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New research shows heat exposure may impair immune function and increase inflammation, which could raise the risk of heart disease. PraewBlackWhile/Getty Images
  • New research shows heat exposure may impact the immune system’s ability to fight pathogens like viruses and bacteria, which may lead to inflammation.
  • The findings suggest impaired immune function and increased inflammation could raise the risk of heart disease in some individuals.
  • To stay safe during hot weather, avoid the sun during peak hours, stay hydrated, and use sun protection.

New research shows how exposure to hotter temperatures over a short duration could affect the immune system and drive inflammation.

These effects could make the body more vulnerable to infections and accelerate the development of cardiovascular disease.

The research underscores the potential dangers of heat exposure as global temperatures continue to rise.

The findings were presented during the American Heart Association’s EPI|Lifestyle Scientific Sessions 2024 in Chicago March 18–21.

“Most research only considers temperature as the exposure of interest, which may not be adequate to capture a person’s response to heat,” lead study author Daniel W. Riggs, PhD, an assistant professor of medicine in the Christina Lee Brown Envirome Institute at the University of Louisville in Louisville, KY, said in a news release.

“In our study, we used alternative measurements of heat in relation to multiple markers of inflammation and immune response in the body to investigate the short-term effects of heat exposure and produce a more complete picture of its health impact,” Riggs said.

For this study, researchers looked at blood samples of Louisville participants to analyze immune system function and how it was affected by heat.

Effects of heat exposure included temperature, net effective temperature (taking into consideration relative humidity, air temperature, and wind speed), and the Universal Thermal Climate Index (UTCI).

The findings show an increase in inflammation markers for every 5-degree increase in UTCI.

These markers included increases in immune cells involved in inflammation, such as monocytes and eosinophils, as well as rises in certain pro-inflammatory cytokines, proteins that promote inflammation in the body.

There was also a decrease in B-cells, which help the immune system identify and fight pathogens like viruses and bacteria.

These findings suggest that increases in inflammation and reduced immune function due to heat exposure may increase susceptibility to infections and could worsen cardiovascular disease.

The new research explores a link between elevated body temperatures resulting from heat exposure and the body’s ability to fight off infection.

This should not be confused with fever, however, which is how the immune system responds to bacterial or viral infection or inflammation.

“Our immune system response to infection raises the temperature of the body above the normal state,” Dr. Hilary Fairbrother, emergency medicine physician with UTHealth Houston, told Healthline.

“There is no known connection between an elevated body temperature and the ability to fight off an infection. It is this very theory that this article was considering on a cellular level,” Fairbrother noted.

Dr. Anjali Bharati, emergency medicine physician at Lenox Health Greenwich Village, explained that environmental heat exposure could worsen an existing inflammation or infection.

“Elevating the body’s temperature too high can impair the immune system’s ability to focus on fighting the infection. In the study, it implies that this known effect can be compounded by additional risk to the cardiovascular system,” Bharati told Healthline.

When the body is overexposed to extreme heat, heat-related illnesses may occur, ranging from mild to severe.

“Mild heat-related illnesses can cause a heat rash or a mild sunburn. More severe sunburns can cause dehydration due to the water loss from the burned skin,” Bharati explained.

Common side effects of heat exposure may include:

  • Heat cramps: Overexposure to heat may cause painful muscular cramps, which may occur during excessive exercise or environmental exposure.
  • Heat exhaustion: Dizziness, nausea and vomiting, and heavy sweating may occur during heat exhaustion. People may develop cold and clammy skin and report weakness, fatigue, and decreased energy levels.
  • Heat stroke: This is a serious heat-related illness that occurs when the body’s temperature exceeds 103°F. Bharati said symptoms may include hot red, dry skin, nausea, vomiting, confusion, disorientation, and loss of consciousness.

To help prevent such outcomes, the body is designed to regulate itself when exposed to heat, producing sweat to help cool its core temperature.

‘We produce a water-based fluid on our skin, and this helps us use evaporation to get rid of excess heat that builds up in our body,” Fairbrother explained.

“Another mechanism that our body has is to direct blood to our skin, giving us a flushed appearance, and this allows for a higher amount of heat to escape outside of the body. Finally, our body also can slow its metabolism to decrease the amount of heat generated by the person’s body,” Fairbrother noted.

When environmental heat exposure becomes too hot, however, these compensatory mechanisms may begin to break down, failing to effectively cool the body’s core temperature.

As a result, the body’s temperature could rise to life threatening levels. Fairbrother said in severe cases of heat exposure, body temperatures well above normal may result in seizures or coma.

Above 107°F, the body begins to shut down, and the organs fail, which may result in death, Fairbrother added.

Young children and older adults are particularly susceptible to heat-related illnesses.

“Young babies under the age of 2 have decreased thermal regulation. This means that their bodies cannot always self-adjust to the effects of environmental temperature,” Bharati said.

Inflammation caused by environmental heat exposure may affect the aging population in particular.

Heat has additional risks to people with existing heart disease and may be an independent risk factor for heart disease, Bharati noted.

Additionally, people with multiple health conditions, such as a weak heart, may be more susceptible to the effects of heat on the body, Fairbrother explained.

“Findings of the study, although limited, could impact how we counsel patients about their heat exposure,” Bharati said.

To minimize the risk of heat-related illness, Bharati recommends the following:

  • Stay out of direct sun during peak hours, 12 p.m. to 4 p.m.
  • Avoid exercise or activities outdoors during peak sun. Try to exercise in the early morning or evening.
  • Keep hydrated and cool by drinking water and using a cooling fan or sprays when outside, and rest in shaded areas when spending time outside.
  • Wear a hat, loose fitting and lightweight clothing. Avoid clothing layers.
  • Wear sunscreen and sun protective clothing.
  • Avoid high sugar and alcohol to prevent dehydration.

Overexposure to heat can affect the immune system’s ability to recognize specific pathogens, which can cause inflammation and raise heart disease risk.

Extreme heat could elevate the body’s temperature to unsafe levels, which can negatively impact the immune system’s ability to combat viruses and other germs.

Young children, older adults, and people with heart disease and other chronic health conditions are most vulnerable during extreme heat.

Experts suggest avoiding the sun during peak hours, staying hydrated, avoiding alcohol, and using sun protection.