Heat Intolerance: Tracking Heat Sensitivity
Heat intolerance is a disproportionate reaction to warmth. A hot room, a long shower, a summer walk, or exercise leaves you weak, dizzy, foggy, flushed, or drained while people around you feel fine. Some people notice old symptoms return for an hour or two. It shifts with the season, the menstrual cycle, medication changes, and illness, so a running record of when heat affects you, and how badly, is far more useful than memory.
Why track this symptom?
- Heat thresholds are personal, and a log shows the conditions where you actually start to struggle.
- Season by season, severity patterns show whether heat is affecting you more than last year.
- A dated record turns "heat wipes me out" into evidence a clinician can review and act on.
How Trace Health helps
Hot days are exactly when you have least energy to write anything down. Trace Health is one tap for mild, moderate, or severe, and you can add environment, exercise, sleep, stress, and medications to the same timeline. Across a summer, that shows which conditions flatten you and which cooling steps actually help.
Common causes
Most heat intolerance has an everyday explanation: dehydration, poor sleep, alcohol, deconditioning after an illness, or a body not yet acclimatized early in the season. Medications are a common and overlooked cause, including anticholinergics and some antihistamines and antipsychotics that reduce sweating, beta blockers that blunt the heart rate response, and diuretics that lower circulating volume. Hormonal causes matter too, particularly hyperthyroidism and the vasomotor symptoms of perimenopause. In multiple sclerosis, a small rise in core temperature can temporarily slow conduction in demyelinated nerves and bring back familiar symptoms, known as Uhthoff's phenomenon. Heat also worsens POTS and other forms of dysautonomia, where vasodilation pools blood in the legs. Less often, mast cell activation syndrome, reduced sweating from small fiber neuropathy, or anemia play a part.
When to see a doctor
Heat stroke is an emergency. Call emergency services for confusion, slurred speech, agitation, seizure, fainting that does not resolve quickly on lying down, skin that is hot with sweating that has stopped, repeated vomiting, or a core temperature at or above 40°C (104°F). Start cooling immediately rather than waiting for help to arrive. Seek same-day advice for chest pain, breathlessness, or a heart rate that stays high after 20 to 30 minutes of rest in a cool place. Book a routine appointment if heat intolerance is new over recent weeks, comes with weight loss, tremor, palpitations, or a racing pulse on standing, follows a medication change, or is limiting your summer.
Frequently Asked Questions
Why does heat make my MS symptoms worse?
In multiple sclerosis, raising core temperature by even a few tenths of a degree can slow conduction along nerves that have lost myelin, so symptoms you already have, such as blurred vision, weakness, numbness, or fatigue, temporarily intensify. This is called Uhthoff's phenomenon. It is a transient effect rather than a relapse, and it usually settles within 30 to 60 minutes of cooling down. Precooling before exercise, cooling vests, cold drinks, and moving activity to the early morning all help. Log the trigger, which symptoms returned, and how long recovery took, since that pattern is useful at your next neurology review.
Why do I feel dizzy and lightheaded in hot weather?
Heat causes blood vessels near the skin to widen so the body can shed warmth. If your autonomic nervous system does not compensate well, blood pools in the legs and abdomen, less returns to the heart, and the heart rate climbs to make up for it. Sweating also reduces plasma volume over the course of a hot day. People with POTS, other forms of dysautonomia, or those on diuretics or blood pressure medication notice this quickly. Fluids, salt if your clinician has advised it, compression garments, cool showers, and lying down with legs raised often help. Log fluid intake, standing time, and how fast the dizziness settles.
What is the difference between heat intolerance and heat exhaustion?
Heat intolerance is a chronic, individual sensitivity: you feel unwell in warmth that others tolerate easily, and you recover within roughly an hour once you cool down, with normal thinking throughout. Heat exhaustion is an acute failure of thermoregulation, with heavy sweating, nausea, headache, cramps, weakness, and a core temperature usually between 38°C (100.4°F) and 40°C (104°F). It requires stopping, cooling, and fluids at once. If confusion, seizure, or a temperature at or above 40°C (104°F) appear, that is heat stroke, which is a medical emergency and needs immediate cooling and emergency care.
When should I see a doctor about heat intolerance?
Book an appointment if heat intolerance is new or has clearly worsened over weeks to months, particularly alongside unintentional weight loss, tremor, palpitations, anxiety, or sweating changes, which may prompt thyroid testing. Also raise it if it began after starting a medication, if you faint or nearly faint in heat, if your pulse rises sharply on standing, or if flushing comes with itching, hives, or gut symptoms. Seek emergency care for confusion, seizure, sweating that stops in a hot environment, or a temperature at or above 40°C (104°F). Bring a few weeks of logged episodes to the visit.
What should I track about heat intolerance to show my doctor?
Record the date, severity as mild, moderate, or severe, the outdoor temperature or indoor setting, what you were doing, which symptoms appeared, and how long recovery took after cooling. Note fluid and salt intake, sleep, menstrual cycle phase, alcohol, and every medication change, since these are the factors a clinician will ask about. Trace Health keeps these on one timeline and exports a doctor-ready PDF, so a summer of episodes fits into a short appointment. Health data stays on your iPhone by default, with optional private iCloud sync, and no account is needed.
Your health data stays on your iPhone.
No account. Private. No personal information is ever requested. Your health data stays on your iPhone by default. A backup option to your private iCloud is available. Trace Health never collects or sells your health data.
What people say
Rated 4.8 out of 5 from 50+ ratings on the App Store.
- “The app is simple, fun, and life-changing for me. I chose it mainly so that my data is never shared, and because it’s the easiest to use. I used to write my symptoms down in a notebook; now I do it every day on the app to track how they change. Being able to also log my everyday activities to see their influence is very handy. I recommend it!” (Solex94, France)
- “Out of all the symptom tracker apps, I found this to be the easiest and most straightforward. It “writes” to the health app, which is exactly what I wanted. All the others only “read” parts of the health app data, and didn’t truly write any data back. This helps me visualize my chronic symptoms soooo well.” (EB1700, United States)
Read the complete guide: How to Track Heat Intolerance Day by Day →