The human body reacts to exposure to – and generation of – heat by activating various system responses. The nervous, cardiovascular, respiratory, and exocrine systems are key players in the physiological behavior of workers subject to heat stress. Effective thermoregulation requires that these systems operate in highly-interconnected ways.
This installment of the “Thermal Work Environments” series provides an overview of the human body’s thermoregulatory functions that are activated by heat stress and introduces the heat balance equation. Each component of the heat balance equation is described in terms of physiological and environmental factors that impact thermoregulation.
Core body temperature is regulated by the hypothalamus, located at the base of the brain (see Exhibit 1); its functions are divided between two areas. The anterior hypothalamus manages heat-dissipative functions, such as vasodilation and sweat production.
Vasodilation results in increased blood flow to the outer regions of the body, transferring heat from the core to the skin. A corresponding rise in heart rate increases the rate of heat transfer from the core to extremities.
Rising skin temperature prompts sweat production. Evaporation of sweat from the skin is the largest contributor to heat loss from the body; improving its efficiency is a common goal in hot environments. It is also the reason that proper hydration is critical to maintaining well-being in a hot environment.
Respiration also contributes to heat loss, as inhaled air is warmed by the body before being expelled. That is until the ambient temperature reaches or exceeds that of the body, at which time, it begins to increase heat stress. Respiration plays a lesser role in humans than in other animals. Dogs, for example, pant to increase respiratory heat loss; it is a larger contributor for them, relative to other mechanisms, than for humans.
These are the primary control mechanisms that act in concert to regulate core body temperature. These controls are activated automatically, often without our conscious awareness. Other responses to heat stress require active engagement, such as monitoring physical and environmental conditions, adjusting clothing and equipment, and developing work-rest cycles and contingency plans. These factors are relevant to the pursuit of heat balance.
Homeothermy requires a balance between the heat generated or absorbed by the body and that which is dissipated from the body. In “perfect” equilibrium, the net heat gain is zero. Zero heat gain implies that the body’s thermoregulatory response functions (i.e. heat strain) are sufficient to maintain a constant core temperature in the presence of heat stress.
As mentioned in Part 1, heat stress and heat strain are quantifiable, typically presented in the form of a heat balance equation. The form of heat balance equation used here 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 hot environments.
The “perfect” equilibrium mentioned above and, thus, constant core temperature is achieved when S = 0. This situation is more hypothetical than realistic however. Fluctuations of body temperature occur naturally and, within limits, are no cause for concern. For example, a person’s core temperature varies according to his/her circadian or diurnal rhythm. Despite a range of up to 3°F (1.7°C), these fluctuations go largely unnoticed.
The average “normal” temperature, 98.6°F (37.0°C), is cited frequently. Less common, however, is discussion of a range of “safe” temperatures. Most people maintain normal physiological function in the 97 – 102°F (36.1 – 38.9°C) range of core temperature. It is also worth noting that these values refer to oral temperature; rectal temperatures are usually ~ 1°F (0.6°C) higher. While rectal temperature is a more accurate measure of core temperature, the limitations on its use in most settings should be obvious.
Because the human body is sufficiently resilient to accommodate significant temperature fluctuations, S = 0 can be treated as a target average. Heat storage in the body (S) will vary as the body’s thermoregulatory control “decisions” are executed. Heat storage can become dangerous when S > 0 for an extended period, trends upward, or becomes exceptionally high.
The metabolic rate (M) is the rate at which the body generates heat, corresponding to work demands and oxygen consumption. Precise measurements are typically limited to research settings; workplace assessments of heat stress typically use estimates or “representative values.” Exhibit 2 provides a guide for selecting a representative metabolic rate for various scenarios.
M is always positive (M > 0), representing thermal energy that must be dissipated in order to maintain a constant core temperature. It may also be called the “heat of metabolism;” heat is generated by chemical reactions in the body, even in the absence of physical work.
A number of factors can effect a person’s metabolic rate. Several are presented, in brief, below:
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.
Heat dissipation via convection (C) begins with the circulatory system. Heat from the body’s core and muscles is transferred to the skin, preventing “hot spots” that could damage organs or other tissue. From the skin, heat is transferred to the surrounding air (C is negative), assuming the ambient temperature is lower than the skin temperature. If the reverse is true, C becomes positive, adding to the body’s heat load.
The radiation (R) term refers, specifically, to infrared radiation exchanged between the body and nearby solid objects. The skin acts as a nearly-perfect black body; that is, it efficiently absorbs (positive R) and emits (negative R) infrared radiation. Like convection, the sign (direction) of radiative heat transfer depends on the temperature of the skin relative to that of the surroundings. A person’s complexion has no effect on infrared radiation or radiative heat transfer.
Heat transfer by conduction (K) is not common in workplaces, as it requires direct contact with a solid object. Where it does exist, it is often highly localized and transient, such as in the hands during manual manipulation of an object. It is positive when touching a hot object and negative when touching a cold one. Contact with objects made through clothing is considered “direct contact” for purposes of heat stress assessment.
In this formulation of the heat balance equation, the evaporation (E) term captures the cooling effect of sweat evaporating from the skin. In mild conditions, the amount of sweat produced may be imperceptible, but it is not insignificant. This “insensible water loss” can approach 1 qt (0.9 L) per day, dissipating ~25% of basal heat production. During strenuous physical activity, the body can produce more than 3.2 qt (3 L) of sweat in one hour.
This component is often called evaporative cooling; as this term implies, E is always negative. Several physical and environmental conditions place limitations on the capacity of evaporative cooling. Proper hydration is necessary to sustain the high sweat rates that produce maximum cooling. Clothing and protective gear may limit the interface area available or the efficiency of evaporation.
Ambient conditions significantly impact the body’s ability to cool itself via evaporation. As humidity increases, the rate of evaporative cooling decreases. Increasing air speed enhances evaporation, though no additional benefit is gained at speeds above ~6.7 mph (3 m/s) or air temperature above 104°F (40°C). When air temperature exceeds skin temperature, low humidity is needed for evaporation to compensate for convective heat gain to maintain a net heat loss. In favorable conditions, evaporative cooling is the single greatest contributor to heat loss from the body.
Heat loss due to respiration (Resp) may be difficult to quantify. In many formulations of the heat balance equation, it is included in the evaporation (E) term, as the largest contribution comes from expelling water vapor. There is also heating of the air while in the lungs, though it may be a relatively small heat transfer.
Resp is usually negative, but could become positive in very high air temperatures. Such conditions are not common in workplaces, as this type of environment is often deemed unsafe for various reasons. In most cases, access to such an environment is restricted to individuals with protective gear, such as breathing apparatus, limited in duration, and closely monitored.
Though the respiration component may be difficult to quantify, independent of other mechanisms of heat loss, inclusion of the Resp term in this discussion is useful for practical purposes. Managing heat and fluid loss in a hot environment is aided by simply recognizing that respiration makes a contribution, even if its magnitude is unknown. The direction of heat transfer is usually understood intuitively; this may be the only information needed for workers to take additional precautions to ensure their well-being.
The body’s heat balance is pictorially represented as a mechanical balance scale in Exhibit 3. It presents factors that increase and decrease core temperature as well as the “normal” range of variation throughout the day. A visual reference can be a useful tool, as it is more intuitive than a written equation, promoting deeper understanding that aids practical application of information.
For additional guidance or assistance with 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).
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[Link] “Thermal Indices and Thermophysiological Modeling for Heat Stress.” George Havenith and Dusan Fiala. Comprehensive Physiology; January 2016.
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[Link] “NIOSH Criteria for a Recommended Standard Occupational Exposure to Heat and Hot Environments.” Brenda Jacklitsch, et al. National Institute for Occupational Safety and Health (Publication 2016-106); February 2016.
Jody W. Phelps, MSc, PMP®, MBA
JayWink Solutions, LLC
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