Thermoregulation, or the maintenance of a fairly steady body temperature even under a variety of external conditions, is important to all animals because each species has a preferred body temperature at which functioning is optimal. These external conditions can include changes in temperature, vapor pressure, air velocity, and insulation among other factors that affect the temperature of the skin. Cold-blooded animals regulate their body temperature by selecting an appropriate external environment. Warm-blooded animals also rely on physiological mechanisms which can produce or dissipate heat [1].
Animals have evolved such that they can adapt to changes in their thermal environments, but thermalizing energy deposited in the body by exposure to RF electromagnetic radiation poses a different kind of problem. It is most similar to the action of skeletal muscle, which can also deposit large amounts of thermal energy into deeper tissues though) [1].

How does the body function under normal thermal circumstances?

Most body organs are most efficient at relatively constant temperatures, near 37°C (98.6°F). The normal range from 35.5 to 40°C ( 96°F to 104°F) allows for considerable variation induced by circadian variation, vigorous exercise, variations in ambient temperature, results of food intake, age factors, menstrual variation in women, and emotional factors. When body temperature is outside this range, it is an indication of a disease state, unusual activity, or extraordinary environmental conditions [1]., which can lead damage of body tissues.

There are two control systems for temperature regulation in endotherms (warm-blooded animals). The behavioral system involves conscious voluntary acts to adjust physical characteristics of the air-skin interface. An example of this would be moving out of the hot sun and into the shade, or putting on a sweater to keep warm. The physiological system consists of involuntary responses of the body that generate or dissipate heat. Sweating is a large component of the physiological system [1].

In a steady-state situation, the heat produced by the body is balanced by the heat lost to the environment. An equation for the body heat balance can be written as

M ± W = ± R ± C ± E ± S [W/m²]

where M is the rate at which thermal energy is produced by the body through metabolic processes, W is the rate of work produced by or on the body, R is the rate of radiant heat exchange with the surroundings, C is the rate of convective heat exchange with the surroundings, E is the rate of heat loss due to evaporation of body water, and S is the rate of heat storage in the body. Numerous studies have confirmed that in many species, an absorbed dose of microwave energy equivalent to the resting metabolic heat production, elevates the deep body temperature of the animal by 1 degree or more. S should ideally be close to zero in order to prevent body temperature changes [1].

What are the means of heat loss from the body?

Radiation, convection, and evaporation are means of heat loss that are directly related to the surface area of the body. Conduction is usually insignificant, but in combination with convection, is a very important form of heat loss. The rate of conduction is a function of body surface area, dry bulb air temperature, and the heat transfer coefficient, which also depends on the ambient air motion. A common everyday example of this is that wind makes a hot day feel much cooler because heat is removed from your body more efficiently.

Radiation heat transfer between two objects is independent of the dry bulb temperature and related to the difference in surface temperatures of the objects and the properties of their surfaces. Evaporation of water is a very important means of heat loss. The latent heat of evaporation of water at normal body temperature is 0.58 kcal/g, so that with the evaporation of each gram of water from the body surface, the body loses 0.58 kcal of heat. This water loss occurs mostly through sweating, but also through water that is breathed out or diffuses through the skin. The rate of evaporation depends on the relative humidity (RH) of the air, and only occurs when RH < 100% [1].

How much heat does the body produce?

The Basal Metabolic Rate (BMR) is defined as the heat production of a human in a thermoneutral environment (33°C or 91°F) at rest mentally and physically more than 12 hours after the last meal. The standard BMR for a 70 kg man is approximately 1.2 W/kg, but it can be altered by changes in active body mass, diet, endocrine levels. It is probably not affected by living in hot climates [1].

The range of endogenous heat production, M, dependent on the work being performed, age, sex, size, physical fitness, and level of activity, is about 40 to 800 W/m² (or 1-21 W/kg for a standard man) [1]. If deep body temperature is altered, either by heat storage from being in a warm environment, or by febrile disease (having a fever), then M changes as well. In cold environments, for example, shivering induced by the body can increase heat production up to four or five times the normal resting level. Further increase can be induced by exercise.

What is vasomotor control?

Vasomotor control is a fancy expression for the convective heat transfer that occurs via the circulatory system. In cold environments, vasoconstriction (constriction of the blood vessels) limits heat loss from the body core to the skin in peripheral vasculature, such as in the hands and feet. This is commonly experienced in the fact that hands and feet are the first to feel cold on a cold day. This is evidence of the body's attempt to retain heat in the body. Residual heat flow is reduced to 5-9 W/m² per °C difference between the body core and skin in the peripheral areas [1]. In thermoneutral environments, peripheral vessels are vasodilated (expanded) so that each liter of blood at 37°C that flows to skin surface and returns 1°C cooler, releases 1 kcal or 1.16W×hr of heat [1]. During vigorous exercise, peripheral blood flow can increase up to ten times, which is crucial in eliminating the increased metabolic heat produced in working muscles.

Tissue conductance refers to the combined effect of conduction though layers of muscle and fat, and convective heat transfer by the blood. In the cold, conductance is minimized to 6-9 W/m²/°C, and in heat, increases a lot.

What role does sweating play in thermoregulation?

Evaporative heat loss through sweating is a very efficient means for balancing metabolic heat production and heat absorbed from surroundings by radiofrequency radiation and convection. Secretion of sweat occurs when the ambient temperature rises above 30-31°c, and/or when internal body temperature rises above 37°C [1]. Those who are physically fit or used to warm environments, show a better response of the sweating mechanism in response to exercise as sweating begins at a lower internal body temperature.

Humans, some primates, and a few other species sweat. Cats and dogs pant to cool by evaporation of water, while other species such as rodents have no physiological mechanism of thermoregulation and hence rely only on behavioral responses [1].

How is thermoregulation controlled?

The physiological system for thermoregulation operates like an automatic control system that responds to negative feedback. The body temperature is regulated at a set reference temperature, and temperature sensors throughout the body respond to the central controller in the medial preoptic/anterior hypothalamic region of the brainstem, which then adjusts heat production and loss accordingly.

Thermosensitive neural tissue consists of specialized receptors in the outer layers which detect the temperature of the skin. Temperature changes in localized sites trigger behavioral as well as physiological thermoregulatory responses. Under ordinary environmental conditions, the body is quick to perceive temperature changes and trigger a response that tells the body, for example, to begin sweating or to move to a cooler environment. In the presence of radiofrequency electromagnetic radiation, however, heat is absorbed by tissues below the skin and is therefore not sensed by temperature sensors until a finite time has elapsed. Absorption patterns in the body can be complex and result in uneven heating. These problems are discussed further below.

What is the difference between heating from RF radiation and other (natural) sources?

The response from heating by RF radiation is actually not that different from heating from other sources. The basic challenge in dealing with excess heat in the body in general is whether the heat-loss capability of the body is good enough to prevent heat storage. A heat balance equation is given above.

The ratio of evaporation required to the maximum possible evaporation gives a measure of the percentage of the skin surface wet with sweat. The Heat Stress Index (HSI) is equal to this ratio and provides an indication of thermal comfort. When the HSI is less than 20%, the body is in a state of thermal comfort. For levels greater than 30% but less than 60%, heat levels are uncomfortable, interfere with concentration and fine motor performance, but are tolerable. When HSI > 60%, conditions are intolerable. When the rate of evaporation is lower than the rate required, the amount of heat storage (S) in the body as found from the heat balance equation can be used to predict tolerance times.

<25 kcal -- may not be noticeable if slow enough heating rate
>80 kcal -- voluntary tolerance time
160 kcal -- 50% risk of collapse
240 kcal -- intolerable [4].

RF radiation probably alters tolerance limits the same way as an equivalent metabolic load. Passive heating (diathermy) and active heating (exercise) causes the same effect on thermoregulation. Models are used to predict the amount of radiation (SAR) that will change temperature by so much under a given set of conditions.

One calculation done used the heat balance equation to predict the maximum whole-body SAR that a healthy man could tolerate for 60 minutes with thermoregulatory mechanisms functioning properly. At a dry bulb temperature of 40°C (104°F), and 80% relative humidity, tolerance was reached after 60 minutes with an SAR of 3.11W/kg and an incident power density of ~13mW/cm². The incident plane wave had E-polarization and was at the resonant frequency of 70MHz, ensuring maximal energy absorption [4]. At cellular and PCS frequencies, absorption would be much lower, and hence a much stronger power density would be required to reach maximum tolerance after one hour.

How can one avoid thermal discomfort in an RF radiation environment?

Clothing can alter the resistance to flow of heat from the skin to the environment. How well it acts as an insulation unit is a function of the thickness of the air layer trapped in the clothing. Avoidance of thermal discomfort in an environment with RF radiation can be achieved by minimizing the percentage of body surface covered with sweat by increasing ambient air movement, reducing ambient vapor pressure, and removing as much clothing as possible.

Is RF radiation similar to exercise?

Muscular exercise causes an internal disturbance which causes an offset in the regulated internal body temperature. At the beginning of exercise, heat production in skeletal muscles increases rapidly, causing a temporary large increase in heat storage. This drives heat rate until heat loss equals heat production and the body temperature equilibrates at a hyperthermic level, which will depend on the level of exercise.

Whole body absorption of thermalizing RF energy may have the same thermoregulation effects as exercise, as has been tested by Nielsen, who observed the same temperature increase in each case [1]. The distribution of heat in the body may be different, however. Convective heat transfer via blood flow is very important in distributing heat in both active and passive heating.

The major stimulus for increased heat dissipation during exercise was increased body temperature, as opposed to ambient temperature which controls heat loss via changes in skin temperature. The same is true for RF radiation. We thus may be able to predict the consequences of RF radiation by comparing it to exercise. Head deposited in the body by RF radiation is no different than that produced by the body itself.

Is RF radiation similar to having a fever?

While exercise produces an elevation in body temperature above the regulated normal level, fever increases the regulated body temperature itself, so the thermoregulation system acts in a manner consistent with this elevated set temperature. This was shown in an experiment in which the body temperature in normal and febrile patients went up the same amount with exercise but with respect to a higher base level. In general, the effects of a fever depend on the environment. In a state of fever, the body is induced by a pyrogen to maintain a higher body temperature and will thus adjust accordingly depending on the surrounding temperature. The hyperthermic state induced by RF radiation is therefore more similar to that from exercise.

How much heat can be tolerated by the body?

To get a sense of the heat load that can be tolerated by the body, we first look at the background exposure levels. For 99% of the United States, this is at less than 0.001mW/cm². At the resonant frequency for humans, this represents a whole-body SAR of about 0.03% of the resting metabolic rate (M), which is insignificant. Even the SAR level of 0.4W/kg specified by the ANSI guideline for controlled exposure is only 35% of resting M, which is equivalent to putting on a light sweater, so it is not that large an effect. In fact, most humans can cope with exercise or work loads even in thermally stressful environments (high ambient temperature) equal to fifteen times the resting M. The reason that humans are able to deal with such a large increase, is that this is compensated by a great increase in sweat production, accompanied by efficient behavioral thermoregulation. In combination, these allow only a minimal temperature increase. Because it is so difficult to generate a large temperature increase, it makes sense that no adverse health effects have been shown in exposure to radiofrequency radiation in the few studies done.

Use of microwave energy can also be the beneficial to a body. It is, for example, used for diathermic heating of body tissues and in cancer treatment.

Does thermoregulatory behavior depend on thermal sensation?

What about other factors? While physiological responses such as vasodilation and sweating may be triggered automatically by thermal stimuli, sensation of tissue heating is necessary to provoke behavioral thermoregulation. It is believed that temperature-sensitive neural structures lying within the outermost 0.6mm of skin sense temperature changes. RF/EM radiation may or may not produce a sensation of warmth, depending on frequency modulation, intensity, duration, and exposed surface area.

Infrared (IR) radiation and very high frequency RF (10GHz or higher) radiation are sensed in the skin, but lower RF frequencies are absorbed at other depths, so it is not so clear what happens. In general the shorter the wavelength of radiation, the less energy required to produce a sensation of warmth (e.g. IR has a shorter wavelength). At short durations, stimulus intensity must be increased to provoke sensation of warmth. In unusual circumstances, persistence of the sensation of warmth from RF radiation after the heating source is gone has been observed. This may be due to thermal inertia of tissues and the greater volume of tissue involved when sense penetration is deeper.

There has not been much experimentation done with humans to monitor changes in thermoregulation behavior (voluntary response) in the presence of microwaves. The focus has been on animals, which can be trained to push buttons to adjust heat from an RF source, meaning that the radiation has a behavioral effect (after a threshold of exposure). Exposure at the resonance frequency for the animal, results in a stable hyperthermic offset of the deep body temperature even though behavioral responses regulate skin temperature at the preferred level. This response is identical to the conditions produced by exercise.

There is also the possibility that physiological responses, such as peripheral vasodilation and sweating that shows on the skin surface could influence behavioral regulation. Extrapolation of animal studies where animals are trained so that response to stimuli is quick, leads to the conclusion that education of workers in high-power sources of RF fields, and environmental signals (visual and auditory) associated with equipment function or malfunction, can aid recognition of potential for over-exposure and tell workers to get away from potentially dangerous fields.

What is a threshold effect?

For given environmental conditions and species, there is an intensity of imposed RF radiation at which a thermoregulatory response is triggered. Below this threshold, RF intensities will produce no response. Above this level, response is dependent on intensity. [1] As the dry bulb temperature increases, the SAR threshold decreases.

How does the body metabolism respond to RF radiation?

The metabolic rate, M, is reduced after a threshold SAR has been achieved in whole-body exposure, so body temperature stays within its normal limits. Chronic low-level RF radiation produced no measurable change in the normal metabolism of infant rats, rats exposed for their entire life, or in squirrel monkeys exposed for 15 weeks. [1]

For non-human primates this threshold is ~0.5-1.5W/kg, and is unknown in other species. Exposure of non-human primates at resonance frequencies gives less response so body temperature rises, in a way similar to exercise, but not by much. Resonant exposure is not any more dangerous than other frequencies.

How does the body use vasomotor and evaporative responses?

Vasodilation occurs to increase heat loss and after the threshold intensity. The magnitude of the response is a function of whole body SAR or total heat load.

Once peripheral vasculature is maximum, the evaporative response sets in to cool the body more. Humans lose a lot of body heat through sweating when the dry-bulb temperature, T_db, is greater than 30-31°C (86- 87.8°F) or deep body temperature is greater than 37°C (98.6°F). In contrast, rodents cannot sweat or pant like cats and dogs, and can only lose heat through evaporation by licking their pelt and causing evaporation of saliva. The local sweat rate increases linearly with the temperature of the body core during exercise. At lower skin temperatures, the body temperature must be higher in order to initiate sweating. This is well known as in that on a cold day you don't sweat as easily. Tests done on monkeys confirm that the rate of sweating depends on the temperature of the core, and the surface temperature (ambient or skin temperature).

What happens under exposure to intense RF radiation? What are the limits of human heat tolerance to RF radiation?

p. 420 Exposure to intense RF radiation results in an initial increase in core temperature, followed by a plateau temperature at a hyperthermic level, and eventually leading to thermoregulatory failure if exposure is long enough. The thermoregulatory response can balance the heat absorbed, but only temporarily. Death can result from hyperpyrexia. Environmental temperature is important here, in that there is more tolerance at lower temperatures. In addition, hydration helps extend tolerance in dogs more, presumably because it allows more evaporative loss in panting and aids cooling of the body. Theoretically hydration would thus increase sweating loss in humans, and allow greater cooling. Tolerance increases with the number of times exposed, similar to how individuals get used to warmer climates.

Wrists, ankles, neck (and animal tails) have enhanced energy absorption and hence higher tissue local temperature, but heat transfer through blood flow prevents any damage from occurring by transferring heat to other parts of the body.

What other factors are important in thermoregulation?

If the thermoregulation system is compromised in any way, by drugs or other agents, there may not be good regulation of body temperature in radiation fields.

Naturally, cardiovascular impairment of any sort will lower the maximum SAR possible for thermal equilibrium because of the limits on heat loss through blood flow because of the limits on the pumping system. Ordinarily some level or RF energy (SAR) can be absorbed indefinitely because the body will be able to effectively balance the low levels of extra heat and maintain thermal equilibrium. [1].

There seems to be a potential for hot spots in the cranial cavity, but models predict that this is unlikely because of greatly enhanced heat loss mechanisms that are active during heating. Likewise, poorly perfused tissues such as the ocular lens, gut or resting muscle can increase temperature if high local SARs are generated, but increased heat dissipation mechanisms will spread the heat quickly. (See health effects.)