Monitoring heat strain

Published:  02 November, 2017

Real‐time monitoring of heat strain in high‐risk workers is now possible through new wearable technology, writes Jose Maria Sanchez de Muniain.

Already in use by hazmat responders and oil and gas workers the technology is currently being validated for use in high‐hazard environments commonly encountered by firefighters.

Heat strain is the body’s physiological response to heat stress and includes an increase in heart rate and sweating. If actions such as these do not result in the core body temperature decreasing, the result is heat‐related illness such as heat stroke, heat exhaustion, heat syncope and heat rash. In extreme cases it can result in death.

The US Fire Administration has noted that in 2015, 66.7% of fatal injuries were caused by stress or overexertion. By far the leading cause of death, this category includes deaths that are cardiac or cerebrovascular such as heart attacks and strokes; it is well established that sudden cardiac death is the leading cause of line‐of duty fatalities, and that heat stress increases cardiovascular strain.

The latest figures available from the US FA regarding firefighter injuries (Fire-related firefighter injuries reported to the National Fire Incident Reporting System, 2012‐2014) shows that 27% were caused by overexertion/strain, with most taking place during structural fires. Although the exact role played by heat stress in these injuries cannot be surmised from the evidence, a link has long been suspected, leading to a number of studies including Firefighter fatalities and injuries, the role of heat stress and PPE, by the Firefighter Life Safety Research Center, Illinois Fire Service Institute, and University of Illinois at Urbana‐Champaign.

It is perhaps not surprising that current wearable heat‐strain monitoring technology owes much of its existence to meeting the needs of the medical world. The latest system started life under the Equivital brand in 2011 as a way of monitoring physiology, primarily for research communities in academic and medical circles. Its developer, Cambridge‐based Equivital in southeast England, also made some inroads in high‐performance sports, military and firefighting markets.

The applications and features of the technology were subsequently developed and launched for professional welfare monitoring in 2015 under the system name Black Ghost. In addition to heart rate, breathing rate and body position, this can monitor internal temperature through the ingestion of a small capsule containing a thermistor and radio. “However, our customers in the military, fire services or hazardous first response were not going to swallow a pill X hours before a call out. So we were trying to work out how to measure core temperature and heat strain without having to swallow a pill,” remembers Equivital cofounder and head of product, Ekta Sood.

The alternative solution to the ingested pill has recently presented itself in the form of an intelligent algorithm.  Validated by the US Army Research Institute of Environmental Medicine*, the algorithm can provide an estimated core body temperature using a non‐invasive measurement of heart rate. In its conclusion, the research found that the algorithm’s bias and variance to observed data were similar to that found from comparisons of oesophageal and rectal measurements. Further research by Equivital confirmed that, provided the heart rate measurements were of high enough quality, the algorithm could provide accurate core body temperature estimates in applications where encapsulated clothing is worn.

The Black Ghost system consists of two elements. The first is the EX EQ02+, a body‐worn sensor module that slips into an anti‐static sensor belt that wraps around the shoulders and the chest.  The sensor module, which is FDA, ATEX, ETL AND IECex certified, measures heart rate, respiratory rate, skin temperature, body position and movement. Powered by a rechargeable battery, it has an operating temperature range of ‐10‐50 °C and a water ingress protection rating of IPX7.

Data from the module is either stored internally for later retrieval or transmitted in real time via Bluetooth to a mobile device. In the case of continuous live monitoring of a firefighter, the wearer is required to carry a Bluetooth‐enabled device; Black Ghost is compatible with Android devices, WiFi, Tetra and satellite comms.

Having passed through a local/private or cloud server, the data is finally displayed on the Black Ghost application on any web‐enabled device. As well as physiological data the system can display an interactive map, for location tracking. The system can also be configured to display third‐party sensor data which, in the case of firefighters, could be breathing‐apparatus data: “For the fire services the big one is BA data, which is a key demand that needs a home right now. It is our next challenge and one that although technically speaking is not necessarily that big, would provide even greater situational awareness,” says Sood.

The latest evolution in the technology has been the development of a heat strain index, introduced in September this year. Using the index, which is displayed in a format that ranges from zero to 12, a supervisor can see the heat‐strain risk of a person wearing the technology, in real time. “It is meant to be a simple measure not just for medics, but for commanders and others to look and say, ‘that person needs to be cycled out,’” explains Sood.

Interestingly, the technology has been further enhanced to provide an estimated future heat strain measurement: “We have taken it further to predict forward 15 minutes, so if the current heat strain index is X, and you carry on working at the same rate, what will be the heat strain risk then? In the kind of applications we are working in, with PPE and breathing apparatus, that is very useful for improving the efficiency and safety of the team,” says Sood.

The system was initially envisaged for use in training establishments as an aid to monitoring the occupational health of instructors and students. However, the potential benefits of seeing physiological data in real time have driven interest further: “A number of fire services became interested in the technology on an operational basis, which has meant developing it so that it fits with their standard operating procedures rather than the other way round,” remarks Sood.

The accuracy of the heat index has so far only been fully validated with wearers in encapsulated PPE and in normal working environments, but Equivital is now in the final stages of validating the system for firefighters and firefighting environments. “In the fire service you have quite different environments because the temperatures will be far higher, so the heat exchange becomes impaired further than if just wearing PPE in a normal environment,” explains Sood.

A number of pilot projects involving firefighting organisations are currently taking place in the Middle East, Australia and US.  A pilot project involving more than one UK fire service has kicked off. Here, physiology, estimated core temperature and heat strain index data are being collected from firefighters that are wearing the body‐worn sensors during training activities. This data is then being compared with the data collected from radio‐containing ingested pills to ascertain the accuracy of a specific algorithm for firefighters.

The results of the firefighter pilot project are expected in the next few months.

* Real-time core body temperature estimation from heart rate for first responders wearing different levels of personal protective equipment. Buller, MJ, Tharion, WJ, Duhamel, CM, & Yokota, M. (2015). Ergonomics, 58(11), 1830‐1841.

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