Explainer: Heat’s Toll on the Human Body

Exposure to extreme heat without proper knowledge and preparation can have disastrous short-term and long-term effects on our bodies.


Being overheated is an uncomfortable feeling most people know. You start to sweat, your face gets flushed and you may feel more lethargic than usual. Now on top of that, imagine having to operate heavy machinery, lacking access to water, or having to wear tight and constrictive protective clothing. For a number of reasons, reactions to heat in the workplace can go way beyond regular discomfort and lead to serious health consequences — even death.

Dr. Brenda Jacklitsch is a research health scientist and occupational heat stress subject matter expert at United States Center for Disease Control (CDC) and National Institute for Occupational Safety and Health (NIOSH), a federal agency overseeing the research and recommendations for the prevention of work-related injury and illness. Jacklistch has spent years of her career researching the toll heat can take on the human body. She said being aware of the risks of heat is more important than ever. 

“Heat stress has always been a concern. But I think it’s becoming more and more of a concern to more people,” she said. “As we are starting to see the impacts of climate in the United States and other continents and countries, what we’re seeing are areas that are now experiencing earlier summers or hotter summers, and more extreme heat events, like heat waves.” 

The way different people react to heat can be hard to predict; different conditions and factors collide to create health risks. Research in this area also still has a long way to go, Jacklitsch said. But here’s what we do know about how a body under heat stress communicates with us. 

🟢 Early indicators*

“Basically anytime somebody gets too hot or they start to dehydrate…you start to suffer and you may not physiologically respond as you should,” Jacklitsch said.

The amount of time it takes for symptoms to become serious is also hard to pinpoint — it’s a cocktail of the “heat burden” on the body (which can vary depending on the person and their acclimatization), plus personal risk factors. 

Thirst: Here’s a good rule of thumb — “if you’re thirsty, you’re already dehydrated,” Jacklitsch said.

Heavy sweating: Sweating is the body telling you that it’s hot and trying to activate some heat loss by causing your sweat to evaporate to cool you down. 

Irritability: Heat exposure can cause behavioral changes, such as becoming easily agitated. 

Headaches/Dizziness/Vertigo: Some people may experience headaches, dizziness or vertigo related to dehydration.

Weakness: Doing normal activities might feel more exhausting.

Urine changes: Pay attention to decreased urine output or changes to the color of your urine (the more dehydrated one is, the darker urine will be). 

* Symptoms like headaches, nausea, vertigo, weakness, thirst, heavy sweating, irritability, and a decreased urine output are common to both heat exhaustion and the early stage of heat stroke, according to NIOSH and the CDC’s “Occupational Exposure to Heat and Hot Environments” report. 

🟡 Acute heat illness

According to Jacklitsch, it starts to become really “worrisome” when someone faints or passes out, or begins experiencing cognitive effects like being confused or slurring their speech. Usually others will be able to tell that something is “very off” with another worker, and this indicates that person might be on the precipice of or already having a heat stroke. But in addition to heat stroke — one of the most serious illnesses associated with heat — there are other symptoms to look out for. 

Heat rash: When sweat gets trapped in the skin, it can cause small papules (red spots) and itching all over the body.

Heat oedema: Causes swelling in the lower extremities (typically the ankles). 

Heat syncope: Fainting or dizziness often affecting those who have been standing for long periods or who have stood up suddenly from sitting or lying down, generally during the first days of exposure to heat.

Heat cramps:  Losing a high level of electrolytes or dehydration can lead to cramping.

Heat exhaustion: Mild to moderate heat-related illness. The main symptoms include an  irregular pulse, shallow breathing, intense thirst, discomfort, weakness, anxiety, fainting and headaches. 

Heat stroke: Very serious illness requiring swift medical attention. Body overheats (temperature can reach 40ºC/104ºF) and the pulse becomes rapid; skin can become dry, red and hot, plus intense headache, confusion and unconsciousness. Some people may also experience nausea, low blood pressure and fast breathing. Damage to internal organs, intestines and muscles may occur because the body reacts during heat stroke with a “generalised inflammatory response”. Severe cases can quickly lead to serious “dysfunction of the central nervous system”; left untreated, this results in death. 

🔴 Chronic heat illness

Research has found that in addition to acute heat illnesses, heat exposure can increase a person’s risk of heart and respiratory diseases, and permanently damage organs such as the kidneys and liver.

Chronic kidney and liver disease: Studies show heat stroke causes an inflamatory response in the body that can injure organs such as the kidney and liver.

Cardiovascular illness: Ties have been made between people who suffered heat illness and a high mortality rate from cardiovascular and ischaemic heart disease.

Respiratory illness: Heat can heighten symptoms associated with respiratory illness. 

Fertility: Research indicates heat can lead to infertility in both women and men, and can cause congenital defects in earlier developmental stages of a fetus. 

Long-term cognitive effects: Although Jacklitsch says this area needs more research, there are studies concluding that heat can negatively impact cognitive performance (including memory, attentiveness, information retention and processing, and response and processing speeds). 

Skin cancer: Workers without protection who are exposed to prolonged heat from UV radiation are at higher risk for skin cancer.

Death: Heat stroke, left untreated, can lead to death. And research has pointed to heat stress and illness leading to overall health problems. 

What makes someone more at risk?

  • Being pregnant. For a number of reasons, research has shown that exposure to extreme heat plus overexerting the body can be harmful to pregnant people. 
  • Taking certain medications. Some medications (such as diuretics, antidepressants, anticholinergics and phenothiazines) can affect your ability to sweat or ability to respond to being in a hot situation, and can lead to dehydration.
  • Physical condition. Certain factors like being overweight, having heart issues, or experiencing other underlying health conditions, can put you more at risk. 
  • Advanced age. If you are 65 or older, you could be more susceptible to heat illnesses.
  • Previous heat related illness. If you have had heat-related illnesses before, you’re more likely to have one again, Jacklitsch said.
  • Wearing PPE. All of the heat the body is naturally creating is getting trapped, plus it is sweating. But due to the PPE, the sweat does not have a way to evaporate. The combination of heat and moisture can lead to serious problems and inhibit the body’s ability to cool itself.
  • Operating hot machinery. Being around hot equipment can make it harder for the body to cool down.
  • Not being acclimatized to the environment. If workers are not acclimatized to their work environment, they are more likely to have heatstroke or heat exhaustion.