Heat puts stress on nearly every system in your body. When air temperatures climb, your heart works harder, your electrolyte balance shifts, your sleep quality drops, and your risk of serious illness rises. These effects range from mild discomfort to organ failure and death, depending on the intensity and duration of exposure, your age, and your overall health.
How Your Body Cools Itself, and Where It Fails
Your body sheds heat through four main channels. Radiation, the release of infrared energy from your skin, accounts for roughly 60% of total heat loss under normal conditions. Conduction and convection through the surrounding air handle about 15%. Evaporation of sweat covers the remaining 22% or so, burning about 0.58 kilocalories for every gram of water that evaporates off your skin.
When you get hot, your brain triggers a cascade of responses: sweat glands ramp up, blood vessels near the skin dilate to shuttle warm blood to the surface, and your metabolic rate slows to reduce internal heat production. These systems work well within a range. But when the air temperature approaches or exceeds your skin temperature, radiation and convection become nearly useless. And when humidity is high, sweat can’t evaporate efficiently, eliminating your most powerful cooling tool. At that point, your core temperature starts climbing with no reliable way to bring it back down.
People with conditions that reduce sweating are especially vulnerable. Without the ability to sweat adequately, the body has no effective backup for extreme heat.
Strain on the Heart and Circulation
Cooling the body is cardiovascular work. To move heat from your core to your skin, blood flow to the surface increases dramatically, sometimes reaching two to four times its normal volume. For every degree your internal temperature rises, your heart rate increases by about 10 beats per minute. That means a modest two-degree rise already has your heart beating 20 beats faster, even while you’re sitting still.
For a healthy person, this extra workload is manageable in the short term. For someone with heart disease, high blood pressure, or reduced cardiac function, it can push the system past its limits. The heart is simultaneously being asked to maintain normal blood pressure to vital organs while flooding the skin with blood for cooling. That competition for blood flow is why people feel dizzy, lightheaded, or faint in the heat: there simply isn’t enough circulation to go around.
Heat Exhaustion vs. Heatstroke
Heat illness exists on a spectrum, and knowing where you fall on it matters. Heat exhaustion develops when your body is losing the battle against rising temperature but hasn’t yet crossed into dangerous territory. Your core temperature sits between 101°F and 104°F (38.3°C to 40°C). Symptoms include muscle cramps, headache, dizziness, weakness, fatigue, nausea, vomiting, pale skin, and a rapid heart rate. Moving to a cool environment, drinking fluids, and resting typically resolves it.
Heatstroke is a medical emergency. Core temperature climbs above 104°F (40°C), and the body’s cooling systems begin to shut down. The skin may become dry and red because sweating has stopped entirely. The hallmark of heatstroke is neurological: confusion, slurred speech, aggression, hallucinations, seizures, or loss of consciousness. Without emergency treatment, heatstroke can cause permanent organ damage or death. If someone shows these neurological symptoms after heat exposure, call emergency services immediately.
Electrolyte Loss and Dehydration
Sweat doesn’t just carry water out of your body. It carries sodium. During heavy sweating, you can lose 4.8 to 6 grams of sodium, the equivalent of 10 to 15 grams of table salt. One study found that people lost an average of nearly 7 grams of salt in just 1.5 liters of sweat during a single exercise session.
If you replace that lost fluid with plain water but don’t replace the sodium, your blood sodium levels can drop too low, a condition called hyponatremia. This is one of the most common electrolyte disorders in hospitals, affecting roughly 30% of inpatients. Mild cases cause nausea, headache, and confusion. Severe cases can trigger seizures and brain swelling. This is why sports drinks or electrolyte solutions matter during prolonged heat exposure, not just water alone.
Effects on Sleep
Heat doesn’t just affect you while you’re awake. Your body naturally drops its core temperature as part of falling asleep, and a hot environment fights that process directly. Research shows that ambient temperatures above about 84°F (29°C) increase nighttime wakefulness and reduce both deep sleep and REM sleep, the two stages most important for physical recovery and memory consolidation.
The ideal temperature zone for sleep is surprisingly narrow. The microclimate between your body and bedding typically needs to stay around 89°F to 93°F (32°C to 34°C) with 40% to 60% humidity for normal sleep. When the room is too warm, your body can’t cool itself enough to stay in those deeper stages. Adding humidity makes things worse: hot, humid conditions suppress deep sleep even more aggressively while also preventing the core temperature drop your body needs. The result is fragmented, shallow sleep that leaves you unrestored, which compounds the cognitive and physical effects of heat the following day.
Mental Health Effects
Extreme heat doesn’t just make people uncomfortable. It appears to destabilize mental health. A large case-crossover study of US adults with health insurance found that days of extreme heat were associated with higher rates of emergency department visits for mental health reasons. This fits a broader pattern observed across research: heat waves correlate with increased aggression, irritability, and psychiatric crises. The mechanisms likely involve both direct effects on brain chemistry and the indirect toll of disrupted sleep, dehydration, and physical discomfort compounding existing vulnerabilities.
Risks During Pregnancy
Heat exposure during pregnancy carries measurable risks for both the mother and the developing baby. A large systematic review covering 198 studies across 66 countries found that for every 1°C increase in heat exposure, the odds of preterm birth rose by 4%. During heat waves specifically, the odds of preterm birth jumped by 26%. This was the most extensively studied outcome, and 78 out of 84 analyses pointed in the same harmful direction.
The risks extend beyond preterm delivery. High heat exposure was linked to a 48% increase in congenital anomalies, a 28% increase in gestational diabetes, and a 13% increase in stillbirths. Overall, the odds of experiencing any obstetric complication rose by 25% during heat waves. These findings showed a dose-response pattern: more heat meant more risk, consistently across diverse populations and geographic locations.
Why Older Adults Face the Greatest Danger
Aging reduces the body’s ability to thermoregulate. Sweat production declines, cardiovascular responses slow, and the sensation of thirst becomes less reliable, meaning older adults may not drink enough even when they’re becoming dangerously dehydrated. Medications common in older populations, including those for blood pressure and heart conditions, can further impair the body’s heat response.
The numbers reflect this vulnerability starkly. Globally, heat-related mortality among older adults hit a record high in 2023, rising 167% compared to the 1990s. A study of over 73,000 nursing home resident deaths between 2010 and 2023 found that extreme heat significantly increased the odds of death in facilities without air conditioning compared to those with it. Air conditioning isn’t a luxury for this population. It is a survival tool.