A rider collapses at the side of the road. Somebody says heatstroke. Somebody else says dehydration. A third person pours a bidon of tepid water over his head. Nobody calls an ambulance because he is still conscious. He is talking, sort of. Slurred words. Odd behaviour. Aggressive when approached.
This man is dying. Not melodramatically, not hypothetically. His core temperature is above 40 degrees and rising, his organs are cooking, and every minute without aggressive cooling pushes his odds in the wrong direction.
We have covered how heat affects performance, the acclimatisation protocols, and the age-specific risks for masters riders elsewhere on this site. This post is different. This is the clinical piece. What happens when things go wrong, how to recognise it before it becomes catastrophic, and what to do in the minutes that determine whether someone walks away or gets airlifted.
If you ride in summer — especially group rides, sportives, or training camps — you need this information. Not because disaster is likely. Because when it happens, the people standing around the casualty are almost always other cyclists. And those first ten minutes are the ones that count.
What Happens When Core Temperature Rises
Your body at rest sits around 37 degrees Celsius. During hard cycling, metabolic heat production can exceed 1,000 watts. Your cooling systems — sweating, skin blood flow, convection from airspeed — work to keep core temperature below 39 degrees. Most of the time they succeed.
When they fail, the progression follows a predictable and well-documented path.
37-39 degrees. Normal exercise range. Heart rate elevates, sweating increases, skin flushes. Performance may decline but the body is coping. This is where every hard ride lives.
39-40 degrees. The brain begins throttling motor output. Fatigue deepens. Concentration narrows. You feel awful but you can still think clearly, still answer questions, still make decisions. This is heat exhaustion territory. It is unpleasant. It is not yet dangerous if you stop and cool.
40-40.5 degrees. The line. Central nervous system function begins to degrade. Confusion. Irrational behaviour. Poor coordination. The hypothalamus — your thermoregulatory control centre — starts to malfunction, which means the very system responsible for cooling you down is the one breaking. Douglas Casa at the University of Connecticut, the leading researcher on exertional heat stroke in athletes, calls 40 degrees the clinical threshold. Above it, with CNS dysfunction present, you are dealing with a medical emergency.
Above 40.5 degrees. Cellular damage accelerates. Gut barrier integrity fails, releasing endotoxins into the bloodstream. Rhabdomyolysis — muscle tissue breakdown — begins. The coagulation cascade destabilises. Organ failure follows if cooling is not initiated rapidly. Lawrence Armstrong's research at UConn documented that mortality rates exceed 50 per cent when cooling is delayed beyond 30 minutes after collapse. When cooling starts within 10 minutes, survival rates climb above 95 per cent.
Those numbers should sit with you. The difference between a near-miss and a fatality is a short window of time and knowing what to do inside it.
Heat Exhaustion vs Exertional Heat Stroke
These are not two points on a sliding scale. They are different clinical entities with different mechanisms, different treatments, and — critically — different consequences. Confusing them can kill someone.
Heat exhaustion is the body losing the thermoregulatory battle but still fighting. Core temperature is elevated, typically 38-40 degrees. The rider sweats profusely. They feel nauseated, dizzy, fatigued. They may have a pounding headache. They want to stop. The critical feature: mental function is preserved. They know who they are, where they are, what day it is. They can follow instructions. They are miserable but cognitively intact.
Treatment: stop riding. Get to shade. Remove excess clothing. Drink cool fluids. Apply cold water to skin. Rest. Recovery happens within 30-60 minutes in most cases. It is self-limiting if managed promptly.
Exertional heat stroke is the thermoregulatory system collapsing. Core temperature exceeds 40 degrees. The hallmark — the single feature that separates it from everything else — is altered mental status. Confusion. Combativeness. Bizarre behaviour. Seizures. Loss of consciousness. The rider who was fine ten minutes ago is now unable to tell you his name.
Other signs that frequently accompany exertional heat stroke: loss of coordination (ataxia), vomiting, hot skin that may be dry or wet (the old teaching that heat stroke means dry skin is wrong — Sebastien Racinais and Julien Periard have documented cases where profuse sweating continues well into heat stroke), and in severe cases, collapse.
Treatment: this is a time-critical emergency. Cool first, transport second. That order matters. Casa's research group has published extensively on the principle of "cool first, transport second" — initiating cooling at the scene, before the ambulance arrives, before loading the patient, before doing anything else. Every minute of cooling delay increases the risk of permanent damage.
The distinction between these two conditions is not academic. A rider with heat exhaustion who is treated as though they have heat stroke will receive aggressive cooling they do not strictly need — uncomfortable but not harmful. A rider with exertional heat stroke who is treated as though they have heat exhaustion — offered a drink, sat in the shade, told to rest — may die.
When in doubt, treat it as heat stroke. The consequences of over-treating are trivial. The consequences of under-treating are catastrophic.
The Warning Signs Most Cyclists Miss
Heat illness rarely announces itself dramatically. It creeps. And the early signs are easy to dismiss as normal suffering on a hard, hot ride.
Personality change. The most reliable early indicator and the one most commonly missed. A rider who is normally sociable goes quiet. A calm person becomes irritable or aggressive. Someone starts making odd decisions — taking wrong turns, refusing to drink, insisting they are fine when they are clearly not. These are not character flaws. They are early central nervous system dysfunction. If a riding partner's behaviour shifts in the heat, pay attention.
Cognitive errors. Inability to do simple mental arithmetic. Forgetting the route despite having ridden it before. Difficulty following conversation. Slurred or confused speech. These indicate the brain is overheating. By the time you notice these, core temperature is almost certainly above 39.5 degrees.
Loss of coordination. Wobbly bike handling. Unclipping clumsily. Difficulty standing after getting off the bike. Staggering. This is the nervous system losing fine motor control. It is dangerous both because of the heat illness itself and because a rider with impaired coordination is a crash risk to themselves and others.
Cessation of sweating. The classic textbook sign, though not as universal as once taught. When it does occur, it means the thermoregulatory system has shut down. The body has stopped trying to cool itself. This is late-stage and extremely serious. But do not rely on its absence — many heat stroke victims continue sweating.
Goosebumps and chills in the heat. Paradoxical cold sensation in hot conditions. Your hypothalamus is malfunctioning, misreading signals. If someone on a 35-degree day says they feel cold, that is an emergency.
A Roadside Recognition Algorithm
You do not need a rectal thermometer to recognise heat stroke in the field (though Casa's group considers rectal temperature the only reliable field measurement of core temperature — forehead strips, ear thermometers, and temporal scanners are inaccurate during exercise). What you need is a structured assessment.
Step 1: Context. Is the temperature above 28 degrees? Has the rider been exercising hard for over 60 minutes? Have they been drinking adequately? If the context is right for heat illness, proceed with suspicion.
Step 2: Mental status check. Ask three questions. What is your name? Where are we? What day is it? If the rider gets any of these wrong, or cannot answer, or gives bizarre responses, assume exertional heat stroke. Do not negotiate. Do not wait to see if they improve. Begin cooling immediately.
Step 3: Physical assessment. Check coordination — can they stand unaided? Observe skin — is it flushed, hot to touch? Are they sweating or dry? Check for vomiting, seizure activity, or collapse.
Step 4: Decision point.
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If mental status is normal but the rider is nauseated, fatigued, sweating heavily, and feeling awful: heat exhaustion. Move to shade, remove clothing, cool with water, encourage fluid intake. Monitor for 20-30 minutes. If improvement occurs, consider assisted transport home. If symptoms worsen or mental status changes, escalate.
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If mental status is altered in any way — confusion, aggression, incoherence, loss of consciousness: assume exertional heat stroke. Call 999 or 112 immediately. Begin aggressive cooling while waiting. Do not move the rider further than necessary. Do not give oral fluids to someone with altered consciousness (aspiration risk).
Emergency Cooling Protocols
You are standing at the side of a road in southern Spain. Your riding partner has collapsed. He is confused, combative, and hot. The ambulance is 15 minutes away. What you do in these minutes determines the outcome.
Gold standard: cold water immersion. Submerging the body in cold water (1-15 degrees) cools at approximately 0.2 degrees Celsius per minute. This is the method with the strongest evidence base and the highest cooling rate. In a race or event setting, organisers sometimes have immersion tubs available. On a training ride, you almost certainly do not. So you improvise.
What you actually have available:
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Water bottles. Pour every bidon you have over the rider's head, neck, torso, and groin. Send someone to find more water — a house, a cafe, a petrol station, a stream. Volume matters. You need litres, not sips.
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Wet clothing as cold packs. Soak jerseys, base layers, arm warmers — anything absorbent — in the coldest water available and drape them across the neck, armpits, and groin. These are the areas with the highest density of blood vessels near the surface. Cooling these areas transfers heat from the blood most efficiently.
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Fanning. Air movement across wet skin accelerates evaporative cooling. Use a jacket, a feed bag, a map, anything flat. If two people are available, one pours water and the other fans.
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Ice from a nearby source. If there is a shop, a restaurant, a petrol station within reach, ice is worth sending someone for. Pack ice into the neck, armpits, and groin. Wrap it loosely — direct ice-to-skin contact causes vasoconstriction and can slow heat transfer. A thin layer of wet fabric between ice and skin is ideal.
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Shade. Not sufficient on its own but important as part of the package. Move the rider out of direct sun if possible without excessive handling.
What not to do:
- Do not give oral fluids to an unconscious or confused person.
- Do not immerse the head if the rider is not fully conscious.
- Do not assume improvement means the crisis has passed — core temperature can continue rising even after external cooling begins (a phenomenon called afterdrop, though this is more common in hypothermia; the concern in heat stroke is that organ damage continues until core temperature drops below 39 degrees).
- Do not stop cooling until emergency services arrive and take over, or until the rider is clearly lucid and oriented.
When to Call 999/112 vs Manage on Scene
This is a binary decision with a clear dividing line.
Call an ambulance if any of the following are present: altered mental status (confusion, aggression, incoherence), loss of consciousness, seizure, cessation of sweating with hot skin, core temperature above 40 degrees if you have a way to measure it, or any symptom that is worsening despite cooling efforts.
Manage on scene if: the rider is mentally clear, communicating normally, able to drink, and symptoms are improving with shade, rest, and cooling. This is heat exhaustion. It resolves with supportive care.
The grey zone — the rider who seems confused but might just be exhausted, the rider who is irritable but insists they are fine — should be treated as heat stroke until proven otherwise. Call, begin cooling, and let the paramedics make the final assessment. You do not lose anything by being wrong in the direction of caution. You lose everything by being wrong in the other direction.
When you call, give the dispatcher: your exact location (GPS coordinates from your phone, or a road name and landmark), the number of casualties, a description of symptoms ("cyclist collapsed, confused, not making sense, hot to touch, we are cooling him with water"), and whether the rider is conscious.
Pre-Cooling as Prevention
Everything above describes response. Prevention is cheaper, less dramatic, and far more effective.
Julien Periard and Sebastien Racinais — two of the leading researchers on exercise in the heat, based at Aspetar in Qatar — have published extensively on pre-cooling strategies. The principle is simple: lower your starting core temperature so the ceiling is further away when you begin exercising.
Ice slurry ingestion. Drinking 500ml of crushed ice mixed with water or electrolyte solution 20-30 minutes before riding lowers core temperature by 0.3-0.5 degrees. The phase change from solid to liquid absorbs heat from the gut. Periard's work shows this delays the onset of critical thermal strain by 15-20 minutes in 35-degree conditions. A blender and ice are all you need.
Ice vest. Wearing a phase-change or ice-packed vest for 15-20 minutes before the start. Effective, practical, and used routinely by World Tour teams. Remove it before riding — the additional insulation during exercise would be counterproductive.
Menthol mouth rinse. This one surprises people. Ross Tucker — the South African exercise physiologist behind The Science of Sport — has discussed the research showing that menthol activates cold-sensitive TRPM8 receptors in the mouth. It does not lower core temperature. What it does is reduce the perception of thermal discomfort, which allows riders to maintain higher work rates for longer before the brain applies the thermal handbrake. Swill 25ml of 0.1 per cent menthol solution for 10 seconds and spit. The effect lasts 15-20 minutes.
Cold water pre-loading. Drinking cold water (4-6 degrees) in the hour before riding provides a small but measurable cooling benefit. Less effective than ice slurry but easier to source.
These strategies stack. A rider who combines an ice vest, an ice slurry, and a menthol rinse before a hot event starts with a meaningful thermal buffer that a rider who simply clips in and goes does not have.
Acclimatisation as Prevention
The single most effective defence against heat illness is not a product, not a supplement, and not a cooling strategy. It is a physiological adaptation that takes 10-14 days to build.
We cover the full acclimatisation protocols elsewhere. The focus here is on acclimatisation specifically as a safety mechanism against heat illness.
When you heat-acclimatise, three things change that directly reduce your risk:
Your resting core temperature drops. By 0.2-0.5 degrees after a full acclimatisation block. This means you start further from the danger zone. A rider who begins exercise at 37.0 degrees instead of 37.4 degrees has an additional buffer before reaching 40 degrees.
You start sweating earlier and more. An acclimatised body activates sweating at a lower core temperature and produces more sweat per gland. Racinais and colleagues have shown that sweat rate increases by 10-25 per cent after acclimatisation. More sweating means more evaporative cooling, which means slower core temperature rise.
Your plasma volume expands. By 10-12 per cent. More blood volume means better cardiac output, which means the heart can simultaneously supply working muscles and the skin without the zero-sum competition that drives cardiac drift. Lawrence Armstrong's group has documented this expansion consistently across acclimatisation studies.
The practical protocol: five to seven consecutive days of 60-90 minutes at zone 2 intensity in hot conditions (above 30 degrees, or on an indoor trainer in a heated room). Full adaptation occurs at 10-14 days. The adaptations decay at roughly 2.5 per cent per day without continued heat exposure, so a protocol completed 7-10 days before an event still leaves substantial protection.
The critical point for this post: acclimatisation does not make you immune to heat illness. It raises the threshold. It gives you more margin. But a fully acclimatised rider who ignores hydration, pacing, and warning signs can still develop exertional heat stroke. Acclimatisation is the foundation. The other layers still matter.
Age-Specific Vulnerability
If you are over 40, your heat tolerance is measurably lower than it was at 25. We have covered this in detail in the age-specific heat tolerance piece. The summary relevant to heat illness prevention:
Sweat output declines. Both the total volume and the rate of onset. Older riders start sweating later and produce less of it. The primary cooling mechanism is running at reduced capacity.
Skin blood flow decreases. The vasodilatory response — widening blood vessels near the skin surface to radiate heat — becomes sluggish. Less blood reaches the surface, less heat escapes.
Thirst blunts. By the time a rider over 50 feels thirsty, they may already be 1-2 per cent dehydrated. That level of dehydration impairs thermoregulation and elevates core temperature at any given intensity.
Cardiac reserve narrows. Lower maximum heart rate and reduced cardiac output mean less capacity to simultaneously power the legs and cool the body. The competition for blood flow becomes more severe, earlier.
The compounding effect: a 50-year-old rider in 35-degree heat reaches critical core temperature faster, with less warning, than a 30-year-old in the same conditions doing the same power. This is not opinion. It is measured physiology from dozens of studies.
What to do about it: acclimatise with a longer, gentler ramp (three to four weeks rather than two). Hydrate proactively to a schedule, not to thirst. Reduce power targets by an additional 5 per cent beyond the standard heat adjustment. And carry more fluid than you think you need.
Medication Interactions
This is the section that applies to a significant percentage of the Roadman audience. If you are 35-55 and riding seriously, there is a reasonable chance you or someone on your group ride takes one of the following:
Beta-blockers (atenolol, bisoprolol, metoprolol). Prescribed for hypertension, anxiety, and heart rhythm disorders. These drugs blunt the heart rate response to exercise and reduce cardiac output. In heat, the heart needs to increase output to supply the skin for cooling. A beta-blocked heart cannot respond fully. Core temperature rises faster.
Diuretics (hydrochlorothiazide, furosemide, indapamide). Prescribed for blood pressure and fluid retention. These increase urine output and reduce blood volume — the opposite of what you want in the heat. A rider on diuretics starts their ride with less circulating fluid and loses more through the kidneys. Dehydration risk is significantly elevated.
Antihistamines (cetirizine, loratadine, diphenhydramine). Used for hay fever, allergies, and sinus issues. First-generation antihistamines (like diphenhydramine) cross the blood-brain barrier and can impair sweating. Second-generation versions are less problematic but still carry some thermoregulatory effect.
Stimulant medications (for ADHD). Amphetamine-based medications increase metabolic heat production and can impair the perception of thermal discomfort. Riders on these medications may push harder than their thermal state warrants because the subjective feedback is muted.
Anticholinergics (some bladder medications, certain antidepressants). These reduce sweating directly by blocking the neurotransmitter that activates sweat glands.
If you take any of these medications, your heat ceiling is lower. Not by a small amount. Enough that the standard heat guidelines — which assume an unmedicated, healthy adult — do not apply to you as written. Reduce intensity further. Hydrate more aggressively. Monitor yourself more closely. And talk to your prescribing doctor about exercise in the heat. Most GPs will not raise this proactively. You need to ask.
Group Ride Responsibilities
Running events, triathlon, and military training all adopted formal heat illness protocols years ago. Cycling lags behind. There is no reason for this except inertia.
If you lead group rides in temperatures above 28 degrees, the following should be standard operating procedure. Not optional. Not "if convenient."
Designate a heat-aware rider. Someone who has read this post, or something like it, and knows the recognition algorithm. This person rides at the back where they can observe struggling riders and is empowered to stop the ride if needed.
Share live location. Every rider should have their phone location shared with at least one other person on the ride. If someone collapses, you need to give the ambulance an exact location. GPS coordinates. Not "somewhere on the N340 past the petrol station."
Carry extra water. Beyond personal hydration. Enough to pour over a casualty. Two extra bottles between the group.
Establish stop criteria before the ride. "If anyone shows signs of confusion, personality change, or stops sweating, we stop the entire group and assess." Saying this out loud before you roll changes the culture. It gives people permission to speak up without feeling like they are ruining the ride.
Know the medical infrastructure. On unfamiliar roads — training camps, tours, sportives in foreign countries — know where the nearest hospital is. Have the local emergency number saved (112 works across the EU). Know the word for ambulance in the local language.
Check on quiet riders. Silence in the heat is a red flag. Someone who normally chats and goes quiet may be conserving energy. Or they may be sliding into cognitive dysfunction. Ask them a question. A direct, specific question. "What did you have for breakfast?" If the answer is wrong or absent, stop.
The Numbers That Matter
Commit these to memory:
- 40 degrees Celsius core temperature with altered mental status = exertional heat stroke. Call for help. Cool aggressively.
- 0.2 degrees per minute = cooling rate with cold water immersion. Gold standard.
- 30 minutes = the window. Cooling initiated within 30 minutes of collapse reduces mortality to below 5 per cent.
- 10-14 days = full heat acclimatisation timeline. Five days for meaningful adaptation.
- 750-1000ml per hour = target fluid intake above 32 degrees Celsius.
- 28 degrees WBGT = threshold above which elite events modify courses. Your threshold should be at least this cautious.
- 112 = European emergency number. Works on any mobile phone, in any EU country, even without a local SIM.
This is not information you file away and forget. It is information you carry with you, literally, on every summer ride. Write the recognition steps on a piece of tape on your top tube if that helps. Tell your riding partners what to look for. Talk about it before the ride, not after something happens.
The riders who survive heat emergencies are the ones who ride with people who recognised the signs and acted in the first ten minutes. Everything else — the ambulance, the hospital, the IV fluids — is secondary to that initial recognition and cooling.
Ride in the heat. Train in the heat. Race in the heat. But know what you are dealing with, carry the knowledge to respond, and build the physiological defences that keep you on the right side of the line.
If you want structured heat preparation and a community of riders who take this stuff seriously, the Roadman Cycling Skool community is where we share protocols, discuss training in conditions, and hold each other accountable. Summer riding is better when someone has your back.