Heat Doesn't Wait For The Unfit

PerformanceSprintsHeat Training
Written By Cat Hoh

Chicago's 2007 marathon hit 25°C on race day. 30.7% of the field didn't finish, the highest withdrawal rate of any major marathon in a ten-year dataset of 1.79 million performances. Heat doesn't wait for the unfit runner at the back. It slows and drops people at every level of the field, and most of what beats it is entirely trainable, in a week or two, without a heat chamber.

Temperature is the biggest lever nobody trains for

Pull 1.79 million individual marathon finishes from six major races over a decade, and one environmental variable towers over the rest: air temperature. Not course profile, not field size, not the weather on the start line looking mild. Temperature alone (El Helou et al. 2012El Helou N, Tafflet M, Berthelot G, Tolaini J, Marc A, Guillaume M, Hausswirth C, Toussaint JF (2012). Impact of environmental parameters on marathon running performance. PLOS ONE 7(5):e37407.).

The optimal race temperature sits somewhere between about 4°C and 10°C depending on sex and ability tier, and every runner, from the fastest woman in the field to the slowest man, gets slower as conditions move away from that window in either direction. Heat does something colder weather doesn't: it doesn't just slow people, it takes them out of the race.

Figure 01

Chicago 2007: the hottest major marathon in the dataset

30.7%

Did not finish, Chicago Marathon 2007 (25°C)

Optimal window

3.8–9.9°C

Peak marathon speed happens here, across sex and ability tier. Both hotter and colder hurts times.

Explained by heat alone

36%

Of the variation in how many runners withdraw, race to race, across the same six marathons.

Data from 1.79 million individual performances across six major marathons, 2001–2010 (El Helou et al. 2012). Every finishing level was affected, not only the athletes at the front.

You can train for this. It just isn't training, exactly

Heat acclimation is the body adjusting its cooling system to a hotter environment: plasma volume expands, sweating starts earlier and at a lower core temperature, heart rate at a given pace drops, and the whole system copes with more heat stress before things go wrong. None of it needs a treadmill session harder than what you'd already do. It needs heat exposure, repeated, for about two weeks.

Most of the benefit lands in the first week

The international consensus statement on training and competing in the heat puts the useful minimum at a few days and full benefit at up to two weeks, with daily heat sessions of 60 to 100 minutes (Racinais et al. 2015Racinais S, Alonso JM, Coutts AJ, et al. (2015). Consensus recommendations on training and competing in the heat. Scandinavian Journal of Medicine & Science in Sports 25(S1):6-19.). Trained athletes get there in roughly half the time of untrained people, because a fitter cardiovascular system has less distance to travel.

Figure 02 / Interactive

What's actually adapted, by day

Days into heat exposure / tap to change

Day 3 Day 7 Day 14

Heart rate at a given pace early drop

Sweat onset just starting to shift earlier

Plasma volume expansion underway

First adaptations are already measurable. This is the point most people quit, right before the return on the next week gets much bigger.

Heart rate at a given pace solidly down

Sweat onset clearly earlier, more fluid

Plasma volume expansion most of the way there

Cardiovascular and sweating adaptations are most of the way built. A trained athlete may already be close to full benefit here.

Heart rate at a given pace near full adaptation

Sweat onset near full adaptation

Plasma volume expansion complete for most athletes

This is the two-week mark the consensus statement points to for maximising benefit. Beyond here, gains are marginal.

Illustrative, not measured. Shows the general adaptation curve described in the consensus literature (Racinais et al. 2015), not day-by-day data from one tracked cohort. Individual adaptation speed varies, and trained athletes typically reach each stage faster.

You don't need a heat chamber. A hot bath works

Most club athletes have no access to a heat chamber or a training camp somewhere warmer. The literature has an answer that fits in a bathroom: run in normal conditions, then get in a hot bath afterwards, for about a week.

Figure 03 / Interactive

Two ways to get the same adaptation

Protocol / tap to change

Heat chamber Post-run hot bath

What it takes. A controlled hot room, typically 38 to 40°C, for 60 to 100-minute exercise sessions across 1 to 2 weeks, with core temperature monitored and held above about 38.5°C to maximise the adaptation signal.

What it needs. A facility. Most club-level athletes and their coaches don't have one within reach, which is exactly why this protocol stays confined to national teams and well-funded programs.

FacilityRequired
Duration1–2 weeks
Session60–100 min

What it takes. A normal 40-minute run in ordinary conditions, followed by up to 40 minutes in a bath at 40°C, on six consecutive days.

What it produces. Resting and end-exercise core temperature down about 0.36°C and exercising heart rate down 14 beats per minute, still detectable two weeks after the last bath (Zurawlew et al. 2019Zurawlew MJ, Mee JA, Walsh NP (2019). Post-exercise Hot Water Immersion Elicits Heat Acclimation Adaptations That Are Retained for at Least Two Weeks. Frontiers in Physiology 10:1080. Protocol originally established in Zurawlew, Mee & Walsh (2016), Scandinavian Journal of Medicine & Science in Sports.), which is a genuinely accessible strategy rather than a lab-only one.

FacilityNone
Duration6 days
SessionRun + 40 min bath

The bath protocol was tested on trained runners doing a standardised lab run, not on team-sport athletes or elite ultra-endurance athletes specifically. Treat it as a demonstrated floor for what's possible without a facility, not a ceiling on what heat acclimation can do.

Losing it costs far less than building it

Once you stop heat exposure, heart rate and core-temperature adaptations decay at roughly 2.3 to 2.6% per day rather than falling off a cliff, and the majority of benefit holds for 2 to 4 weeks (Daanen, Racinais & Périard 2018Daanen HAM, Racinais S, Périard JD (2018). Heat Acclimation Decay and Re-Induction: A Systematic Review and Meta-Analysis. Sports Medicine 48(2):409-430.). Here's the more useful number: getting it back is dramatically cheaper than building it the first time.

Figure 04

Building it the first time versus getting it back

First build

10–14 days

Typical time to reach most of the available heat-cardiovascular adaptation from a standing start.

Re-acclimation, same athlete

8–12× faster

How much quicker heart rate and core-temperature adaptations return after a break, compared with the original build.

Trained vs untrained

~2× faster

Trained athletes typically reach the same adaptation stage in about half the time of untrained people.

A missed fortnight of heat exposure before a hot race isn't a return to zero. Sweat-rate re-induction didn't show the same speed-up in the same analysis, so don't expect every adaptation to snap back equally fast (Daanen, Racinais & Périard 2018).

The adaptation isn't identical for everyone

Women may need more sessions or greater thermal stress than men to reach the same level of adaptation, and the mechanism looks different under the surface, not just slower (Wickham, Wallace & Cheung 2020Wickham KA, Wallace PJ, Cheung SS (2020). Sex differences in the physiological adaptations to heat acclimation: a state-of-the-art review. European Journal of Applied Physiology.).

Figure 05 / Interactive

The same protocol, a different adaptation curve

Sex / tap to change

Men Women

Whole-body sweat rate rises with acclimation

Sweat gland recruitment increases

The standard picture: both measures move together as adaptation builds.

Whole-body sweat rate doesn't reliably follow

Sweat gland recruitment increases

More sweat glands switch on without whole-body sweat rate necessarily rising the same way, one reason the same protocol can read as a smaller effect.

Directional, not measured. Bars illustrate the pattern the review describes, not specific study values. Body morphology (surface area relative to mass) also contributes to the gap, and few of the underlying studies control for menstrual cycle phase. Treat "women may need more sessions" as a reasonable read of current evidence, not a settled number.

Australia's own heat rules have a hole in them

Sports Medicine Australia's heat policy uses wet bulb globe temperature (WBGT) to set four risk tiers, from low risk under 20°C through to extreme risk at 30°C and above, and it explicitly names heat acclimatisation as a way to reduce risk (Sports Medicine Australia, 2007Sports Medicine Australia (2007, reissued). Policy: Preventing Heat Illness in Sport.). The problem is the tiers themselves.

Figure 06 / Interactive

Same strain, different rating (and one that hides entirely)

Official risk category / tap to change

Rated "moderate" Rated "extreme" 46°C dry heat

Conditions 30°C, 60% humidity

Sits in the policy's moderate-to-high band. On paper, this calls for extra breaks and vigilance, not postponement.

Conditions 36°C, 30% humidity

Sits in the policy's extreme band and points toward postponement or cancellation.

Conditions 46°C, 14% humidity, 2017 NSW heatwave

Low humidity keeps the official WBGT reading well down the scale, technically permitting play under the policy's own thresholds. Several sporting bodies cancelled anyway, on their own judgement, not the policy's.

Illustrative bars, not a published strain index. They represent a University of Sydney critique: 30°C at 60% humidity imposes strain comparable to conditions the policy rates extreme despite landing two tiers apart, and the same WBGT formula can rate genuinely brutal dry heat as far milder than it feels (Chalmers & Jay 2018Chalmers S, Jay O (2018). Australian community sport extreme heat policies: Limitations and opportunities for improvement. Journal of Science and Medicine in Sport 21(6):544-548.).

None of this means the policy is useless. It means a WBGT reading in the "moderate" band on a humid day deserves the same caution a coach would give a genuinely extreme reading, regardless of which column it falls into.

Heat illness is the other reason this matters

Heat acclimatisation isn't only about times. It's one of the named risk-reduction strategies against exertional heat stroke, which requires hyperthermia (commonly a core temperature above 40°C) plus central nervous system disturbance to diagnose, not a high temperature reading alone (American College of Sports Medicine, 2023American College of Sports Medicine, Casa DJ, et al. (2023). ACSM Expert Consensus Statement on Exertional Heat Illness: Recognition, Management, and Return to Activity. Current Sports Medicine Reports 22(4):134-149.). One episode carries a 3.3 times relative risk of a second within two years, and the preferred treatment, where available, is ice water immersion.

What this means for programming

  • Start 10 to 14 days out, not the week before. Most of the benefit takes that long to bank, and trained athletes can move faster than that, not slower.
  • No heat chamber, no problem. Six days of a normal run followed by a hot bath produces measurable, weeks-lasting adaptation, and it costs nothing beyond hot water.
  • Don't panic about a missed week. Re-acclimation runs 8 to 12 times faster than the first build, so a short break before a hot race isn't a reset to zero.
  • Read "moderate" WBGT readings on humid days with real caution. The category can understate the actual strain on a squad.

References

All links checked September 2026.

Cat Hoh
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