Brain Under Fire: The Consequences of Excess Heat Exposure

Austin Perlmutter M.D.

The 2023 summer saw record-breaking heat, with one of the hottest months in recorded history occurring in July. It’s well known that excess heat leads to health problems like heat exhaustion, dehydration, and heat stroke. But higher temperatures may also pose significant risks to our brain health—including higher chances of mental health problems and even may increase the odds of violent behavior.

With climate change increasing the odds of heat waves and other extreme heat events, it’s important to understand how our brains may be influenced by exposure to elevated temperatures. Here’s how excess heat may impact our brains, and steps to help protect your brain against heat-related damage.

Studied Associations Between Excess Heat and Brain Health Outcomes:

  • Exposure to extremes of heat is associated with more cognitive decline in socially vulnerable populations
  • Higher temperatures are linked to a higher rate of violent crime, especially in young men
  • Higher temperatures correlate with higher risk for mood disorders, especially during heatwaves of 3 or more days
  • Warmer indoor temperatures are linked to slower reaction times
  • Heat waves may represent a risk factor for stroke
  • Higher temperatures are linked to an increased risk of dementia hospitalization

What Are the Potential Pathways Linking Exposure to Environmental Heat to Worse Brain Health Outcomes?

1. Increased oxidative stress

Imbalances between oxidation and reduction in our cells are thought to represent a key pathway to a number of diseases, including brain diseases. Oxidation and reduction reactions (collectively called “redox” reactions) are essential to the function of mitochondria, our cellular energy factories.

Neurons are particularly rich in mitochondria, which are key to brain energetics and therefore function. Higher temperatures may promote an imbalance in redox and in doing so damage mitochondria and impair brain health. Additionally, excess oxidative stress may harm the protective lining of the brain, the blood-brain barrier

2. Excess brain inflammation

It’s known that in animal models, high temperatures appear to affect the makeup and function of the immune system. Specifically, excess heat may trigger an inflammatory response.

In humans with heat stroke, there’s evidence for excessive inflammation and a suggestion that inflammation may drive some of the brain symptoms of this condition. While the exact pathways linking heat and inflammation are still somewhat unclear, it’s likely that they involve stress pathways and heat-shock proteins (HSPs).

It’s also thought that excess brain inflammation may suppress brain serotonin; some data have directly drawn connections between heat exposure and serotonin function.

3. Excitotoxicity

To maintain a healthy balance of neuronal activation versus deactivation, levels of a neurotransmitter called glutamate must be closely controlled by our brain cells. In animal models, it has been shown that excess heat may promote an overabundance of available glutamate, leading to a brain-damaging state called “excitotoxity.”

Why Is the Brain So Sensitive to Temperature Fluctuations?

As endotherms (warm-blooded animals) humans regulate internal body temperature. Contrast this with ectotherms (cold-blooded animals), who rely on the outside environment for their temperature. Our core body temperature remains quite constant (around 97-98 °F or 36-37°C regardless of our surroundings. For ectotherms (like an iguana or a snake), core body temperatures rise and fall with the temperature outside.

Being warm-blooded means we’re able to keep our bodies and brains fully operational regardless of what’s happening around us. But it’s also incredibly expensive energetically to make enough heat, and when our temperature regulation system is overwhelmed, things go downhill for our brains very quickly.

Our body’s temperature regulation system is based in our brains. Specifically, it’s housed in the hypothalamus, the “thermostat” of our bodies that constantly adjusts our core temperature.

When we’re exposed to excess heat or cold for too long our bodies’ temperature regulation systems are overburdened. The hypothalamus itself may be damaged (in the context of high or prolonged heat exposure), energy is exhausted (in the context of hypothermia) and the body and brain are damaged.

In response to higher temperatures, our bodies have two major mechanisms for getting rid of excess heat. First, we increase blood flow to our skin. As the blood comes closer to the environment, it dissipates internal heat (almost like blowing on a cup of hot coffee to cool it).

Heat also causes us to start sweating. When sweat is pumped onto our skin, the water evaporates, leading to a cooling effect. When these processes are overwhelmed, excess heat builds up and our brains can be affected.

What About the Benefits of Intentional Heat Exposure, Like Saunas?

Over the last few decades, research has indicated that there may be health benefits realized through intentional exposure to higher temperatures, including better brain health. One study looking at roughly 14,000 adults found dramatically lower rates of Alzheimer’s disease in frequent sauna users (compared to infrequent users).

A number of studies also describe a link between whole-body hyperthermia and reduced depressive symptoms. Interestingly, recent work suggests that one of the drivers of this benefit may be an increase in the inflammatory marker IL-6. How do we reconcile this seemingly conflicting data?

In heat-exposure interventions, including saunas, heat exposure is time-limited, and hydration is encouraged. Additionally, since intentional, therapeutic heat exposure is planned for, there’s an ability to remove oneself from heat if it becomes apparent that the body is being overloaded.

But perhaps most notably, much research around heat exposure focuses on a topic called “hormesis,” or the beneficial effect of short-lived stressors. The concept is that short bursts of heat may actually enhance health resilience through transient and relatively milder activation of stress pathways, including brief increases in inflammation. This would indicate the potential for an inverted “U” shaped curve, where mild and time-limited doses of heat may provide health benefits, while longer and higher heat exposure could lead to significant health consequences.

How Can We Decrease Brain Risks Associated with Excess Heat Exposure?

Independent of brain outcomes, research is clear that our overall health benefits from avoidance of excess heat exposure. Layering in the potential for increasing risk for brain issues only further highlights the value of this concern. Here are practical steps to help mitigate the effects of unhealthy heat on your brain and body:

  • Wear loose/light clothing. This can help to dissipate heat and allow for easier cooling
  • Stay hydrated, and make sure to replace your electrolytes if you’re doing a lot of sweating (note: you probably don’t need to replace your sugar along with your electrolytes, despite what marketing would tell you)
  • Avoid high-energy outdoor activities that may dramatically increase heat-related strain on your body and brain
  • Use indoor fans and air conditioning, or find places where these are available
  • Minimize the use of alcohol, which can exacerbate dehydration and impair your awareness of heat-related symptoms
  • Minimize additional indoor heat from cooking if possible
  • Use your curtains to cut back on extra heat from the sun
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