Heat waves are becoming amplified as the climate changes – lasting longer, becoming more frequent and getting just plain hotter. 

Death Valley hit a temperature of 128° Fahrenheit on July 16, 2023 – not the world’s hottest day on record, but close. Phoenix broke a record heat streak with 19 straight days with temperatures above 110° F, and had more in the forecast, accompanied by several nights that never got below 90° F. Globally, Earth likely had its hottest week on modern record in early July 2023.  Experts are increasingly weighing in on what global warming means for the future of the planet, but what does it mean for us right now.  At what point is it unsafe for a healthy person to engage in normal outdoor activity? 

How Hot is Too Hot? 

Heat exposure is generally measured by calculating a “wet-bulb” temperature, which takes into account both heat and humidity.  For example, a wet-bulb temperature of 95° F is equal to a temperature of 95° F at 100% humidity, or 115°, F at 50% humidity.  As a matter of fact, a wet-bulb temperature of 95 has generally been considered the upper heat limit for normal healthy human functionality. 

For the past several years Penn State University has been conducting controlled studies to determine exactly what happens in the healthy human body at different wet-bulb temperatures.  It’s called the HEAT project and researchers have healthy young subjects swallow a small telemetry pill that continuously monitored their deep body or core temperature. They then sat in an environmental chamber, moving just enough to simulate the minimal activities of daily living, such as showering, cooking and eating. It turns out we start losing functionality at wet-bulb temperatures below 95. 

How our Bodies Respond to Heat 

You most likely know that your internal body temperature should be about 98.6. But do you know why? It’s because that’s the ideal temperature where your body doesn’t have to adjust by exerting extra energy to warm up or cool down. This “thermoneutral” zone’s lower limit is 82.4 F, at which time the body must expend energy to maintain its ideal temperature, utilizing mechanisms such as shivering. At higher temperatures, the body uses cool-down mechanisms such as sweating and vasodilation of blood vessels at the skin surface to increase heat loss through evaporative cooling. 

Studies show that when the human body is exposed to temperatures greater than 104 F with 50% humidity, metabolic rate is increased by almost 50% (the reason many people suffer through hot yoga!). Unfortunately for some people, this is also the point where the body can lose the ability to get rid of excess heat and perform vital functions. 

When ambient temperatures are high, the body uses mechanisms to cool down, such as sweating and vasodilation of blood vessels at the skin surface to increase heat loss and evaporative cooling. Interestingly, researchers noted that drinking water did not help to cool the body, but most definitely helped with the dehydration effects that accompany high heat and humidity. 

Overexposure To Heat Is Categorized into Three Levels: 

Early signs of general heat illness include having cool, pale or even flushed skin as well as headache, lightheadedness or weakness.  After that, from least to most serious, these are the three generally accepted categories of overexposure:  

1. Heat cramp 

Although hard on the body, heat cramp does not require medical attention. It tends to subside with plenty of rest and rehydration. Symptoms include elevated body temperature, dizziness and muscle pain or stiffness. 

        2. Heat exhaustion

Heat exhaustion requires medical attention if the symptoms last longer than an hour or worsen over time. In addition to the symptoms of heat cramp, a person may experience vomiting, headache, reduced concentration or impaired judgment.  

        3. Heatstroke 

Heatstroke is a true medical emergency and is often fatal if left untreated. The symptoms of heatstroke include the symptoms of heat cramps, as well as seizures, difficulty maintaining consciousness and liver failure. 

Who Is at Risk of Overheating? 

As the HEAT Project showed, anyone will overheat at certain wet-bulb temperatures which are becoming increasingly common this summer.  But adults over 65 years, children, and people with heart or lung problems have the highest risk of overheating. Also, anyone without easy access to means of cooling down is also more at risk. 

Which Is Worse – High Temperatures or High Humidity?  

It’s a toss-up.  While humid heat is more dangerous than dry heat because it prevents your sweat from evaporating and cooling you down, both are uncomfortable. Dry heat often results in the ground, buildings, and objects sitting in the sun and heat to retain temperatures into and over 150 F. 

How Persistent Heat Can Lead to Chronic Health Problems  

While much of the focus on extreme heat is on the immediate risks to human health, chronic heat can cause a range of chronic health problems. 

• Heart Health 

 Your ability to thermoregulate in the face of heat relies primarily on your cardiovascular system and kidneys, increasing the stress on these systems. When your body overheats, the blood vessels in your skin dilate, sending hot blood to the surface to cool off. But this can also cause blood pressure to drop, which can be dangerous for people with heart disease. In response to low blood pressure, the heart rate increases, thereby increasing strain on the heart, which is especially worrisome in people with an underlying heart condition. Statistics show that about 50% of deaths that occur during heat waves are due to heart attacks. 

• Kidney Health 

 Sweating is the body’s favorite and primary cooling mechanism, but sweating causes dehydration if the fluids are not replenished. As dehydration develops and progresses, it places more and more strain on the kidneys, which filter waste and excess fluid before removing it from our body. Over time, chronic dehydration and heat stress can lead to kidney damage and chronic kidney disease. 

• Mental Health 

Warmer temperatures are associated with higher rates of mental health ER visits, especially for substance use disorders, mood disorders and self-harm. 

Heat also affects sleep quality, which has an impact on both physical and mental health. A recent study estimated that we are losing an average of 44 hours (about 2 days) of sleep each year because of hotter nights. 

You Can Adapt to the Heat — Up to a Point 

The body, in general, will always strive for homeostasis, or stability throughout varying conditions. For example, people who live in warm climates tend to acclimatize and therefore won’t increase their body temperature and metabolic rate when exposed to high heat. However, if we limit ourselves to air-conditioned environments, our bodies will never really be able to acclimatize to warmer temperatures.  

Just as you can train your muscles to tolerate more weight, you can train your cardiovascular system to tolerate more heat. Studies show that heat acclimatization takes at least seven days.  Acclimatization speeds up and increases sweating, allowing more evaporative cooling, and the more physically fit you are, the more likely you are to withstand the heat. 

Also, studies show re-adapting to the heat occurs much faster than the initial adaptation, meaning the human body can remember how to adapt to the heat if it has done it before. Additionally, exposing yourself to low heat over longer periods of time can increase your resilience to future heat exposure. One simple way to acclimatize to heat is to spend time outdoors every day at the same time to adjust to the high temperatures. 

The Bottom Line: 

 Exposure to high temperatures and humidity can become uncomfortable, and potentially unhealthy. To stay safe, make sure to monitor your time in the sun, and retreat to a cool place, out of direct exposure to cool off. Wear loose-fitting clothing and a hat, stay hydrated and avoid being enclosed in hot spaces, like a vehicle. By being aware of your body’s heat response and taking simple steps, you and your loved ones, especially young children or older adults, can stay safe while enjoying the summer heat.