Introducing the AIHA Heat Stress Mobile App
New Tool Facilitates Management of Heat Hazards
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Pictured above: Mock-up of the AIHA Heat Stress Mobile App. The app was undergoing testing as this issue of The Synergist went to press. Final design may differ.
In October 2022 and February 2023, members of AIHA and ACGIH participated in brainstorming discussions about managing heat-related hazards. Asked to share their experiences with heat stress in the workplace, participants reported that workers under their care had suffered a range of heat illnesses including cramping, rashes, loss of consciousness, and convulsions. Others talked about worker hospitalizations and fatalities. “I had to participate in a line-of-duty death investigation for a wildland firefighter who died in Texas,” one participant said, according to a summary of the sessions. “He was only 23 and his parents wanted to know why he was dead.”

As these stories suggest, heat-related illnesses, injuries, and fatalities are becoming more common. According to the United States Bureau of Labor Statistics, from 2011 to 2019, an estimated 31,560 occupational heat-related illnesses and injuries required time away from work, an average of 3,507 per year. Excessive environmental heat stress caused the deaths of more than 900 workers in the United States between 1992 and 2019. During that time, the three-year average of heat-related occupational fatalities increased more than 100 percent, according to an analysis published by NPR.
What to do about it? Workers who are acclimatized to the environment are less likely to experience severe heat-related illnesses, but, as more than one participant in the AIHA/ACGIH sessions observed, it can be difficult to determine who is acclimatized and who isn’t. Another complication is that the gold-standard for quantifying environmental heat to evaluate thermal stress, the wet-bulb globe temperature (WBGT), has historically required specialized instruments to measure. WBGT incorporates air temperature, relative humidity, air movement, and radiant heat, whereas the far more common heat index accounts only for temperature and humidity.
Some participants in the discussion sessions suggested that what’s needed is a way to reach workers, who aren’t trained in the complexities of the science. Many agricultural and construction workers who are among those most exposed to severe environmental heat are employed by small companies that don’t have OEHS professionals on staff. “We need a tool that [can] make the WBGT data simplified, or some kind of phone app,” one participant said. “Something that is simple, that has it all in one place, that is easy to read and easy to understand.”
Such a tool will soon become reality. This month, a beta version of the AIHA Heat Stress Mobile App is undergoing user testing. It is anticipated that the app will be ready for wide release on both Android and iOS platforms by the middle of July, perhaps earlier. A project of AIHA’s Thermal Stress Working Group, the AIHA Heat Stress Mobile App improves upon the Heat Safety Tool released by OSHA and NIOSH in 2017. The two apps have many similar features, but while the OSHA/NIOSH version is based on heat index, the AIHA app uses WBGT index, a more accurate environmental metric of heat stress.
A GREAT OPPORTUNITY “Heat index has its uses,” said Jo Anne Balanay, PhD, CIH, a professor at East Carolina University and an original member of the Thermal Stress Working Group. But because heat index does not account for the effects of sunlight, its readings can be off by as much as 15 degrees Fahrenheit, research has shown. And as Balanay pointed out, heat stress guidance from OSHA, NIOSH, and ACGIH all rely on WBGT.
Balanay is the coauthor of a study, published in the Journal of Occupational and Environmental Hygiene in 2020, that assessed the accuracy of the OSHA/NIOSH Heat Safety Tool. The study used WBGT monitors to log the WBGT values over 44 days from April through August 2019 at two outdoor agricultural sites in North Carolina, while the Heat Safety Tool was used to obtain hourly heat index levels and the app’s corresponding risk determinations. Balanay and coauthor Danielle Dillane assigned each logged WBGT value to one of the five risk levels designated by the Heat Safety Tool. By comparing the WBGT data to the Heat Safety Tool output, Balanay and Dillane could determine how often the tool’s risk levels agreed with risk levels derived by WBGT.
The AIHA Heat Stress Mobile App not only calculates the current WBGT index but also forecasts what the WBGT index will be up to five days ahead of time.
Balanay and Dillane found that the Heat Safety Tool performed adequately in low-risk conditions. But as heat stress risk and workload increased, the tool’s accuracy diminished. It performed worst when the risk was highest—that is, when users would need it most. Balanay and Dillane concluded that “the use of the app to assess occupational risk to heat stress in agricultural settings is not recommended.”
Not long after the study was published, Balanay and Sinan Sousan, an assistant professor at ECU who had experience in coding, participated in an app development training program at the university. Through connections made at the event, Balanay and Sousan received funding from ECU to create a web app prototype that used what’s known as the Liljegren algorithm, developed by Argonne National Laboratory, to compute WBGT from weather data.
Because it used WBGT, the prototype was a significant improvement over the Heat Stress Tool. Still, as Balanay explained, she and Sousan recognized that the app would have to do more than just provide the WBGT index. “Say the WBGT is 35 degrees Celsius,” she said. “What does this mean?” Workers wouldn’t know what to do with this information. So, borrowing a page from the Heat Safety Tool, Balanay and Sousan designed the prototype to translate the WBGT into qualitative risk levels and recommend preventive measures.
With the university funding exhausted, Balanay and Sousan weren’t sure what would become of their work. Then AIHA’s Thermal Stress Working Group decided to solicit proposals from app developers to create a heat stress mobile app. “We thought, ‘This is a great opportunity for us to improve this prototype further,’” Balanay said. The same connection who had helped secure funding for the prototype now put Balanay in touch with Dualboot Partners, an app development firm. Dualboot and Balanay submitted a proposal, and the Thermal Stress Working Group accepted it.
Balanay was thrilled. “I was confident that if there is a group that I would trust that would improve the prototype, it’s AIHA,” she said. “I had no doubt about it because of the expertise of the members of AIHA.”
THINKING WITHOUT LIMITS From the moment the Thermal Stress Working Group was created in 2021, a heat stress app was on its agenda. With more than 80 members, the working group is one of AIHA’s larger volunteer groups. A subgroup of 16 members led the app development project. This subgroup was chaired by Kyle Hubregtse, the CEO of Kenzen, a company that develops wearable technology for preventing heat illness at work sites.
Everyone in the subgroup wanted an app based on WBGT, and Balanay’s prototype fulfilled this major requirement. What else the app should do was the subject of intense discussion. In addition to allowing users to specify the severity of their workload, should the app also permit user input on the kind of clothing they’re wearing? What about ambient weather conditions? Some members felt strongly that the app should address acclimatization, a huge factor in determining a worker’s risk for developing a heat-related illness. Others recognized the importance of acclimatization but struggled with how to explain this complex phenomenon to laypeople and worried about the potential legal liability of declaring someone acclimatized. “It’s very complicated to categorize the worker as to whether they are acclimatized or not,” Balanay said.
Such debates are common when launching new technology, according to Hubregtse, who has participated in many development projects at Kenzen. “We always try to think without limits and then pare it down based on the resources we have,” Hubregtse said. “There were a lot of really great ideas about how to incorporate different things in the app, but then you kind of have to pick and choose what’s realistic for this version and the audience you’re building it for.”
A key consideration was whether the app would store user data such as preexisting medical conditions. Workers who are overweight or who have high blood pressure or heart disease are at increased risk for developing heat-related illnesses. Certain medications can also make a worker more susceptible. Data such as these would increase the specificity of guidance and algorithms in the app. But the subgroup ultimately decided that asking users to provide such sensitive information could limit the app’s reach. For the same reason, the subgroup felt that the app should not require users to create an account.
“When you’re talking about individual interactions and individual data, you want to be able to give [users] some functionality to modify certain things unique to their environment,” Hubregtse said, like the intensity of their workload, the amount of personal protective equipment they wear, and the degree of cloud cover. “But we wanted to stop short of asking or storing any health-related or personally identifiable data. That’s really not something we wanted to be involved in or collect, because for the purpose of this app, which is to provide guidance on heat stress, these data are not important. The planning aspect of the tool is the most important.”
FEATURES OF THE INITIAL RELEASE The planning aspect is a distinguishing feature of the AIHA Heat Stress Mobile App, which not only calculates the current WBGT index but also forecasts what the WBGT index will be up to five days ahead of time. “That’s a very important feature that even the traditional heat stress monitor cannot do,” Balanay said. Knowing the forecasted WBGT index allows employers to schedule work accordingly. “If the supervisor sees that the heat stress level at three o’clock in the afternoon would be an extreme risk level, then they could shift the heavy work to the morning and the lighter work to the afternoon,” Balanay explained.
Also part of the app’s initial release is a clothing adjustment factor. By default, the app assumes users are wearing a t-shirt and jeans, but users can choose among a few protective ensembles; the more impermeable the clothing, the greater its potential contribution to heat stress. The app also accepts user input regarding cloud cover. If nothing is selected, the app assumes the user is working in direct sunlight. And users can select one of four workloads—light, moderate, heavy, or very heavy—and set reminders to rest and hydrate based on their work schedules.
Based on the inputs for clothing, ambient weather, and workload, the app adjusts the user’s WBGT index and the resulting risk level. Once the risk level is determined, the app delivers appropriate protective measures. Users can access links to authoritative resources on heat stress and an explanation of how to help coworkers who may be experiencing heat-related illness.
The app can access weather data throughout North, Central, and South America, and it is being released not only in English but also in French, Portuguese, and Spanish. Eventual expansion to other parts of the world is possible. “The integration with the weather data would be the key to making it available in different regions,” Hubregtse said. “I think in future iterations, [overseas expansion] could be potential for us.”
REAL-WORLD PROBLEM SOLVING As this issue of The Synergist went to press, beta testing for the AIHA Heat Stress Mobile App was on track to begin June 1 and was expected to last two to three weeks, though it could take longer, depending on what the testing reveals about the app’s performance. According to Mike Boehm, product director at Dualboot, the goal of beta testing is to put the app through its paces in as many devices and locations as possible. “Any solution is only as valuable as the problems that it actually solves for real people,” Boehm said. “Our goal in beta testing is to make sure what we believed to be solutions to real problems provide the value we had hoped for.”
If all goes well, the app will become widely available by mid-July. Further development is planned, with updates released on a regular, perhaps annual, basis. Features that didn’t make it into the initial release will be considered for inclusion in future versions. Balanay suggested that the working group will further discuss a way to address acclimatization.
But the success of the app may depend less on its technological sophistication than on its ability to connect with the people who use it. There are no easy solutions to heat stress, particularly as extreme weather becomes more common. “If we can help everyone understand the risk and complications associated with heat, we would love to make that happen,” Hubregtse said. He then clarified that the goal of the app is ultimately educational. “The more people understand about the short- and long-term risk of heat exposure and heat illness—if we could prevent long-term chronic conditions and acute issues, like, that is a game changer, I think. So, if we could achieve those milestones, I think we’ll be doing something great.”
ED RUTKOWSKI is editor-in-chief of The Synergist.
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AIHA: “Defining the Science Members Guide: Initiative Background and Conceptual Research Ideas” (PDF, November 2023).
Federal Register: “Heat Injury and Illness Prevention in Outdoor and Indoor Work Settings” (October 2021).
Journal of Occupational and Environmental Hygiene: “Comparison Between OSHA-NIOSH Heat Safety Tool App and WBGT Monitor to Assess Heat Stress Risk in Agriculture” (April 2020).
NIOSH: “Heat Stress.”
NPR: “Heat Is Killing Workers in the U.S.—And There Are No Federal Rules to Protect Them” (August 2021).
OSHA: “Heat Injury and Illness Prevention in Outdoor and Indoor Work Settings Rulemaking.”
OSHA: “Heat Safety Tool.”