For approximately two decades, a single figure has dominated scientific and policy discussions regarding the upper limits of human tolerance to extreme heat: 35 degrees Celsius measured on the wet-bulb thermometer. The theory posits that above this point, the body's primary cooling mechanism—perspiration evaporating from skin—cannot function rapidly enough to prevent core body temperature from reaching fatal levels. This physiological principle has been incorporated into peer-reviewed climate models, government risk assessments, and the foundational assumptions underlying infrastructure planning throughout Europe and elsewhere.
A recent investigation in Nature Communications has now demonstrated that this benchmark has been systematically misapplied, treated as a policy threshold rather than a physiological fact. The uncomfortable conclusion is unavoidable: the temperature line that climate science has designated as the dividing point between danger and fatality is not actually where deaths begin occurring. Mortality commences considerably earlier. The 35°C marker represents the theoretical maximum, not the practical starting point.
The research modelled six of the deadliest heatwaves in recorded history—events collectively responsible for tens of thousands of confirmed excess deaths—and discovered that none of them ever crossed the 35°C threshold. Not a single one. Yet all six produced mass mortality. The modelling reconstructed the actual biophysical experience of exposed populations using frameworks incorporating clothing, activity levels, building heat retention, solar radiation, and age-related declines in thermoregulatory function, rather than relying solely on atmospheric measurements.
Understanding wet-bulb temperature
Wet-bulb temperature differs fundamentally from the air temperature displayed on conventional thermometers. It represents a combined measurement integrating both heat and humidity—specifically, the temperature reached by a surface as water evaporates from it. In practical application, it approximates the maximum cooling achievable through perspiration: when wet-bulb temperature rises, the surrounding air already contains sufficient moisture that evaporation becomes slower and the body's cooling capacity through sweating diminishes accordingly. As wet-bulb readings increase, the body's ability to regulate its own temperature deteriorates.
Laboratory research on young, healthy adults under controlled conditions established the 35°C wet-bulb figure. Under such circumstances, even a resting, unclothed person with unlimited water access cannot prevent their core temperature from rising. The figure remains physiologically sound for those specific conditions. However, the Nature Communications study reveals the fundamental problem: those conditions do not match the circumstances under which people actually perish during real heatwaves.
Actual heatwave mortality concentrates among populations that are elderly, chronically ill, sedentary, and frequently unable to rely on sweating as their primary defense mechanism. Aging independently reduces thermoregulatory efficiency regardless of external conditions. Cardiovascular disease compounds vulnerability as hearts struggle to redirect blood circulation for cooling purposes. Residents of buildings that absorb and retain daytime heat—encompassing the majority of urban public housing stock across Europe—experience effective wet-bulb exposures substantially exceeding what weather station data indicates. The 35°C standard emerged from an optimal-case scenario yet has been applied to populations facing worst-case circumstances.
The six heatwave events and their findings
The six heatwave events examined in the research span different regions and time periods but share one defining feature: they rank among the most deadly heat episodes in historical records. The modelling reconstructed the actual biophysical conditions experienced by people during those events using frameworks that account for clothing, activity level, building heat retention, solar radiation, and age-related reductions in thermoregulatory capacity, rather than just raw atmospheric readings.
In every instance, the modelled conditions crossed the threshold for physiological unsustainability—the point at which the body cannot maintain safe core temperature—at wet-bulb readings substantially below 35°C. For the most vulnerable populations modelled, the proportion of six-hour windows crossing the survivability threshold ranged from roughly 12% of the event period in Mecca to 24% in Phoenix, in conditions the 35°C wet-bulb standard would have classified as safe. The people dying in these events were not exceeding the safety line. They were dying well short of it, in conditions the existing framework characterised as dangerous but survivable.
The researchers emphasize that this finding does not undermine the 35°C figure as a physiological ceiling. Rather, it invalidates the practice of treating that ceiling as the appropriate planning threshold. The data demonstrates that the effective danger zone—where population-level mortality risk increases sharply—begins well below the theoretical maximum, and that the space between the two is where the majority of heat-related deaths actually occur.
Implications for European policy frameworks
Over the past three years, European governments have substantially overhauled their heat response frameworks, prompted by the mortality data from summers 2022 and 2023. A study in Nature Medicine estimated excess heat mortality across the continent at over 61,000 in summer 2022 alone, a figure the WHO has subsequently referenced in its own regional analysis. Nations including France, Spain, Portugal, and the United Kingdom have modernized their heat emergency protocols, expanded early warning systems, and in certain cases established mandatory cooling provisions for at-risk populations.
The Nature Communications findings suggest that much of this policy overhaul may still operate from an incorrect baseline. If early warning systems are calibrated to activate at thresholds derived from the 35°C ceiling, and actual mortality risk rises sharply at substantially lower wet-bulb readings, the systems are supplying less advance notice than planners believe. The populations facing greatest risk—the elderly, those with chronic conditions, people without cooling access—may already be experiencing dangerous conditions before the official response infrastructure activates.
This carries direct consequences for how European cities approach heat as an urban infrastructure challenge. The EU Mission on Adaptation to Climate Change has encouraged member states to develop heat resilience strategies, but the specific temperature thresholds embedded in those plans may require reassessment based on this new research. The definition of a heat emergency, according to this study's evidence, encompasses a broader category than current frameworks recognize.
Consequences for technology and infrastructure sectors
For technology firms and the broader infrastructure sector, the study presents practical implications extending beyond public health policy alone. Data centres already encounter substantial operational difficulties during heat events due to cooling system demands and electricity grid strain. Although the study does not directly address infrastructure planning, its central finding—that the 35°C wet-bulb ceiling underestimates real-world risk—poses a significant question for any contingency planning relying on that ceiling as a baseline: if the actual danger threshold lies lower than assumed, the planning window for high-risk periods may be larger than current models suggest.
The same logic applies to cold chain logistics, urban transportation networks, and the broad spectrum of outdoor-dependent operations—construction, agriculture, utilities maintenance—that engage large workforces across southern Europe. Occupational heat safety standards, which establish permissible exposure limits for outdoor workers, frequently depend on the same single-threshold approach the study critiques. While the paper does not evaluate any particular occupational standard, its conclusions raise a pertinent question for regulators: whether exposure limits constructed around the 35°C ceiling provide sufficient protection for outdoor workers who are older, have limited access to shade, or already manage chronic conditions—the populations the study identifies as most at risk well below that line.
The threshold has been interpreted as a safe boundary. The study indicates it was always an extreme endpoint, and that the actual risk resides in the gap between the two.
What happens next
The Nature Communications paper will probably stimulate substantial additional research, particularly focused on identifying population-specific effective thresholds: the wet-bulb levels at which mortality risk becomes elevated for elderly populations, for individuals with particular comorbidities, and for people residing in different housing types. Accumulating that research and translating it into revised policy guidance will require time. In the meantime, the study's fundamental finding—that the 35°C line represents an endpoint rather than a boundary—remains available to policymakers, infrastructure planners, and regulators prepared to act upon it.
The authors refrain from proposing a replacement threshold. The data does not yield a single straightforward number suitable for substitution in every planning context. Instead, it produces a more accurate understanding of where heat actually kills, which is consistently in conditions that existing frameworks have classified as manageable.
So which city moves first? Which mayor will publicly acknowledge before next summer that cooling centres open too late, that warning systems activate at a number derived from a laboratory ceiling no actual heatwave has ever required surpassing? The six events in this study killed tens of thousands of people without ever reaching 35°C. The threshold has already proven inadequate. The sole remaining question is how many additional summers planners will spend waiting for it to fail again.
Source: Silicon Canals
