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Over the past six years, a handful of companies have developed smart devices for athletes that they claim can detect heatstoke and other heat-related illnesses before they become dangerous. 

Take the temple-thermometer sensor HotHead Technologies, launched in 2009, that was briefly available in some football helmets from Schutt. A similar technology is now used in a cap made by HotHead's successor, Spree Wearables. VaporEze's BodyTemp is a bandage-like strip with a built-in thermometer. And now a startup called SMRT Mouth is creating a sophisticated mouth guard, set to debut in 2016, that will monitor temperature, hydration, and exertion to warn competitors of impending heat illness. 

This sort of technology has the potential to save lives. Every year, about 2,800 people are hospitalized for heat illness, according to a Centers for Disease Control and Prevention study. And while the elderly are the most vulnerable, athlet es are at risk, too: a 2013 study analyzing summer running events in Israel found that participants were 10 times as likely to suffer heatstoke (the most serious heat-related illness) as any cardiac event—although risk for either was still low. Every year, some 9,000 student-athletes suffer some type of exercise-related heat illness, with the highest rates in football. Heatstroke causes organ failure and is life threatening if not treated quickly. In the Israeli study, two of the 21 heatstroke cases were fatal, and a 2014 paper by the University of North Carolina's National Center for Catastrophic Sport Injury Research noted that there's an average of 2.6 football player deaths per year due to exertional heat stroke. In some severe but non-fatal incidents, there can be lifelong changes to organ function.

SMRT Mouth claims it s mouthguard, which goes on sale next year, can detect heatstroke.   Photo: SMRT Mouth

But here's the catch: none of the tools on the market work, at least not yet, says Douglas Casa, one of the U.S.'s foremost experts on exertional heat illness and the director of the University of Connecticut's Korey Stringer Institute. The only way to accurately identify heat stroke early, he says, is to measure core temperature, which none of these devices are capable of doing. 

Our bodies cool through the skin, via conductive (when heat radiates away from the skin) and convective (when sweat carries heat to the surface and then evaporates) means. But as soon as air temperatures rise past 98 degrees, we lose our conductive ability and rely entirely on evaporating sweat to cool down. Heatstroke occurs when our bodies are no longer able to shed heat. "Despite the fact that we define heat stroke as when your temperature goes past 104 degrees, this is not solely a temperatur e event," says Dr. Lisa Leon, a research physiologist with the U.S. Army's Research Institute for Environmental Medicine. "It's a cardiovascular event." What ultimately causes collapse and the most serious heat injury is too much strain on the cardiovascular system.

It's normal for body temperature to rise during exercise, says Casa. Our normal resting core temperature of 98.6 degrees increases when we work out, particularly when we're outside in warm weather. Conditioned and acclimatized athletes can typically handle periods of exercise-induced hyperthermia—where body temperature greatly exceeds normal. "This morning, I went for a run at 10 a.m. and I went hard for an hour and a half. My core temp when I finished was 104.3," he says. "When we worked with soccer players in the World Cup, it was pretty normal for them to get to 103.5 or 104.5 during practice or games."

The problem is when core temperatures rise above 104 degrees and stay elevated for longer than 30 minutes. "The great thing about the human body is that you have about a 30-minute window to be over that critical threshold with no long-term complications. A highly trained athlete who goes hyperthermic in a half-marathon, they know that if they have a half-mile left, they can get hot for those last minutes as long as they can immediately cool down afterward."

So, 104 or less, for less than a half hour. Simple, right?

Not really, say Casa and Leon. The only way to accurately tell if you're pushing the 105-degree mark is to measure core temperature. The accepted standards are rectal and esophageal, both of which are impractical for athletes to monitor during practice or games. Ingestible thermometers that transmit wirelessly, called thermistors, can be accurate, but only once they've passed into the intestine, says Casa. Otherwise, cold fluids can interfere with the reading. And that's where most of the devices mentioned above fall short.

No skin- or mouth-temperature readings have been found to consistently correlate to core temperature, says Casa, and he cautions against relying on them to monitor heat stress. "We've found nothing else that works," besides directly measuring core temp, he says. Leon says that USARIEM has in some situations been able to use heart rate changes to predict core temperature, but she reiterates her point that heat stroke, particularly in relation to exercise, is a more complex issue than that one metric can show.

That's the second reason avoiding heat injury isn't as simple as tracking core temp. Especially when dealing with exertional heat stroke "there are so many factors that go into your susceptibility," Leon says. Are you acclimatized to heat? Are you fit or do you have extra weight that makes it harder to cool yourself? Do you have a pre-existing heart condition that limits your cardiovascular system's ability to shunt heat from the core to the skin? � �Multiple individuals at exactly the same core temperature experience heat injury to different degrees of severity," she says. "One person may be fine at 104 degrees and another is collapsing at not much over 100."

Of the various devices listed above, SMRT Mouth seems the most promising, partly because it won't rely exclusively on temperature monitoring. Cofounder Dana Hawes says that the device will measure pulse rate, exertion based on exhalation force, and hydration, using fluid osmolality of saliva. The data will be sent to a proprietary application on a tablet using low-power wireless transmission. "The triangulation of those data points help paint a clearer picture for what is happening with the athlete," he says. The idea is to let coaches and trainers monitor athletes and "provide a predictive tool" that helps prevent heat injury.

But the device is still very much in prototype stage and Hawes admits that SMRT Mouth needs to go through extensive testing to show that it can accurately measure what it claims to measure and that the links to heat illness are consistent and clear. 

As director of the Stringer Institute, Casa has spoken with a number of companies in SMRT Mouth's position and is skeptical of their broader claims. "I do think this has potential to help with hydration," he says, "but the leap to body temperature is less relevant. I don't know that a mouthpiece can give you an accurate indication of core body temp when someone just chugged a liter of 52-degree Gatorade." He also points out that hydration is only partly related to heat illness. "A one percent loss in body mass due to dehydration will increase your core body temperature by about half a degree," he says. But dehydration isn't a prerequisite. "You can have heatstroke without being dehydrated," he says, noting that he's seen runners with heatstroke at the New York marathon in temperatures as low as 65 degrees. In fact, the 2014 football fatality report from the National Center for Catastrophic Sport Injury Research found that two athletes died from over-hydrating to prevent heat illness. That's where SMRT Mouth's multiple data points may help the most.

Brett Ely, a Graduate Teaching Fellow at the University of Oregon and former researcher at USARIEM, has co-authored several studies on using saliva osmolality to track hydration and says that, as long as you recognize the limits of the method, it's a viable metric. "But it doesn't work in terms of absolute thresholds," she says. Even a sip of water can throw off short-term measurements. The key is to get lots of data by tracking over time—within the workout and over many workouts. She also points out that even if it isn't directly related to heat injury, tracking hydration alone can be a valuable tool. To an observer, "the symptoms of dehydration and over-hydration look about the same," she says. "The saliva osmolalit y could give you an indication of which it is so you know how to respond." Ely added that one of the biggest benefits might be simply the mental shift that accompanies tracking data. "It's similar to heart-rate monitors, where the awareness that you're monitoring can be a reminder to take your easy day. This might serve as a reminder to take more breaks."

Leon adds that she's cautiously optimistic about tracking devices, if only for the reason that they can increase awareness. "We have specific guidelines we give to soldiers, and the basics are to know your environmental conditions, and the work you're performing, and most of all, know your physical ability."

While Casa and Leon say no wearable can currently monitor heat stress accurately, both are hopeful that will change soon. "I love the potential," says Casa. "Someone is going to solve this problem in the next five years." 

We're just not there yet.

Your best bet at preventin g heat illness? Avoiding it in the first place. Prevention comes down to these five basic rules: 

Acclimatization

"Most heat illnesses happen in the first two to four days of exercise in the heat. Ramp up your exposure over a two-week period: the body is capable of some pretty amazing physiological changes to adapt," says Casa.

Hydrate

Drink lots of fluids and not just water—you need electrolytes, too. "We see functional changes in how the body performs with dehydration around two percent loss of body mass," says Ely, adding that you can reach that in as little as an hour of hard exercise. Dehydration is progressive, so if you're only exercising an hour, you won't notice the effects, but if your workout is longer and you're not replacing fluids, the changes get increasingly severe.

Cool Yourself

Go for any tools you can use to lower your core temperature—shade, cold, wet towels during breaks, misting fans, and post-workout cold-immersion tubs .

Modify the Workout

"You can do the same intensity for a shorter period, or with more rest breaks, or you can do less intensity," says Casa. This is key with structured workouts in team sports where athletes are asked to do a certain workout over a certain time. A 220-pound guy with almost no fat and a 320-pound lineman doing shuttle runs for 30 seconds at full speed are working at vastly different intensities. Take frequent breaks when it's hot, and employ those cooling options.

Be Aware

Leon calls it paying attention to "skin in/skin out" conditions. Skin in consists of knowing your fitness and capabilities. Skin out is everything from what you're wearing (like football pads or black clothing that absorbs heat) to the environmental conditions. In team sports, awareness needs to be collective, as teammates and staff look out for each other. The standard for assessing weather that poses a heat injury risk is a wet bulb globe thermometer, which is a relatively expensive piece of equipment but likely a good investment for any decent-sized organized exercise program like a high school athletics department or even large running club. Casa is dismissive of old-school coaching techniques where athletes are assigned to run laps or do other hard conditioning work as punishment. Coaches need to move past the workout-as-punishment mindset, he says, which can also make athletes less likely to speak out about being in discomfort.

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