Scientists have known for decades that extreme heat is becoming more frequent. Both the research record and lived experience across the world document that shift. What has been far less understood is when these events occur within the year. Most research on seasonal warming has concentrated on average temperatures or on shifts in the timing of the seasons themselves. Summer conditions in mid-latitude regions, for example, have lengthened by roughly six days per decade since 1990. But the timing of individual extreme heat events—a different question entirely—had received little if any rigorous attention. Extremes may not move in step with the seasonal average, and that disconnection is precisely what the new study set out to measure.

The researchers expected that rising global temperatures would uniformly make it easier to cross dangerous heat thresholds throughout the year, widening the extreme heat season symmetrically at both ends. That is not what they found. Instead, the expansion is lopsided. “In some places, we have a larger expansion of extreme heat during the spring, before the traditional heat season starts. In other places, there’s a much faster expansion of the heat season into fall,” Ivanovich explains. The asymmetry means that different regions of the world are experiencing fundamentally different transformations of their heat regimes.

The methodology behind the findings was deliberately careful. The team counted extreme heat events on the six inhabited continents between 1980 and 1989, defining “extreme” as days falling in the hottest 5 percent of daily temperatures. They performed this counting twice, using two distinct measures of heat. The first was standard thermometer readings, a measure of dry heat. The second was wet bulb globe temperature, a more sophisticated metric that combines humidity, solar radiation and air temperature to quantify heat stress as the human body actually experiences it. This distinction matters enormously: humid heat limits the body’s ability to cool itself through sweating, making it considerably more dangerous to people, while dry heat is harder on crops and ecosystems. Adapting to one is not the same as adapting to the other.

The authors then compared those 1980s baseline figures to the most recent decade in the record, 2015 through 2024. The results were striking. Extreme heat seasons had expanded significantly across just over half of the world’s land area for dry heat, and just under half for humid heat. In the western United States, eastern China, northern Africa and eastern Europe, extreme heat events became more common more rapidly in the two months following their historical heat seasons. The opposite held in western Europe, southern Africa and northwestern India, where extreme heat arrived predominantly in the two months before the traditional season began. “There are very clear asymmetries in how extreme heat seasons are expanding in different parts of the world,” Ivanovich says. “Extreme heat is starting to become something different in a lot of these regions.”

To be certain the pattern was real and not an artifact of a single dataset, the researchers ran their analysis with two independent sources—one compiled by NASA and the other by the European Centre for Medium-Range Weather Forecasts. The pattern largely held up across both. This kind of replication is essential when studying extreme events, which are by definition rare. Extreme heat outside its season is rarer still, which makes changes in its timing statistically difficult to pin down. Ivanovich emphasizes that the findings should be taken as a compelling first line of evidence, and a next step is to repeat the comparison using climate simulations. Models can generate many more theoretical versions of reality under the same climatic conditions, providing a much larger sample of extreme events. If the models agree with what the observations show, that will strengthen the case that the observed changes represent a genuinely new pattern.

The timing of extreme heat matters for reasons that are both physiological and practical. Heat is harder on the human body when it arrives before people have acclimated to the season, or after they have already endured months of it. It is also harder for communities to prepare for, because cooling centers, heat alerts and public health campaigns are built around a summer calendar. A city that expects its heat season to end in September may find itself unprepared for a deadly hot spell in October. The data from Phoenix, Arizona, illustrate the phenomenon vividly. The city recorded 183 extreme heat days by temperature in the baseline decade of the 1980s, and 338 in the decade ending in 2024. None of the 1980s events fell after the heat season had ended, yet between 2015 and 2024, 6 percent did. The median date of the city’s dry heat extremes moved ten days later in the year, while its humid heat extremes moved 5.5 days earlier.

Phoenix’s recent experience underscores the stakes. After 113 consecutive days above 100 degrees Fahrenheit in 2024, stretching from late September into mid-October, the city went on to tie or break daily temperature records 21 days in a row. Maricopa County, where Phoenix is located, recorded 608 heat-related deaths in 2024, with 46 percent of them in July, the hottest month of the year. In 2023, July accounted for 64 percent of that year’s 645 deaths. And in March of this year, after the study period had ended, the city recorded nine days that topped 100 degrees Fahrenheit—something that had happened only once before in March over the entire historical record. The message is unambiguous: the shoulder seasons, once safe from dangerous heat, are no longer off-limits.

One of the most important questions the researchers addressed is what is driving the pattern. When they tested whether ordinary warming alone could reproduce it, rising average temperatures accounted for changes in the heart of the heat season but not for the lopsided expansion at its edges. Regional factors such as shifting rainfall patterns or changing land use are likely also at work. “We can’t confirm what share of the signal is due to climate change using observations alone,” Ivanovich says, but “it’s certainly the primary component of the story.” Climate models should help determine the respective contributions of human-induced climate change and natural variability, untangling how much of the asymmetry reflects a warming world and how much reflects local dynamics like soil moisture, irrigation and vegetation change.

The implications extend well beyond the heat season itself. Changes in the seasonal timing of extreme heat make it more likely that hot days will intersect with other seasonal hazards: peak wildfire season in the western United States, or peak hurricane season in the Southeast. When multiple hazards coincide or arrive in rapid succession, “they are much more dangerous and impactful than if these events happened in isolation,” Ivanovich says. A wildfire season that overlaps with an extended heat season strains emergency services, power grids and human health simultaneously. The study suggests that cities and public health agencies may need to rethink the entire architecture of heat preparedness, moving away from a summer-only framework toward one that treats dangerous heat as a year-round possibility in half the world’s land area. For the billions of people who live there, the hottest days of the year are no longer where the calendar says they should be.

News Publication Date: 10-Sep-2026

Web References: Not provided

References: Ivanovich, C., Cook, B., & McDermid, S. S. Dangerous Hot Days Are Spreading Beyond Summer. AGU Advances. https://www.eurekalert.org

The investigation, published as Strengthening England’s Heat Resilience: From Warnings to Long-Term Adaptation, combines survey data from adults in England with interviews involving professionals responsible for communicating heat risks. Around seven in ten people surveyed said they had encountered a Heat Health Alert. Yet the most common explanation among those who did nothing was not that the message was inconvenient or impossible to follow. Instead, many respondents said they did not consider themselves personally vulnerable to high temperatures. That response points to a psychological barrier at the centre of the problem: people may recognise that heat can be dangerous in general while excluding themselves from the group they believe is at risk.

The results arrive after a summer that placed England’s heat-health infrastructure under exceptional pressure. On 26 June, a provisional maximum temperature of 37.7°C was recorded at Lingwood, Strumpshaw Hill, breaking a June record that had stood since 1976. England was also placed under only the second Red Heat Health Alert since the current alert system was introduced. At Hastings, the minimum temperature reached 23.2°C, setting a new June record for the warmest night. Such high overnight temperatures are medically important because the body depends on cooler conditions during sleep to dissipate stored heat and recover from daytime thermal stress. When homes remain hot through the night, physiological strain can accumulate, especially among older adults and people with underlying illnesses.

Another spell of extreme heat in August prompted widespread amber alerts across England, again bringing attention to the country’s exposure to increasingly intense heat events. Heat is not simply an issue of discomfort. The human body maintains its core temperature through mechanisms including sweating and increased blood flow to the skin. In hot conditions, evaporation of sweat becomes the principal route for losing heat, but that process becomes less effective when humidity is high or when people cannot access water, shade or ventilation. Older people may have a reduced ability to sweat or regulate circulation, while cardiovascular, respiratory, renal and neurological conditions can make thermal stress more dangerous. Some medications can also affect hydration, sweating or blood pressure, meaning that heat risk depends on health, housing and social circumstances as well as the temperature shown on a weather forecast.

The UNU-INWEH analysis found that only 18% of people in England reported a high perception of heat risk. Approximately 30% said they knew little or nothing about the actions that could protect them. A similar proportion had encountered no heat alerts or heat-protection information at all, indicating that two different failures can occur: some people never receive a warning, while others receive one without understanding its relevance or knowing how to respond. The report also identifies uncertainty about the alerts themselves. Some respondents did not recognise what they were looking at, while others found the message difficult to interpret or were unclear about the practical steps expected of them. A warning may therefore fail before it becomes a decision, particularly when the language is abstract or does not connect the risk to everyday experiences such as sleeping in an overheated bedroom, travelling on public transport or caring for a family member.

“People aren’t ignoring these alerts because they don’t care,” said Dr Mehri Khosravi, a research fellow in behavioural change and risk communication at UNU-INWEH and lead author of the policy brief. “They read them and think, this isn’t about me.” Khosravi said heat continues to be perceived as good weather or as a threat limited to the very old and very ill. That perception can produce what risk researchers describe as a gap between hazard recognition and personal susceptibility. If people do not identify themselves as vulnerable, they are less likely to change routines, check on neighbours, drink more fluids, seek cooler environments or alter strenuous activity. The challenge is therefore not only to distribute technically accurate forecasts, but also to make the causal pathway from heat exposure to health effects visible and personally meaningful.

Interviews with professionals involved in England’s heat-risk system suggest that its communication architecture has been designed primarily for coordination among institutions. Among the practitioners interviewed, 76% identified informing the public as a communication objective, while substantially less emphasis was placed on motivating people to act or enabling them to do so. During a heatwave, alerts help national agencies, local authorities, health services and emergency responders coordinate their activities. That institutional function is essential, but it does not automatically translate into household-level protection. A message can move efficiently through government and health networks while failing to reach a person living alone, working outdoors, caring for an older relative or occupying a poorly ventilated home.

Digital communication has increased the speed and scale of heat warnings, but the report argues that speed should not be confused with equal access. Online platforms can distribute information rapidly, yet their reach depends on connectivity, digital confidence, language, trust and the habits of different age groups. Social media was identified as the least trusted source of heat information, particularly among older adults. The channels most heavily used by authorities may consequently be least effective for some of the people most exposed to harm. The researchers point to libraries, faith groups, voluntary organisations, social-care providers and other trusted intermediaries as ways to carry heat information into communities. These organisations can do more than repeat an alert: they can explain what it means, identify who may need help and connect general advice to the physical realities of local homes and neighbourhoods.

The report’s central argument is that heat resilience cannot be created by warnings alone. Housing conditions, deprivation, existing health problems and environmental exposure determine how much heat people experience and how effectively they can escape it. A warning cannot cool a top-floor flat that stores heat, provide shade in a treeless neighbourhood or repair a home with inadequate ventilation. The authors call for heat resilience to be incorporated into housing standards, planning policy and retrofit programmes, alongside stronger enforcement and implementation of Part O of England’s Building Regulations, which addresses overheating in residential buildings. They also recommend investment in local-authority and community capacity, because protective action often depends on organisations that know residents personally and can respond before a heatwave becomes a medical emergency.

“A warning tells you a heatwave is coming. It doesn’t tell you how to get through the night in a flat that traps heat,” said Professor Kaveh Madani, director of UNU-INWEH and a co-author of the policy brief. The researchers argue that future evaluation should measure whether alerts prompt protective behaviour rather than simply counting how many people saw them. That shift would treat communication as part of a technical intervention whose success can be tested through outcomes: whether people understand their personal risk, know what to do, can access a cooler place and receive support when they need it. England’s warning system may be effective at detecting and announcing dangerous heat, but the new findings suggest that the next stage of heat adaptation must move beyond notification. As extreme temperatures become a recurring public-health hazard, the decisive question will be whether warnings reach people in forms they trust—and whether the places where they live allow them to act.

Subject of Research: England’s heat-health warning system, public responses to heat alerts, risk communication and long-term heat adaptation.

Article Title: Strengthening England’s Heat Resilience: From Warnings to Long-Term Adaptation

News Publication Date: 14 August 2026

Web References: https://unu.edu/publication/strengthening-englands-heat-resilience-warnings-long-term-adaptation; https://unu.edu/inweh

References: Khosravi, M., Assan, A. and Madani, K. (2026). Strengthening England’s Heat Resilience: From Warnings to Long-Term Adaptation. United Nations University Institute for Water, Environment and Health. DOI: 10.53328/INP26PMK001. Support paper: Assan, A. A., Khosravi, F. and Osei, G. (2026). “The heat is on: Understanding public responses to heat-health alerts in England.” Energy Research & Social Science, 135, 104685.

Keywords: Heat waves, climatology, climate change, climate change adaptation, climate change effects, environmental health, human health, public policy, risk management, risk communication, behaviour modification, disaster management.

Current climate data underscores the gravity of the heat crisis. Approximately 3.6 billion individuals reside in regions highly vulnerable to climate fluctuations, with heat-related mortality exceeding 480,000 annually between 2000 and 2019. The implications extend beyond mortality; elevated temperatures degrade cognitive function, reduce work productivity, and exacerbate mental health through increased stress hormone levels and impaired sleep quality. The prevalence of more frequent and intense heat waves amplifies the necessity for personal cooling mechanisms that are both effective and environmentally sustainable.

Professor Dahua Shou, serving as the Limin Endowed Young Scholar in Advanced Textiles Technologies at PolyU, recently published a seminal article in Science that elucidates the integration of advanced materials science and artificial intelligence in personal cooling solutions. His research introduces innovative methodologies for manipulating and regulating human thermoregulation using apparel that dynamically interacts with the wearer’s physiology and external environment. By synergizing conventional and cutting-edge cooling mechanisms, his team has pushed the boundaries of textile science.

Central to this research is the exploitation of the four primary modes of heat transfer: radiation, conduction, convection, and evaporation. Each mechanism contributes uniquely to thermal management and, when intelligently integrated, can deliver superior cooling performance. The study proposes an AI-driven closed-loop architecture that interlinks sensing technologies, predictive algorithms, and actuators embedded within garments to maintain optimum thermal comfort—tailoring responses in real time to fluctuating ambient conditions and individual physiological signals.

The shift from passive to active cooling in textiles is a pivotal innovation in this domain. Spectrum-selective fabrics form the foundation, engineered to emit mid-infrared radiation efficiently while blocking incoming ultraviolet and visible spectrums, thus minimizing direct solar heat absorption. This selective radiative cooling not only conserves the body’s thermal balance but also mitigates urban heat island effects. Alongside this, conduction-tunable fillers enable garments to adjust thermal conductivity, enhancing insulation when necessary or promoting heat dissipation dynamically.

Complementing radiative and conductive elements, moisture-responsive fibers augment evaporative and convective heat loss. These specialty fibers intelligently manage perspiration by routing sweat away from the skin’s surface, facilitating rapid evaporation, and maintaining a dry microclimate. This moisture management is critical since accumulated sweat can impede cooling by increasing garment weight, reducing breathability, and diminishing radiative efficiency. By overcoming these limitations, these textiles sustain continuous cooling even during intense physical activity or prolonged heat exposure.

Integral to the system’s efficacy are lightweight wearable modules integrating thermoelectric, electrocaloric, and variable emittance devices powered by flexible solar cells and on-body energy storage. These components empower adaptive cooling controlled by AI algorithms that optimize energy use while maximizing comfort. This smart textile ecosystem operates with unprecedented agility to detect temperature rises, initiate cooling responses, and modulate thermal properties autonomously, thus extending comfort zones and reducing reliance on energy-intensive air conditioning.

Despite this exciting progress, the research highlights ongoing challenges. Achieving seamless real-time thermoregulation necessitates interdisciplinary collaboration across textile engineering, thermodynamics, flexible electronics, and machine learning. Scalability and sustainability in manufacturing remain paramount, requiring recyclable materials and user-tailored design attributes such as durability, washability, and aesthetic appeal. Furthermore, establishing standardized, user-centric performance metrics, including cooling power per watt and subjective thermal sensation, is essential to guide future development and consumer adoption.

One of the hallmark innovations emerging from Professor Shou’s team is the iActive™ intelligent sportswear. This garment leverages artificially engineered “sweat glands” powered by low-voltage actuation, coupled with a root-like liquid network that mirrors natural sweat distribution pathways, allowing rapid expulsion of perspiration droplets. This biomimetic design not only keeps the skin dry but also removes sweat at a rate surpassing human physiological limits by up to threefold, mitigating discomfort and enhancing cooling efficiency.

Another breakthrough is Omni-Cool-Dry™, a breathable, skin-like fabric engineered to direct sweat unidirectionally while employing spectrum-selective cooling functionality. This material actively reflects solar and ground radiation and enhances mid-infrared heat emission from the body, resulting in a significant reduction of skin temperature—by approximately 5°C compared to conventional textiles—thereby offering substantial relief in sun-exposed environments.

For industries operating in extreme heat conditions, the thermo-adaptive Soft Robotic Clothing integrates temperature-responsive soft actuators within textile structures. These actuators expand in response to temperature increases, augmenting fabric thickness to trap insulating still air, considerably elevating thermal resistance. This dynamic insulation yields thermal resistance values ranging from 0.23 to 0.48 K·m²/W and maintains inner garment temperatures up to 10°C cooler than standard insulating clothing when external temperatures reach up to 120°C, offering unprecedented protection and comfort.

The SweatMD wearable represents a convergence of textile science and biotechnology, featuring a completely textile-based, non-invasive microfluidic system that channels fresh sweat through biomimetic networks. Coupled with skin-friendly sensing yarns, it quantitatively tracks individual biomarkers such as glucose and potassium in real time, delivering actionable insights related to fatigue and hydration status directly to a connected smartphone, thereby facilitating personalized health monitoring alongside thermal management.

Together, these innovations form an AI-enabled ecosystem where physiological sensors gather continuous data streams, predictive models interpret thermal demand, and adaptive garments execute precise cooling responses. This system facilitates self-sustained personal cooling solutions that are not only energy-efficient but also seamlessly integrated into daily life—from casual wear and sportswear to protective industrial gear—expanding possibilities for heat resilience worldwide.

PolyU’s translational research strategy leverages partnerships across Mainland China and interdisciplinary centers, such as the PolyU-Xingguo Technology and Innovation Research Institute and the Research Centre of Textiles for Future Fashion. These collaborations facilitate the accelerated transformation of scientific breakthroughs into scalable, market-ready products adaptable to diverse climatic and occupational contexts, reinforcing the global impact of these technologies.

The significance of Professor Shou’s research is further validated by multiple prestigious recognitions, including Gold Medals at the Geneva Invention Exhibition (2024 and 2025) and the TechConnect Global Innovation Award. Additionally, his receipt of The Fiber Society’s Distinguished Achievement Award marks exceptional accomplishment, underscoring the transformative potential of his work in the field of textile science and personal thermal management.

Subject of Research: Sustainable personal cooling using advanced textiles and intelligent wearables to mitigate health risks from extreme heat.

Article Title: Sustainable personal cooling in a warming world

News Publication Date: 28-Aug-2025

Web References: DOI link

Image Credits: © 2025 Research and Innovation Office, The Hong Kong Polytechnic University. All Rights Reserved.

Keywords: Heat waves, Artificial intelligence, Textiles, Fibers, Sweating, Body temperature regulation