← Daily DoseThe Wet-Bulb Line — Daily Dose No. 034
Daily Dose / Aug 19, 2026 / Human Heat Limits
The Wet-Bulb Line
A thermometer wrapped in a wet sock tells you something an ordinary thermometer cannot: whether a human body can still shed the heat it makes. Above a certain reading, sweat stops working. Not "uncomfortable" — thermodynamically impossible. That number turns out to be lower than the famous one, and the planet has already crossed it, briefly, at least fourteen times.
35 °C
The wet-bulb temperature long treated as the ceiling of human survivability. At 35 °C wet-bulb, the air is so warm and so saturated that a resting, naked, fully hydrated person in full shade cannot lose metabolic heat by any route. Core temperature climbs until it stops.
Theoretical limit — Sherwood & Huber, PNAS, 2010
The measured limit is lower. 30.6 °C — Vecellio et al., J. Appl. Physiol., 2022
01 / The Scale
What a wet-bulb reading measures
Wet-bulb temperature is what you get when evaporation has cooled a thermometer as far as the air will allow. In dry air it sits far below the air temperature. In saturated air the two converge — and the body's only cooling mechanism, evaporating sweat, converges on nothing.
20 °CPleasant
26 °CHumid summer day
30.6 °CMeasured limit (2022)
33 °C~80 events since 1979
35 °CTheoretical ceiling
Sequential scale, 20–36 °C wet-bulb. Zone labels from Sherwood & Huber (2010), Raymond et al. (2020), Vecellio et al. (2022).
02 / The Correction
The real ceiling is lower than the famous one
The 35 °C figure came from a thermodynamic model, not from people. In 2022, Penn State researchers put young, healthy volunteers in a climate chamber doing light activity and found the point where heat stress becomes uncompensable: a mean wet-bulb temperature of 30.55 °C in humid conditions — and lower still in hot, dry air. Older adults, the ill and the working almost certainly fail earlier.
Humid summer day
26.0 °C
Measured limit, young healthy adults
30.6 °C
Theoretical ceiling
35.0 °C
Jacobabad, Pakistan observed peak
35.0 °C+
Bars begin at 20 °C wet-bulb, not zero — the interesting range is narrow. 35.0 °C figure is theoretical (Sherwood & Huber 2010); 30.6 °C is measured (Vecellio et al. 2022).
The body's margin
Normal core temperature is 37 °C. Clinical heat stroke is defined at roughly 40 °C core plus neurological failure. The entire survivable range is about three degrees wide, and the only thing defending it is evaporation.
Why sweat works
Every litre of sweat that actually evaporates carries away roughly 580 kcal — about 2.4 megajoules. Sweat that drips off carries away nothing. In saturated air, none of it evaporates.
03 / Already Happening
Fourteen times, in five places
Analysing hourly weather-station records back to 1979, Raymond, Matthews and Horton found that extreme humid heat is not a projection for 2100. Readings at or above 35 °C wet-bulb have already occurred fourteen times — all at coastal stations around the Persian Gulf and in Pakistan's Indus valley, and all lasting less than about six hours. Worldwide, the frequency of wet-bulb readings above 30 °C has roughly doubled since 1979.
Station-hours ≥ 31 °C
~1,000
Station-hours ≥ 33 °C
~80
Occurrences ≥ 35 °C
14
Linear scale, 1979–2017 weather-station records. Source: Raymond, Matthews & Horton, Science Advances, 2020.
Schematic map, not survey-accurate. Event counts and locations: Raymond, Matthews & Horton, Science Advances, 2020 — stations in Pakistan, Saudi Arabia and the UAE.
04 / The Count
The line is already being crossed in death certificates
Long before wet-bulb 35, ordinary heat kills. In the United States, deaths with heat listed on the certificate more than doubled between 1999 and 2023 — and the steepest climb came after 2016. These are only the deaths a coroner attributed to heat; excess-mortality estimates run several times higher.
21,518 heat-related deaths recorded in total, 1999–2023. Source: Howard et al., JAMA, 2024, using CDC WONDER death-certificate data.
05 / The Bill
What heat costs before it kills
The wet-bulb line is the hard edge. The soft edge — hours not worked, crops not harvested, sleep not slept — arrives far earlier and is measured in trillions.
489,000
Heat-related deaths per year, 2000–2019 Modelled
+167%
Heat deaths in over-65s in 2023 vs the 1990s
$2.4 TN
Annual cost of lost working hours by 2030 Projection
5.6 BN
Air conditioners in use by 2050, from ~2 bn now Projection
Where the lost hours fall — share of global working hours lost to heat, 2030
Agriculture
60%
Construction
19%
All other sectors
21%
Projection assuming warming held to 1.5 °C and outdoor work performed in shade — the ILO calls this a conservative case. Equivalent to 2.2% of all working hours, or 80 million full-time jobs. Source: ILO, 2019.
Why it matters
Heat is the deadliest weather there is, and the most preventable
Know the humid number, not the dry one. 38 °C at 20% humidity and 33 °C at 70% humidity are not the same event. Heat index and wet-bulb globe temperature encode the difference; the headline temperature does not.
Fans have a ceiling too. Once air temperature exceeds skin temperature and humidity is high, moving hot air over skin adds heat rather than removing it.
The vulnerable are identifiable in advance. Over-65s, outdoor workers, people on certain medications, anyone without cooling. The 167% rise in older-adult heat deaths is a list of names, not a curve.
Cooling is a climate problem shaped like a solution. Air conditioning already draws over 2,000 TWh a year. Efficiency standards decide whether the fix compounds the cause.
Shade is infrastructure. Tree canopy, awnings and reflective surfaces cut the radiant load that no amount of sweat can offset.
Check on someone. Most heat deaths happen indoors, alone, over several days — not dramatically, and not outside.
Sources & notes
Sherwood, S. C. & Huber, M. "An adaptability limit to climate change due to heat stress." PNAS, 2010. — Origin of the 35 °C wet-bulb survivability limit. Theoretical model, not measured
Vecellio, D. J., Wolf, S. T., Cottle, R. M. & Kenney, W. L. "Evaluating the 35 °C wet-bulb temperature adaptability threshold for young, healthy subjects (PSU HEAT Project)." Journal of Applied Physiology, 2022. — Measured critical wet-bulb temperature of 30.55 ± 0.98 °C in humid conditions; lower in hot-dry conditions.
Raymond, C., Matthews, T. & Horton, R. M. "The emergence of heat and humidity too severe for human tolerance." Science Advances, 2020. — ~1,000 station-hours ≥ 31 °C wet-bulb and ~80 ≥ 33 °C since 1979; 14 occurrences ≥ 35 °C at five stations in Pakistan, Saudi Arabia and the UAE; Jacobabad recorded on six dates between 1987 and 2012; frequency of 30 °C+ readings roughly doubled since 1979.
Zhao, Q. et al. "Global, regional, and national burden of mortality associated with non-optimal ambient temperatures from 2000 to 2019." The Lancet Planetary Health, 2021. — ~489,000 heat-related deaths per year; 45% in Asia, 36% in Europe. Modelled estimate
The 2024 Report of the Lancet Countdown on Health and Climate Change.The Lancet, 2024. — Heat-related deaths among people over 65 in 2023 were 167% above the 1990s baseline; average exposure of 13.8 heatwave days per person in 2023.
Howard, J. T. et al. "Trends of Heat-Related Deaths in the US, 1999–2023." JAMA, 2024. — Annual series used in the trend chart; 1,069 deaths in 1999 rising to 2,325 in 2023; 21,518 total. Death-certificate counts under-record heat as a cause.
International Labour Organization. "Working on a Warmer Planet: The Impact of Heat Stress on Labour Productivity and Decent Work," 2019. — 2.2% of global working hours lost by 2030, equal to 80 million full-time jobs and US$2.4 trillion a year; agriculture 60% and construction 19% of lost hours. Projection
International Energy Agency. "The Future of Cooling," 2018, and IEA space-cooling tracking. — About 2 billion air-conditioning units in use consuming over 2,000 TWh of electricity a year; stock projected to reach 5.6 billion by 2050. Projection for 2050
Physiological reference values (normal core temperature 37 °C; heat stroke defined at roughly 40 °C core with CNS dysfunction; ~580 kcal removed per litre of sweat evaporated) are standard clinical and thermal-physiology figures, drawn from the NCBI Nutritional Needs in Hot Environments reference volume and heat-stroke clinical literature.