Exposure to sunlight benefits physical and mental health by stimulating the body to produce Vitamin D. However, prolonged exposure to the hot summer sun can place too much stress on the body, which negatively impacts cognitive function. While the effects are usually temporary, permanent cognitive impairment is possible in severe cases.
Heat stress induced impairments to cognitive and motor functions increase the risk for accidents [1; 2]. The impact of heat stress on performance is related to both the complexity of the task and the ability of a person’s body to maintain a constant internal temperature, called thermoregulation [3]. Performance on simple tasks is relatively unaffected, while complex tasks that require sustained attention are more heavily impacted. Significant impairments to cognitive and motor performance become apparent when the internal body temperature reaches 101.3oF (38.5oC) [2]. Sustained neurological impairments can occur under conditions of heat stroke, when the internal body temperature reaches 104oF (40oC). Individuals who have a difficult time dissipating heat are more likely to experience a rise in internal body temperature, called hyperthermia, following heat exposure [3]. Since the cognitive effects are largely related to changes in internal body temperature, these individuals are more likely to be vulnerable to heat-related cognitive dysfunction.
Age plays a major role in determining an individual’s ability to withstand heat stress. A variety of age-related changes reduce the ability of the body to regulate internal temperature. These include a reduced capacity to sweat and the decreased ability to divert blood flow toward the skin and away from the core [4]. Certain blood pressure medications (e.g. vasoconstrictors, beta blockers, and diuretics) can also impair thermoregulation [5]. As a result, older adults are more likely to experience cognitive dysfunction following heat exposure and are at a greater risk for severe reactions like heat stroke.
Heat stroke induces a series of responses through the brain and body which produce neurological effects that can lead to long-term cognitive impairment. In response to overheating, there is a disruption in the blood-brain-barrier? which allows for the infiltration of immune cells into the brain, leading to neuroinflammation [6; 7]. In order to shunt heat, there is also a decrease in blood flow to the brain, which can starve it from the oxygen and nutrients it needs to function properly. These changes can lead to acute neurological effects, including confusion, loss of consciousness, and seizures [7]. If the brain/body is not cooled down quickly enough, there can be permanent changes to attention, memory, and personality.
People with reduced capacity for temperature regulation also have increased risk for heat-related cognitive impairment in response to heightened temperature variability, even when the heat does not reach extreme levels. A study involving Medicare-enrolled individuals living in New England found that each 2.7oF (1.5oC) increase in summer temperature was associated with a 12% increased risk for dementia-associated hospitalization [8]. A similar effect was seen in years when there was a greater degree of temperature variability, suggesting that when the ability to regulate internal body temperature is impaired, changes to external temperature can place undo stress on the brain, and facilitate pathogenic processes.
With more extreme seasonal temperature fluctuations due to climate change, it has become increasingly important to take precautions against heat stress. Some of these include avoiding sun exposure at peak mid-day hours, wearing loose light-colored clothing, and avoiding strenuous physical activity in the heat [9]. Hydration status can also impact cognition, and remaining hydrated can mitigate some of the negative effects associated with heat stress. When conditions reach heat advisory levels, access to air conditioning is critical to prevent overheating, as fans alone are insufficient to cool temperatures to safe levels during extreme heat. In the event of suspected heat stroke, it is vital to seek immediate medical attention to ensure the best chance for a full cognitive recovery.
Betsy Mills, PhD, is a member of the ADDF's Aging and Alzheimer's Prevention program. She critically evaluates the scientific evidence regarding prospective therapies to promote brain health and/or prevent Alzheimer's disease, and contributes to CognitiveVitality.org. Dr. Mills came to the ADDF from the University of Michigan, where she served as the grant writing manager for a clinical laboratory specializing in neuroautoimmune diseases. She also completed a Postdoctoral fellowship at the University of Michigan, where she worked to uncover genes that could promote retina regeneration. She earned her doctorate in neuroscience at Johns Hopkins University School of Medicine, where she studied the role of glial cells in the optic nerve, and their contribution to neurodegeneration in glaucoma. She obtained her bachelor's degree in biology from the College of the Holy Cross. Dr. Mills has a strong passion for community outreach, and has served as program presenter with the Michigan Great Lakes Chapter of the Alzheimer's Association to promote dementia awareness.
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