causes the greatest heat loss.
Heat loss via conduction (K) is typically associated with handling of tools and materials and walking on cold surfaces. Gloves and boots appropriate for the type of work being performed should be worn to minimize this. If dexterity requirements prohibit the use of gloves, appropriate work/warming cycles must be implemented to prevent loss of dexterity and cold injury. Even when conductive heat loss is small relative to total heat loss, cold hands and feet can have a disproportionate effect on comfort and performance.
Evaporation (E) remains a significant source of heat loss. In cold stress situations, the goal is to minimize evaporative cooling; it cannot be eliminated and must be managed. “Insensible water loss” continues in cold conditions; this moisture must be dissipated to avoid wetting clothing and degrading its insulating properties. The challenge becomes greater if strenuous activity induces sweating. Rapid heat loss due to the combined effect of reduced insulation and excessive evaporation could lower one’s body temperature to an uncomfortable, if not dangerous, level. While the humidity in the surrounding air has a profound impact on evaporative cooling under conditions of heat stress, it plays no appreciable role in cold environments.
Heat loss due to respiration (Resp) also requires management. For example, a face covering that allows exhaled water vapor to dissipate while retaining some of the heat transferred to the air while in the lungs could be a helpful complement to a worker’s wardrobe. In addition to any respiratory heat that may be recaptured, the heat loss due to convection and radiation are also lowered by the reduced skin exposure. Perceived comfort may also improve significantly by warming the nose, cheeks, and ears.
The analogous representation of the human body’s heat balance as a mechanical balance scale, as shown in Exhibit 3, continues to be a helpful visualization. It is quite intuitive and identification of the diurnal range on the scale provides an important reminder that some variation in core temperature is normal and unrelated to our thermal work environments.
For additional guidance or assistance with Safety, Health, and Environmental (SHE) issues, or other Operations challenges, feel free to leave a comment, contact JayWink Solutions, or schedule an appointment.
For a directory of “Thermal Work Environments” entries on “The Third Degree,” see Part 1: An Introduction to Biometeorology and Job Design (17May2023).
[Link] Human Factors in Technology. Edward Bennett, James Degan, Joseph Spiegel (eds). McGraw-Hill Book Company, Inc., 1963.
[Link] Kodak's Ergonomic Design for People at Work. The Eastman Kodak Company (ed). John Wiley & Sons, Inc., 2004.
[Link] “Hypothalamus” in Encyclopedia of Neuroscience. Qian Gao and Tamas Horvath. Springer, Berlin, Heidelberg; 2009.
[Link] “Thermal Indices and Thermophysiological Modeling for Heat Stress.” George Havenith and Dusan Fiala. Comprehensive Physiology; January 2016.
[Link] “Hypothermia and Cold Weather Injuries.” Rick Curtis. Princeton University Outdoor Action Program, 1995.
Jody W. Phelps, MSc, PMP®, MBA
JayWink Solutions, LLC
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