Physiological Monitoring as a Determinant of Heat Stress

Physiological Monitoring as a Determinant of Heat Stress

Physiologic monitoring can help protect all workers from heat-related illness.

Physiologic monitoring may be used in conjunction with other environmental measurements as a means to determine the heath impact of heat stress. Such techniques include oral, skin, and aural measurements that can be done at the worksite rather than a clinical setting. The effects from heat stress should be considered whenever the Wet Bulb Globe Temperature (WBGT) exceeds the American Conference of Governmental Industrial Hygienist (ACGIH) Threshold Limit Values (TLVs) or the National Institute for Occupational Safety and Health Recommended Action Limit (RAL) or Recommended Exposure Limit (REL). These occupational exposure limits provide a validated reference for working in hot environments indoors and outdoors. 

Heart rate, skin and core body temperature, and total body water loss from sweating can be measured as a physiologic response to heat exposure. More advanced methods and new tools are available for physiologic monitoring. When the NIOSH Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments was first published in 1972, physiologic monitoring was not considered a viable adjunct to the WBGT index and control of heat stress.  

In 1986, it was proposed that monitoring core body temperature and/or the work and recovery heart rate of workers exposed to environmental conditions in excess of the ACGIH TLVs could be a safe and relatively simple approach. Heat stress indices assume that most workers will not incur heat related illnesses or injuries if they are exposed to hot work conditions that do not exceed the permissible value. Inherent in this assumption is that some exposed workers may still develop heat-related illness. 

The NIOSH REL and the ACGIH TLV are intended to protect nearly all healthy heat-acclimatized workers at heat stress levels that do not exceed the occupational exposure guideline. Physiologic monitoring can help protect all workers, including new hires, workers on medication or suffering from illness or disease along with heat-intolerant workers. 

Oral Physiological Measurements 

Oral temperatures are easily obtained with inexpensive disposable oral thermometers. However, to obtain reliable oral temperature requires a strictly controlled procedure. The thermometer must be correctly placed under the tongue for three to five minutes before taking the reading, mouth breathing is not permitted during this period, nor should hot or cold liquids be consumed for at least 15-minutes beforehand.  

The thermometer must not be exposed to an ambient air temperature higher than the oral temperature before the thermometer is placed under the tongue or until after the thermometer reading is taken. In hot work environments, thermometers must be kept in a cool insulated container or immersed in alcohol. With the advent of digital oral thermometers, oral temperature measurements may be obtained within 30 seconds, thus avoiding some of the issues found with standard oral thermometers. Evaluation of any oral temperature must follow established medical and occupational hygiene guidelines. 

Data indicate that nearly 95 percent of the time, oral temperature was below 99.5 degrees Fahrenheit when the recovery heart rate was 124 bpm or less, and 50 percent of the time oral temperature was below 99.5 degrees Fahrenheit when the heart rate was less than 145 bpm. If the heart rate is below 90 bpm, the heat stress condition is considered satisfactory. When the heart rate approximates 90 bpm and/or the recovery is about 10 bpm, it indicates that the work level is high but there is little increase in the core body temperature. If the heart rate is greater than 90 bpm and/or recovery rate is less than 10 bpm, it indicates a no-recovery pattern—that is, the heat stress exceeds acceptable levels and corrective actions should be taken to prevent heat-related illness. 

Skin Physiological Measurements 

Skin temperature can be used to assess the severity of heat strain and estimate tolerance, which is supported by thermodynamically and field-derived data. To remove body heat from the deep tissues (core body) to the skin (shell), where it is dissipated to the ambient environment, requires an adequate heat gradient.  

As the skin temperature rises and approaches the core body temperature, this temperature gradient is decreased and the rate of heat removed from the body core to the shell is decreased and the rate of core heat loss is reduced. To restore the rate of heat loss or core skin heat gradient, the core body temperature needs to increase. Circulation of warm blood from central (core) body to the skin results in an increase in skin temperature. An increase in skin temperature results in heat transfer to the ambient environment through conduction, convection, and radiation. 

As heat is transferred to the environment, blood near the skin surface cools and returns to the body core resulting in a decrease in core body temperature. Under certain environmental conditions, insufficient heat is transferred from the skin to the environment which can result in an increase in core body temperature. As core body temperature increases above 100.4 degrees Fahrenheit, the risk of an ensuing heat-related illness is increased. From these clinical observations, it has been suggested that a reasonable estimate of tolerance time for hot work could be made from the skin temperature. Where evaporative heat exchange is not restricted, skin temperature should not increase much, if at all. In such situations, the maintenance of an acceptable core body temperature may not be jeopardized, except under very high metabolic loads or restricted heat transfer from clothing.  

When convective, evaporative or radiant heat loss is restricted while wearing impermeable protective clothing or multiple layer ensembles, sufficient time is required for skin temperature to converge with core body temperature to assess heat strain and predict tolerance time. Although skin temperature is generally 2 to 4 degrees below core body temperature, skin temperature can be used to estimate core body temperature. In fact, skin temperature and core body temperature are used for convenience, because temperature varies over different parts of the body. Several authors have attempted to use heart rate and skin temperature to estimate core body temperature, with varying degrees of success. In practice, skin or internal temperatures should be measured separately to manage heat stress. 

Aural Temperature 

Aural temperature is a measurement collected with an infrared (IR) thermometer in the ear canal. Because the IR thermometer should not directly contact the tympanic membrane, it does not provide a true measurement of tympanic temperature. Measurements taken at the peak temperature can be compared with the after-work temperature to compare with the temperature obtained during their work shift to potentially establish work-related hyperthermia. The efficacy of the aural temperature measurement to monitor for heat stress is uncertain because it consistently underestimates core body temperature, but it is simple and noninvasive. Care should be exercised when trying to interpret these physiological measurements. 

Sweat Rate 

Hamidreza H. et. al. looked at sweat rate and WBGT index as a measure of risk in the assessment heat stress to workers in both arid and semi-arid regions. During the spring and summer, a cohort of 136 randomly selected outdoor workers were enrolled in this study. Sweat rate was measured three times a day along with the WBGT index at each work station. The level of agreement between sweat rate and WBGT was poor (κ<0.2). Based on sweat rate, no case exceeded the reference value observed during the study. WBGT overestimated heat stress in these outdoor workers as compared to their sweat rate. Monitoring sweat rate in hot climates alone can underestimate the risk. Even though sweat rate is a good indicator of heat stress, the results from the sweat rate and WBGT showed a poor level of agreement. 

Heart Rate 

Recent studies indicate that body heat generated from a metabolic load can be stored for up to 60 minutes of rest. Although the heat decreases, muscle temperature remains elevated, possibly due to sequestration of warm blood in the muscle tissue. Even in recovery, workers are still under heat stress. This evidence must be taken into consideration when applying any corrective actions (engineering and administrative controls or the use of PPE). 

Historically, obtaining recovery heart rates at 1- or 2-hour intervals or at the end of several work cycles during the hottest part of the workday of the summer season presented logistical problems, but advancements in technology overcomes many of these problems. Wearable sensors, capable of continuously monitoring and recording of physiological responses, have been introduced to the market. Probably the most notable example is the heart-rate-recording wristwatch, which is used by many athletes. It enables continuous automated heart-rate measurements in real time that are accurate and reliable. The data can be stored, downloaded and analyzed at a later time. 

Other Technologies  

Single-use disposable digital oral thermometers can monitor workers at regular intervals. It would not be necessary to interrupt work to insert the thermometer under the tongue and to remove it after four to five minutes. However, inaccuracies can be found by the ingestion of fluids and control of mouth breathing for about 15 minutes before an oral temperature is taken. Oral temperatures are not the most accurate indicator of core body temperature, and such physiological measurements are not practical for anyone who is nauseated, feverish, or has already vomited. 

A more accurate technology involves ingestible capsules capable of recording and telemetering intestinal “core” temperature on a continuous basis. These devices have been used in research for about 20 years. A problem with ingestible temperature sensing capsules is that they must be ingested the evening before use and cease function only after passing from the body. Another drawback is the cost of the capsules and monitoring equipment. Other sophisticated wearable physiological sensor systems are under development. 

Some of the latest technology includes a sweatproof and waterproof wearable device worn on the upper arm. These wearable devices have a photoplethysmography sensor to measure heart rate, along with skin temperature and relative humidity sensors. This device monitors core body temperature continuously during work and alerts supervisors of potential risk. The algorithm accurately predicts core body temperature similar to a gastrointestinal pill or rectal probe based on different indoor and outdoor environmental conditions and work intensities. 


Physiological monitoring is a useful tool in combination with the WBGT indices. With the exception of measuring core body temperature by total body water loss from sweating and aural thermometers, the data can be compared to studies performed on each collection method and referenced against established exposure guidelines. Newer technologies on the market help monitor worker heat stress under variable metabolic rates while working in a hot environment and, thereby, eliminating the need for other physiological test methods. Further evaluation of these new technologies are needed to validate models for worker heat stress under variable metabolic loads both indoors and outdoors.  

This article originally appeared in the September 1, 2022 issue of Occupational Health & Safety.

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