Page 54 - OHS, September 2022

EMPLOYEE HEALTH SCREENING
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.3 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.4 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.
Conclusion
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.
Bernard L. Fontaine is Managing Partner at The Windsor Consulting Group, Inc.
REFERENCES
1. https://www.acgih.org/heat-stress-and-strain-2/
2. https://www.cdc.gov/niosh/docs/2016-106/default.html 3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6466968/ 4. https://pubmed.ncbi.nlm.nih.gov/34948736/
50 Occupational Health & Safety | SEPTEMBER 2022
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