The wet-bulb temperature is a physical indicator that combines air temperature and relative humidity. Unlike the dry temperature we usually read on thermometers, wet-bulb temperature reflects the actual ability of the air to absorb and dissipate heat from the human body through evaporation.
This measurement is crucial for understanding the physiological limits of the human body. Our organism maintains an internal temperature of about 37°C through sweating: when sweat evaporates from the skin, it carries away thermal energy, cooling us down. But this evaporation mechanism only works if the surrounding air is not already saturated with water vapor.
The concept of wet-bulb temperature was formalized in the 19th century with the invention of the psychrometer, an instrument consisting of two thermometers: one with a dry bulb and the other with a wet bulb wrapped in a water-soaked wick. The temperature difference between the two allows determining the relative humidity of the air and deducing the wet-bulb temperature, an essential indicator for assessing heat stress.
Scientific research has established that a wet-bulb temperature of 35°C represents an absolute theoretical limit for human survival. At this threshold, even a perfectly healthy person, at complete rest, in the shade, and with unlimited access to water, can no longer dissipate the heat produced by their basal metabolism. This limit can be reached through different combinations of dry temperature and relative humidity. Heat becomes deadly long before reaching extreme temperatures when humidity is high.
| Temperature (°C) | Relative Humidity (%) | Tolerance Duration | Comment |
|---|---|---|---|
| 35°C | 100% | 6-8 hours max | Absolute theoretical survival limit |
| 35°C | 70% | Several hours to 1 day | Moderate discomfort, rest and hydration required |
| 40°C | 90% | 1-2 hours | Immediate danger, high risk of heatstroke |
| 40°C | 60% | 3-4 hours | Severe heat stress, risk of heatstroke |
| 40°C | 30% | 6-8 hours | Tolerable at rest with adequate hydration |
| 45°C | 70% | 1-2 hours | Immediate danger |
| 45°C | 40% | 2 hours | Limit even for acclimatized individuals |
| 45°C | 20% | 2-3 hours | Extreme desert conditions, rapid dehydration |
| 50°C | 50% | 30-60 minutes | Severe heat stress, urgent evacuation required |
| 50°C | 30% | 1-2 hours | Deadly danger even at rest, possible loss of consciousness |
| 50°C | 10% | 2-3 hours | Extreme aridity, respiratory burns, critical dehydration |
Climate change is dangerously amplifying the frequency and intensity of extreme humid heat episodes. Tropical and subtropical coastal regions are among the most vulnerable, as they combine high temperatures and high atmospheric humidity, a particularly deadly combination for the human body.
Climate projections identify several critical zones where the 35°C wet-bulb temperature threshold could be regularly exceeded by the end of the century if greenhouse gas emissions continue at the current rate. The Persian Gulf tops this concerning list: the United Arab Emirates, Qatar, Kuwait, and some coastal regions of Iran are already experiencing sporadic peaks approaching this deadly threshold. The Indian subcontinent is another major area of concern, with Pakistan, India, and Bangladesh experiencing increasingly intense heatwaves. The Ganges Delta, densely populated, combines extreme humidity and scorching temperatures during the pre-monsoon season.
Southeast Asia, including Thailand, Vietnam, the Philippines, and Indonesia, also shows increasing vulnerability, as does the Red Sea region and the Horn of Africa. Northern Australia, some coastal areas of Central America and the Caribbean, and the southern United States complete this geography of risk.
Recent data confirm the worsening of the phenomenon. In 2021, the Pakistani city of Jacobabad recorded a wet-bulb temperature of 34°C, nearing the human survival threshold. That same year, the heat dome that struck the Pacific Northwest caused hundreds of deaths in Canada and the United States, particularly in British Columbia, where the dry temperature exceeded 49°C in Lytton. Although wet-bulb temperatures remained slightly below 35°C, the massive mortality underscores the lethality of these events, even without reaching the absolute theoretical limit.
Climate models predict a alarming increase in extreme humid heat events during this century. According to the IPCC, in a +2°C warming scenario compared to the pre-industrial era, billions of people could be regularly exposed to wet-bulb temperatures exceeding 31°C.
With warming of +3°C or more, some densely populated regions could experience wet-bulb temperatures reaching or exceeding 35°C for several hours, or even several consecutive days. Such a situation would render these areas simply uninhabitable without permanent air conditioning.
A study published in Nature in 2023 estimates that by 2100, in the worst-case scenario (+4°C), up to 1.2 billion people could live in areas exposed each year to at least one deadly humid heat event. These projections exclude technological adaptation capacities but highlight the potential scale of the crisis.
Source: Science Advances – The emergence of heat and humidity too severe for human tolerance (Colin Raymond et al., 2020), NCBI – Evaluating the 35°C wet-bulb temperature adaptability threshold (PSU HEAT Project), NCBI – Greatly enhanced risk to humans as a consequence of empirically determined lower moist heat stress tolerance, NCBI – Temperature and humidity based projections of a rapid rise in global heat stress exposure, IPCC – Reports on Climate Change, Nature Scientific Data – Daily Max Simplified Wet-Bulb Globe Temperature dataset (1940-2022), NOAA Climate.gov – Brief periods of dangerous humid heat arrive decades early.
Wet-Bulb Temperature: Humanity's Climate Time Bomb 
The Sky Ordered Our Week: Monday, Tuesday, Wednesday... 
Why Doesn't Earth's Atmosphere Escape into Space? 
Origins of Earth: Magmatic Chaos and the Birth of the Solid World 
Earth's Atmosphere: The Invisible Shield Protecting Life 
The Three Ages of Terrestrial Water: Multiple Origins 
From Carbon 14 to Uranium-Lead: The Science of Dating 
The Boundary of Space: Where Does It Really Begin? 
The Leap Second 
Planetary Alignments: A Fascinating but Relative
Phenomenon 
All Deserts of the World 
Earth's Paleoclimates and Carbon Dioxide 
Three Gorges Dam and Day Length 
International Date Line 
Start Date of the Seasons: A Complex Celestial Mechanism 
Reversal of the Earth's magnetic field over time 
3D Simulator: Revolutions of the Planets 
Geological Time and Mass Extinctions: From the Ordovician to the Cretaceous 
Earth History: Timeline Compressed into 24 Hours 
Earth's Water: An Extraterrestrial Origin? 
Reading the Sky: Understanding Declination and Right
Ascension 
Inversions of the Earth's Magnetic Field 
Earth's Magnetosphere: The Invisible Shield 
Fujita Scale: Classifying the Fury of Tornadoes 
Earth's Radioactivity: The Internal Energy of Our Planet 
The Deep Sea: Exploring the Deepest Ocean Trenches 
Why Are Days Getting Longer? 
The Depths of the Earth: From the Lithosphere to the Core 
Earth's Movements: How Earth Traces Its Spiral Through the Universe? 
How much is the sea level rising? 
The Life of Earth: Structure and Layers of the Earth 
The Eccentricity of Earth's Orbit: An Ellipse That Changes
Everything 
Little Ice Age: The History of a Natural Climatic Cooling 
The Quest for Time: How Ancient Civilizations Used
Astronomy? 
Van Allen Radiation Belt: A Shield Against Cosmic Particles 
Axis of rotation of the planets or obliquity 
The Cosmos in Our Measure: When Man Invents the Universe 
The Galilean Cut 
What is the pendulum fixed relative to? 
Problem of longitude at sea 
World population, still gallopingin 2008 
The Oldest Image of Earth from Space 
Earth's Obliquity and Variations in Its Axis of Rotation 
The Astronomical Unit Fixed: A Length, No Longer an Orbit 
One Molecule, Three States: Solid, Liquid, Gas on a Single Planet 
Simulator, the round of near-Earth cruisers 
Satellites that measure underwater relief 
Hadean's Hell 
The First Free Flight of an Astronaut in Space 
The first measurement of the Earth-Sun
distance 
La The Announced End of the World on December 21, 2012: A Millennial
Prophecy
The Equinoxes: An Astronomical Event
Milankovitch Cycles: The Astronomical Rhythm of Earth's Climate