Long COVID Heat Intolerance: Why Heat Makes Symptoms Worse

Long COVID heat intolerance is one of the most common but least understood symptoms experienced by people living with Long COVID, POTS and dysautonomia.

Everyone else thinks the heatwave is great. You are lying very still with the curtains shut wondering why a warm sunny day feels like a physiological attack.

You are not being dramatic. There is a clear biological reason why heat hits people with Long COVID, POTS, ME/CFS and dysautonomia so much harder than it hits everyone else. Understanding it will not make the heat more comfortable, but it does mean you can stop blaming yourself for struggling and start making decisions based on what is actually happening in your body.

Why heat is different when your autonomic nervous system is disrupted

In a body with a healthy autonomic nervous system, heat triggers a well managed cascade. Blood vessels near the skin dilate to release heat. Sweat glands activate to cool the surface. Blood pressure and heart rate adjust to compensate for the extra demand. The system handles it so smoothly you barely notice.

In Long COVID with dysautonomia or POTS, that cascade does not work reliably.

Blood vessels dilate in response to heat just as they should. But the compensatory signals that should follow, the ones telling blood vessels elsewhere to constrict, the heart to adjust, the sweat glands to activate, are unreliable or delayed. Blood pools in peripheral vessels and the lower body. Less returns to the heart. Blood flow to the brain drops. The heart rate spikes trying to compensate for a problem the rest of the system is not helping to solve.

This is why standing up during a heatwave with orthostatic intolerance feels like a medical event. The heat has already reduced your circulatory reserve. Standing up takes whatever margin remained.

Dr Svetlana Blitshteyn, director of the Dysautonomia Clinic at the University of Buffalo, has highlighted that many people with POTS have sweat glands that are damaged or function differently, meaning the body cannot cool itself through sweating in the first place, as reported in this Sick Times piece on Long COVID, POTS and climate change. When sweating is impaired, internal heat builds faster than it appears externally. You can look entirely fine and be genuinely overheating.

Heat as a hidden energy expenditure

Heat is also a trigger for post exertional symptom exacerbation, which catches many people off guard.

The body treats heat as a physical stressor and burns energy managing it. A hot day spent lying still uses more physiological resources than a cool day doing the same nothing. This is why a heatwave can produce a crash the following day even when you feel as though you barely moved.

The delay is what makes it confusing. You managed the hot day. You paced carefully. You rested. And then the next morning you wake up significantly worse and cannot immediately connect it to the heat because it was yesterday. This is the same delayed post exertional pattern that occurs after physical overexertion, just triggered by a different stressor, as covered in more detail in our article on post exertional malaise in Long COVID.

Why Humidity Can Be Even Worse Than Heat

Many people notice that humid weather makes Long COVID symptoms worse even when the temperature is not exceptionally high. This is because humidity reduces the body’s ability to cool itself through the evaporation of sweat.

Normally, sweat evaporates from the skin, carrying heat away and helping to regulate body temperature. In humid conditions, sweat remains on the skin instead of evaporating efficiently. As a result, the body struggles to release heat, core temperature rises more quickly, and the cardiovascular system has to work even harder to compensate.

For people living with Long COVID, POTS, dysautonomia or ME/CFS, this additional physiological stress can significantly worsen symptoms such as dizziness, fatigue, brain fog, palpitations, breathlessness and post-exertional malaise. Many people find that a humid 28°C day feels considerably more disabling than a dry 32°C day because the body’s normal cooling mechanisms become much less effective.

This is why weather forecasts that include heat index, feels-like temperature or humidity levels can often be more useful than the air temperature alone when planning activities.

Which conditions are most affected and why

Long COVID with POTS or dysautonomia. Heat is one of the most consistent and most disabling triggers. A 2025 review in JACC Advances confirmed that POTS is among the most common autonomic presentations in Long COVID, and heat sensitivity is a core feature of virtually all POTS subtypes, as documented in this review of POTS patterns in Long COVID. Climate change adding more extreme heat days each year makes this a worsening public health issue for this group, not a seasonal inconvenience.

ME/CFS. Reduced blood volume, cardiovascular instability, and thermoregulatory dysfunction that closely mirrors Long COVID mean that the extra demand of a heatwave can push people into significant crashes. The physiology here overlaps substantially with Long COVID.

Fibromyalgia. The central nervous system dysfunction underlying fibromyalgia affects temperature regulation. Around 80 percent of people with fibromyalgia report weather changes as a worsening factor, and heat specifically exacerbates fatigue, pain and sleep disruption.

Multiple sclerosis. Heat worsens MS symptoms through a well documented mechanism called Uhthoff’s phenomenon, where a rise in core body temperature temporarily slows nerve conduction. This effect can be reproduced reliably and was historically used as a diagnostic test.

Lupus and inflammatory conditions. Heat can trigger immune activation and inflammation, and sun exposure adds photosensitivity reactions for some people.

The medication factor nobody mentions

Many commonly used medications reduce the body’s ability to regulate temperature, and this is rarely explained to patients.

Antihistamines reduce sweating capacity. Some antidepressants, particularly tricyclics, impair thermoregulation. Certain blood pressure medications and diuretics reduce circulating volume, making heat induced circulatory stress worse. Anticholinergic drugs of various kinds reduce the sweating response directly.

If you are taking any of these and wondering why you overheat faster than seems proportionate, this is very likely part of the explanation. It does not mean stopping the medication. It means factoring in a lower baseline heat tolerance than you might otherwise assume, and planning accordingly.

What Does This Mean for Me?

If you find yourself feeling dramatically worse during hot weather, you’re not imagining it, and it doesn’t mean your Long COVID is necessarily getting worse permanently. Heat places additional demands on the autonomic nervous system, making existing problems with circulation, temperature regulation and energy production much harder to compensate for. Understanding that heat is a physiological stressor not simply uncomfortable weather can help you plan ahead, pace more effectively and reduce the risk of a significant post-exertional crash.

Why tests often look normal

The maddening thing about heat intolerance in Long COVID is that it is a dynamic, demand dependent problem. At rest, in a cool clinical environment, cardiovascular tests and autonomic measures can appear perfectly normal. The dysfunction shows up under the specific stressor of heat or upright posture, which is not how most medical assessments are conducted.

This is why the explanation of “your tests are normal” after a heat related crisis is genuinely frustrating and genuinely unhelpful. The tests were done in conditions that do not replicate the problem.

Understanding this physiology is the foundation for everything in our companion article on how to actually survive a heatwave with Long COVID and chronic illness. The practical strategies in that article are not generic summer tips. They are specifically designed around what is happening mechanistically in the conditions described here.

Heat intolerance is one of the most common ways temperature dysregulation presents in Long Covid, but it is not the only one. Many people also experience feeling unusually cold, sudden flushing, chills, or rapid swings between feeling too hot and too cold. If you’re looking for a broader explanation of why Long Covid affects body temperature, read our guide to Temperature Dysregulation After COVID.

Key Takeaways

• Heat increases the workload on the autonomic nervous system.
• Blood vessels widen, reducing blood returning to the heart and brain.
• People with Long COVID, POTS and dysautonomia have less ability to compensate.
• Heat itself can trigger post-exertional malaise.
• Cooling strategies and pacing are evidence-based approaches rather than simple comfort measures.

Frequently asked questions

This article is for general information and education. It does not replace personalised medical advice. If heat is consistently triggering significant worsening of your symptoms, please discuss this with your GP or a clinician familiar with Long COVID or dysautonomia.

Long Covid Journey is an independent educational resource dedicated to helping people understand Long COVID through evidence-informed articles, emerging scientific research and lived experience. Our aim is to explain complex medical topics in clear, accessible language while acknowledging that research into Long COVID continues to evolve.

Every article is carefully researched using peer-reviewed studies and reputable clinical sources whenever possible. Information is reviewed and updated as new evidence becomes available, with the goal of supporting patients, families and healthcare professionals in making informed discussions with their own medical teams.

While our articles are educational and do not replace personalised medical advice, we believe that understanding the science behind Long COVID can help people make sense of symptoms, reduce uncertainty and advocate more confidently for their care.This article focuses on the biological mechanisms behind heat intolerance, autonomic dysfunction and thermoregulation in Long COVID.

Sources Climate change, Long COVID and POTS: https://thesicktimes.org/2024/08/12/climate-change-is-worsening-symptoms-of-long-covid-and-pots-heres-how-people-are-navigating-it/ POTS patterns in Long COVID, JACC Advances 2025: https://pmc.ncbi.nlm.nih.gov/articles/PMC12399250/ Worsening of POTS symptoms after COVID infection: https://pubmed.ncbi.nlm.nih.gov/38340983/ ME/CFS and heat, ME Association guidance: https://meassociation.org.uk/medical-matters/items/how-to-cope-with-a-heatwave-if-you-have-me-cfs/ ENDOFFILE