Wearable Wisdom
The Promise and Challenge of Wearable Sensors for Heat Stress Management
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Heat stress is a substantial, recognized physical hazard for many workplace environments. Construction, mining, energy extraction, manufacturing, agriculture, fishing: these are just a few of the industries whose workers are at greater risk of heat stress. Exposure to heat stress can lead to heat strain, resulting in a variety of health effects, including death. These effects are mostly acute, and for this reason the risk management of heat stress requires timely intervention strategies and associated monitoring. Heat stress is currently a policy priority of AIHA and the AIHA Thermal Stress Working Group.
As the impact associated with heat stress in the workplace increases, OEHS professionals are asked to make difficult decisions about worker health and safety. Various methodologies are available to help with decision-making, but conducting a systematic review of these methodologies can be overwhelming to practitioners who need to simultaneously become experts on thermal stress, implement a heat stress management system, and communicate the rationale and results to a variety of stakeholders.
THE SCIENCE OF HEAT The science of heat can be uncomfortable. The ACGIH Threshold Limit Value is based on a body weight of 70 kilograms (154 pounds), while the average adult weight in the U.S. is 171 pounds for women and 200 pounds for men, according to the National Center for Health Statistics. This difference may mean calculation adjustments are necessary for the prescribed method of estimating metabolic rate. Moreover, the TLV for heat stress and strain is based on maintaining body core temperature within 1 C of “normal” (37 C) for the average person. In reality, each person’s “normal” core temperature and their response to heat stress are influenced by extensive inter-individual (including age, sex, chronic disease) and intra-individual (such as medication use, fitness, acclimatization status) factors. Work-rest rotations, which are effective, immediate, and simple, are often seen as the control of last resort due to their cost and are particularly unpalatable in an economy struggling with staffing that is also obsessed with lean workforces and just-in-time logistics. OEHS professionals who need to explain these principles to an organization for the first time may struggle, especially if they are not well grounded in thermal physiology.
From the perspective of monitoring, heat is unique since both the hazard (heat stress) and the health effects (strain) can be measured in a variety of ways right in the field. Practitioners have the flexibility and responsibility to select and adopt monitoring strategies to fit their needs, but they must also be aware of the benefits and limitations of each strategy. This is in line with the evolution of industrial hygiene practice toward “IH4.0” as proposed in an article published in the February 2022 Synergist.
Instruments that measure heat stress may report environmental data such as relative humidity or forms of combined temperatures such as ambient, dry bulb, wet bulb, or globe. Heat strain measures the way in which heat stress affects a person, which is often indicated through physiological, perceptual, cognitive, or performance measures. For both heat stress and heat strain, practitioners can adopt a variety of real-time detection systems and direct-reading methodologies. If on-site measurement is not feasible, heat stress can also be measured in terms of heat index or humidity index (humidex), which can be derived from weather station data. This is the approach used by the NIOSH-OSHA Heat Safety App.
One of the major challenges of using any heat stress and heat strain measurement is translating the data collected into useful insight, especially if the information is being used to make decisions about the acceptability of exposures. Current heat stress/strain safety guidelines are often targeted to a small population of workers (those who are healthy, hydrated, and heat acclimatized) and are supported by limited scientific evidence.
PHYSIOLOGICAL MONITORING To address the complexity of individual responses to heat stress, many OEHS professionals are turning to personal physiological monitoring systems to assess heat strain and evaluate risk. While physiological monitoring can provide valuable information in terms of heat strain, professionals should first consider the consent of the workers, the privacy of the data collected, and other ethical issues. Core body temperature is considered the most significant indicator of the risk of heat-related injury and illness, but direct measurement methods such as rectal thermometers or ingestible thermometer pills are appealing only from a data integrity perspective; challenges with invasiveness and ethics often preclude their use. Devices that monitor employee core temperature through in-ear thermometry are no longer available due to practical challenges with their usage and concerns about their validity. As a result, OEHS professionals must usually turn to other methods of measuring human physiological parameters to indirectly estimate core body temperature, combined with or informed by estimates of work dynamics.
Several wearable devices that measure heat strain promise to fill this difficult and uncomfortable gap. Devices often provide automatic data visualization or interpretation, warning signals, or even direct instructions for the user to rest. The simplicity of these outputs is one of the strengths of these devices, but it can lead users—OEHS professionals and workers—to become overconfident regarding the quality and accuracy of the information. Clear messages with no ambiguity or expressions of uncertainty may be comforting, but they don’t facilitate understanding of the variable actually being measured and the accuracy of the predictive models used to estimate core body temperature.
Heart rate and skin temperature are among the most common measurements used by wearable devices to estimate core body temperature, but concerns remain about their accuracy for this purpose. How much are these variables influenced by factors such as caffeine intake, smoking, hypertension, medications, the location on the body at which they are measured, or the tightness of the strap on the instrument? These questions matter when using indirect methods of measurement, and each factor contributes to sampling and analytical errors. Too much error, and the device loses all value for warning or monitoring, instead functioning more as an in-flight recorder—that is, valuable only as an element to be investigated following a critical illness.
Whenever core body temperature is calculated based on heart rate, skin temperature, or any other measurement, the OEHS professional needs to understand the association between the two values. Any predictive model, including those implemented by a monitoring device, has some intrinsic error between true core body temperature and the measurement taken. But the models and algorithms used by device manufacturers are often proprietary; unless they are disclosed or examined, the OEHS professional may be forced to assume that the intrinsic error is limited enough to rely on for health and safety.
Some manufacturers have coordinated evaluation of their devices against a control group, assessing measurement validity in scenarios that reflect known environmental conditions and metabolic demands. One recent study (PDF) recruited firefighters to perform practical exercises while wearing a thermal strain monitoring device based on heart rate. Data was simultaneously collected by the wearable monitor (BioTrac) and by a rectal thermometer (DataTherm II), which is considered to have high validity and fidelity for core body temperature. Charting the measured core temperature using the rectal thermometer versus the calculated core temperature from the wearable device reveals a very close match for core temperatures below 38 C. See Figure 1.
Figure 1. Example of the data collected by a wearable monitor (blue line) and rectal thermometer (orange line). Reprinted with permission from SlateSafety. Click or tap on the figure above to open a larger version in your browser.
A subsequent study published in the journal Sensors used a Bland-Altman analysis, which assesses the agreement between two methods of measurement, to compare data between an ingestible pill thermometer and a wearable arm-band device while nurses performed common tasks in a high heat environment. The study found that the proprietary algorithm calculated a core temperature from measured heart rate that closely resembled the ingestible thermometer on average for individuals with core temperatures below 38 C. Unsurprisingly, the arm band was strongly preferred by users over ingestible thermometers. These kinds of studies, while limited in many ways, help establish the validity of devices and approaches while also enabling proactive partnership with employees.
EVALUATING WEARABLES Comprehensive analysis of device performance to evaluate its fitness for purpose—that is, the benefit of using it for a specific need—requires an advanced skillset in data analysis, which may not be accessible to most practitioners. The use of a third party, such as a research institution, is critical for providing reliable data and evaluating the device’s effectiveness in metrics that matter. Such evaluations can also identify key limitations that may frame the appropriate use of the instrument.
OEHS professionals are advised to ask the following questions when evaluating a heat-related wearable and continue their systematic review based on those answers.
1. What is the device actually measuring?
Common indirect metrics used by devices to measure core body temperature are heart rate and skin temperature. Knowing what is measured is key to determine the validity of measuring it and the potential for interferences. Modern devices commonly include a proprietary algorithm that may obscure the translation between input signal and reported values. If the algorithm is not disclosed, use of the device requires a certain amount of trust in the manufacturer. Any interference—for example, a radio signal, direct sunlight, or new firmware—presents a potential confounding variable in measurements.
2. How well are the sensors within the device measuring what they are intended to measure?
Complex algorithms or predictive models are often used to calculate outcomes related to heat strain, but no consensus exists on their content. Practitioners should understand how accurate the indirect measurements are compared to the true value. Measurements that are extremely variable and differ substantially from the true value may not be appropriate for safety decisions. Unfortunately, there is no standard to determine when an estimated value is considered adequately accurate. Calibration of the device for each worker might improve the estimation but adds time, complexity, and cost.
3. Has the device been validated? Understanding the measurement error starts with understanding the standard used for measurement. This can provide insight into the quality of the components and the thought process behind validation, both of which can be key. Any claim of validation should be substantiated with data collected and analyzed by a third party (and preferably peer-reviewed). Pay special attention to the context of the validation, especially conditions at rest and during simulated or actual exertion.
4. Does the instrument consider user variables? User variables such as age, resting heart rate, body mass, and skin tone can be significant in some sensor implementations or background calculations. While these variables are not needed for all implementations, considering this question helps the OEHS professional determine whether the device treats each user as an interchangeable physiological component in a system or really focuses on the effect of heat stress on the worker.
OEHS professionals should still rely on signs, symptoms, and “gold-standard” measurement tools to recognize and diagnose heat illnesses.
5. What do we intend to do with the information provided by the device? NIOSH recommends that organizations identify the objective of the data obtained by the instrument as the first step in the “right sensors used right” approach. Implementation may look different for devices used as “silent” evaluators—that is, devices that are not set to alarm or trigger actions by the user—in addition to general and job-specific controls. The temptation may be great to conflate use of such a device with some measure of safety. Users and practitioners should be reminded of the limitations of such devices. For example, since few devices can summon emergency responses, most will continue monitoring even during a medical emergency warranting intervention, such as heat stroke. Another limitation is that the devices might be left on the breakroom table by workers who feel they represent an unacceptable level of intrusion. Especially for physiological monitoring with wearable devices, the involvement of workers in the selection and adoption process is critical, and patience is needed. In addition, no device can substitute for human judgment: OEHS professionals should still rely on signs, symptoms, and “gold-standard” measurement tools to recognize and diagnose heat illnesses. Wearable devices are intended to be used for prevention purposes.
6. Are we prepared for the reality of what this device will report? It can be useful to consider what an organization will do with both acceptable and unacceptable results. Principles governing the organization’s response to data from wearables should be determined before the devices are trialed.
7. How will our workforce react to our suggestion that the device be used? It is evident that workers will play an important role in IH4.0 considering the incremental use of sensors and monitors in the workplace. OEHS professionals should strive for partnership with the workers they protect and advocate for transparent discussions about risk and monitoring as well as user privacy. In addition, the use of wearable devices should always be associated with ethical values and principles, as discussed in a post to the NIOSH Science Blog.
8. How will we set and respond to alarms? Users asking this question should refer to AIHA guidance on the alarm-setting process (PDF). The guidance recommends associating specific alarms with specific actions.
AN EMERGING CORE COMPETENCY Wearable heat stress and strain devices have the potential to substantially improve safety for thermal illness if their adoption is carefully considered. When properly evaluated, combined with other tools and methods, and used with proper context and judgment, their potential is significant.
For many OEHS professionals, adoption of wearable devices will entail addressing uncomfortable topics and asking important but difficult questions of both their organizations and wearable device manufacturers. Wearable technology is an emerging core competency in occupational health. The experience we gain addressing these topics in real-time detection systems will pay dividends at the forefront of the changing face of work.
SPENCER PIZZANI, CIH, is the occupational health manager for PepsiCo Global EHS. His professional passions include sensor technology, genetic susceptibilities, and accessibility of industrial hygiene technical information.
EMANUELE CAUDA, PhD, is the director of the NIOSH Center for Direct Reading and Sensor Technologies as well as an adjunct professor at the University of Pittsburgh Graduate School of Public Health.
MAGGIE MORRISSEY, PhD, is director of occupational and military safety at the Korey Stringer Institute, president of the National Heat Safety Coalition, and secretary of the AIHA Thermal Stress Working Group.
WILLIAM MILLS, PhD, MSc, CIH, CChem, is research officer for the AIHA Real-Time Detection Systems Committee and an associate professor in the College of Engineering and Engineering Technology at Northern Illinois University.
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ACGIH: TLV and BEI Documentation, Heat Stress and Strain (2023).
Biochemia Medica: “Understanding Bland Altman Analysis” (June 2015).
Journal of Applied Physiology: “Wearable Physiological Monitoring for Human Thermal-Work Strain Optimization” (2018).
Journal of Athletic Training: “Assessing the Validity of Aural Thermometry for Measuring Internal Temperature in Patients with Exertional Heat Stroke” (February 2021).
Journal of Athletic Training: “National Athletic Trainers’ Association Position Statement: Exertional Heat Illnesses” (September 2015).
National Center for Health Statistics: “Body Measurements.”
NIOSH: “Heat Stress Work/Rest Schedules” (PDF, 2017).
NIOSH Science Blog: “Right Sensors Used Right: A Life-Cycle Approach for Real-Time Monitors and Direct Reading Methodologies and Data. A Call to Action for Customers, Creators, Curators, and Analysts” (May 2019).
Sensors: “Comparison of Slate Safety Wearable Device to Ingestible Pill and Wearable Heart Rate Monitor” (January 2023).
Sensors: “Wearable Sensor Technology to Predict Core Body Temperature: A Systematic Review” (October 2022).
SlateSafety: “Industrial Athlete Core Temperature Accuracy Analysis of Commercial Arm Worn Device vs Rectal Thermometry” (PDF).
The Synergist: “The Challenge for Industrial Hygiene 4.0: A NIOSH Perspective on Direct-Reading Methodologies and Real-Time Monitoring in Occupational Environments” (February 2022).
Temperature: “Indicators to Assess Physiological Heat Strain - Part 1: Systematic Review” (2022).
Temperature: “Indicators to Assess Physiological Heat Strain - Part 2: Delphi Exercise” (2022).
Temperature: “Indicators to Assess Physiological Heat Strain - Part 3: Multi-Country Field Evaluation and Consensus Recommendations” (2022).