When is it too dangerous to be outside? New wearable tech could tell you.

National Geographic

6 hours ago

Illnesses caused by heat—dehydration, cramps, and even strokes—can come on quickly and quietly. New, wearable technology could help alert wearers to lurking danger. Two people pick grapes in a vineyard near Fresno, California. Outdoor farm workers are on average 35 times more likely to experience heat-related illnesses than the general population. Photograph by Karla Gachet, Nat Geo Image Collection
Globally, health experts anticipate a 370 percent rise in yearly heat-related deaths and a 50 percent increase in hours of labor lost due to heat exposure.
Heat is one of the most dangerous forms of extreme weather, but symptoms can develop slowly, leading to delays in recognizing the signs of a more serious illness. That's why scientists are also developing technologies that could help save lives by telling device wearers precisely when they're overheating.
Some of these same technologies are also being developed for the public, in the form of wearable watches or sweat patches that warn when dangerous outdoor conditions are present. A worker on a farm looks out at the hot summer sun from a tractor. Greenhouse gas emissions have made extremely hot days more common. Photograph by Brian L. Frank, Nat Geo Image Collection Farm laborers are facing record high temperatures while working in unshaded fields. Above, an unplanted cotton field in Brownfield, Texas, is damaged by erosion during a heat wave. Photograph by Robb Kendrick, Nat Geo Image Collection
Long before recent heatwaves, scientists have been researching ways to measure heat stress in the body, without relying on an individual's perception of their own risk. What you put in your body can have a big impact on what you keep in your head.
The U.S. military uses an algorithm developed by scientists in 2013 that estimates the chances of heat stroke, heat exhaustion, and debilitating heat cramps by measuring changes in heart rate, skin temperature, and gait.
"This is the future," said Emma Atkinson, a biomedical researcher for the U.S. Army Research Institute of Environmental Medicine, in a 2024 press release announcing a device worn around the chest that resulted from testing the algorithms on 14,000 soldiers. "This is a life-saving device."
The army's core body temperature algorithm is the most advanced to date and is being integrated into products for the general public, said Zachary Schlader , a University of Iowa Bloomington associate professor who studies the implications of heat stress.
He acknowledges there is 'no gold standard' technology for combating heat stress in work settings, but says the most promising are algorithms to estimate internal body temperature. A construction worker covered up from the sun as morning temperatures reached 92 degrees Fahrenheit at a housing development in Marana, Arizona on Friday, July 12, 2024. Photograph by Cassidy Araiza, The New York Times/Redux
'In a clinical setting or lab, to measure the internal temperature of the body requires the measurement of rectal temperature or esophageal temperature, or different blood temperatures,' he said. 'Those aren't feasible in a work setting and in a bigger population setting. But how hot you get it, hot the inside of the body gets, is largely what determines the health risks of heat, as well as the productivity risk.'
For the public, Apple Watch, Galaxy Watch 5, and the Garmin watch can all detect skin temperature, primarily to track menstrual cycles. But to determine if someone is at risk of heat stress, wearable devices need to measure core body temperature, a capability they lack. Core body temperature measures the temperature of internal organs — like your heart, liver and brain, whereas skin temperature only captures your temperature on the surface.
Epicore , a digital health company, has created sweat-sensing patches that let wearers see their electrolyte loss, body temperature, and body movement to inform how much they need to drink to prevent dehydration.
Sony released the REON Pocket Pro , a wearable that sits between your shoulder blades and cools your body via a metal plate, in June . A separate sensor clips to your shirt to measure temperature and humidity. Testing new devices
In real-world work settings, it's unclear how effectively these devices prevent serious heat-related illnesses, according to a 2024 review of research studies of devices .
Early tests have shown some promise. In May, a startup called VigiLife released a heat stress sensor, costing between $100 and $300, that attaches to a bicep or forearm. It measures heart rate, along with core body and skin temperatures. It then gathers information from the wearable and environmental sensors to formulate health and safety insights.
For now, the technology is only available to commercial entities.
Texas-based construction firm Rogers-O'Brien adopted VigiLife's technology across its sites in mid-June. The firm saw zero heat-related illnesses and over $200,000 in savings when it piloted VigiLife's technology from May to September 2023. During a typical summer like 2023, the company might see five or six heat-related incidents that require basic on-site first aid, according to a spokesperson.
Yet, for now, most smartwatches also can't run core body temperature software without draining their batteries, CEO Zachary Kiehl noted. To reduce exposure during the hottest parts of the day, many farms hire harvesters to work overnight or in the early morning hours. Left: Workers pick tomatoes in a field in the early morning in Los Baños, California. Right: Laborers in the Western Cape province of South Africa harvest Pinot Noir wine grapes overnight. Photograph by Karla Gachet, Nat Geo Image Collection (Top) (Left) and Photograph by David Silverman, Getty Images (Bottom) (Right)
Some aren't sold on a wearable-only solution. One skeptic is Jason Glaser, the CEO of La Isla Network , a company that advocates for worker safety. The network pairs weather and health data to model heat strain on workers and recommend the best hours for avoiding outside work.
'A wearable is only an alarm system,' he said, adding that most farm workers can't afford it.
It's also unclear if, given access to wearable technology, workers would be incentivized to listen to its alerts. Intense manual labor and piece-rate payment systems, based on the quantity of produce they harvest, encourage workers to work beyond their limits without adequate access to water, shade, and breaks.
One preliminary study looking at the link between high temperatures and chronic kidney disease also found that implementing a heat safety program—added rest periods, improved access to shade and fluids, placing bathroom facilities nearby and more, at a sugar mill in Nicaragua, decreased kidney injury in workers. Some argue these company-mandated solutions might be more effective than using wearable technology.
Glaser would like to integrate wearables into his research, but that is on hold until the nonprofit receives more funding.
To save money and avoid pushback from workers who don't want to wear devices to avoid lost wages if the device indicates they need to be pulled off the job, Schlader of University of Iowa Bloomington recommends, based on his research , that only the highest risk workers wear devices. Traffic warden Rai Rogers mans his street corner during an 8-hour shift under the hot sun in Las Vegas, Nevada on July 12, 2023, where temperatures reached 106 degrees Fahrenheit amid an ongoing heatwave. Photograph by Frederic J. Brown, AFP/Getty Images
In addition to telling individuals about their own heat risk, wearable technology can also help scientists create algorithms that model the risk of working outside on a given day.
One ongoing project provides wearable biosensors that collect vital signs, measure skin hydration, and analyzes movement data from farm workers in Florida.
Researchers from Emory University in Atlanta and Georgia Tech, using a National Institute of Environmental Health Sciences grant, feed this information to an AI algorithm that may someday accurately predict when a worker could be struck by heat illness.
Experts like Schlader say it's hard to know when these heat-stress technologies will have a true impact on working conditions. It's also difficult to predict when core body temperature measurements will be easily integrated into your Apple Watch or Garmin watch by default, he said.
'To my knowledge, this is a ways off,' he said.
For now, those interested in monitoring their health during periods of intense heat can look for well-documented symptoms, such as cramps, dizziness, nausea, or headaches. Go to the CDC's website for more information on staying safe.