EHS most commonly stands for two very different conditions: exertional heat stroke, a life-threatening medical emergency caused by the body overheating during physical activity, and electromagnetic hypersensitivity, a disputed condition in which people report symptoms they attribute to electromagnetic fields from devices like phones and Wi-Fi routers. The causes, evidence, and medical standing of each are distinct, so both are covered below.
Exertional Heat Stroke: When the Body Can’t Cool Itself
Exertional heat stroke happens when intense physical activity generates more internal heat than your body can shed. During vigorous exercise, your muscles produce heat at 15 to 18 times the resting metabolic rate. Most of that heat moves from muscles into the blood and travels to the body’s core. Without effective cooling, this level of heat production could push core temperature from a normal 37°C (98.6°F) to a dangerous 42°C (107.6°F) in roughly 25 minutes.
Your body normally cools itself by routing blood to the skin and producing sweat. This depends heavily on the cardiovascular system’s ability to manage competing demands: supplying blood to working muscles while also pushing blood toward the skin for heat release. When that system gets overwhelmed, or when environmental conditions make sweating less effective, heat builds up faster than it can escape. The result is a core temperature that climbs dangerously high, paired with central nervous system dysfunction like confusion, loss of coordination, or collapse.
Hot and humid environments make exertional heat stroke more likely, but they aren’t strictly required. The sheer volume of internal heat from intense exercise can trigger it even in moderate weather. Risk climbs significantly when the Wet Bulb Globe Temperature, a measurement that factors in heat, humidity, and sun exposure, exceeds 27°C (about 81°F). Above 31.4°C (roughly 88.5°F), conditions are classified as “danger” level for heat-related illness.
Genetic Factors in Heat Stroke Risk
Some people are genetically more vulnerable to exertional heat stroke. Variants in the RYR1 gene, which controls a calcium-release channel in skeletal muscle, are the best-studied genetic risk factor. These same variants are linked to malignant hyperthermia, a dangerous reaction to certain anesthesia drugs. In one study of heat stroke cases, about 30% of patients carried disease-causing or potentially disease-causing genetic variants across 14 different genes. Beyond RYR1, researchers have identified associations with genes involved in muscle disorders, heart muscle function, temperature sensing, and the cell’s ability to produce energy. One variant in a temperature-sensitive ion channel called TRPM4 was shown to impair the channel’s ability to respond to heat, pointing to a specific molecular pathway that could make someone more susceptible.
Other Risk Factors
Dehydration, poor fitness, sleep deprivation, illness, and certain medications that affect sweating or blood flow all raise the risk. So does wearing heavy clothing or equipment, which is why exertional heat stroke disproportionately affects military recruits, football players, and outdoor laborers. A previous episode of heat stroke also increases vulnerability to future episodes, possibly because of lingering changes to the body’s thermoregulatory responses.
Electromagnetic Hypersensitivity: What the Evidence Shows
Electromagnetic hypersensitivity refers to a collection of symptoms, including headaches, fatigue, difficulty concentrating, sleep problems, and skin tingling, that some people attribute to nearby sources of electromagnetic fields such as cell towers, Wi-Fi routers, power lines, or smart meters. Self-reported prevalence varies widely by country, ranging from about 1.5% of the population in Sweden to as high as 13% in Taiwan, with most estimates falling between 2% and 5%.
The central scientific question is whether EMF exposure actually causes these symptoms. The World Health Organization’s position, based on the weight of available research, is that it does not. In well-controlled double-blind studies, where neither the participant nor the researcher knows whether EMF is on or off, people who identify as electromagnetically hypersensitive cannot detect the presence of EMF any more accurately than anyone else. Their symptoms do not correlate with whether the fields are actually active.
This does not mean the symptoms aren’t real. People with this condition genuinely experience discomfort and functional impairment. The question is what’s driving those symptoms if not the electromagnetic fields themselves.
The Nocebo Hypothesis
The most widely discussed psychological explanation is the nocebo effect, essentially the opposite of a placebo effect. If you believe something will harm you, that belief alone can produce physical symptoms. Because people with electromagnetic hypersensitivity consistently report symptoms based on whether they think EMF is present rather than whether it actually is, researchers proposed that the condition originates from nocebo responses.
The picture is more complicated than that, though. A qualitative study tracing how the condition develops over time found that symptoms typically appear before people start suspecting EMF as the cause. In other words, people first feel unwell, then later come to blame electromagnetic fields. This sequence doesn’t fit a straightforward nocebo explanation, since you’d need to already believe EMF is harmful before it could trigger nocebo symptoms. However, once someone has made the connection between their symptoms and EMF, nocebo responses may reinforce and amplify the pattern going forward. Other psychological mechanisms, including anxiety and heightened attention to bodily sensations, likely play a role as well.
Proposed Biological Mechanisms
A smaller body of research explores whether some individuals might have a genuine biological sensitivity to EMF. The leading hypothesis centers on oxidative stress, which is an imbalance between harmful free radicals and the body’s ability to neutralize them. Multiple labs have demonstrated that EMF exposure produces a mild, temporary increase in free radicals inside cells. In most people, the body’s antioxidant defenses quickly neutralize these. But in individuals with weakened antioxidant protection, the theory goes, this same mild increase could accumulate and cause real cellular damage.
One case report documented a patient with electromagnetic hypersensitivity who showed measurable deficits in cellular antioxidants alongside elevated levels of radical-scavenging enzymes, consistent with chronic oxidative stress. The researchers proposed that in this patient, EMF-induced free radicals were triggering a chain reaction involving oxidized cholesterol particles and immune activation. This mechanism also aligns with earlier observations about EMF affecting calcium signaling within cells, since free radicals are known to influence calcium flow.
These biological findings are preliminary. They come from individual case reports and mechanistic studies rather than large controlled trials, and they haven’t changed the scientific consensus. The condition currently has no code in the ICD-11, the international system used to classify diseases, which effectively means it has no formal diagnostic recognition in mainstream medicine.
Why the Distinction Matters
If you searched “what causes EHS” after hearing the term in a sports or military context, the answer is thermoregulatory failure during intense physical activity, sometimes compounded by genetic susceptibility, environmental heat, or dehydration. It is a medical emergency requiring immediate cooling.
If you searched because you or someone you know experiences symptoms around electronic devices, the evidence points to real symptoms whose cause remains debated. Controlled studies consistently fail to link them to EMF exposure itself, but the symptoms are not imaginary, and the mechanisms behind them, whether psychological, biological, or some combination, are still being investigated.