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    AI performs as well as medical specialists in analyzing lung disease, research shows

    A Nagoya University research group has developed an AI algorithm that accurately and quickly diagnoses idiopathic pulmonary fibrosis, a lung disease. The algorithm makes its diagnosis based only on information from non-invasive examinations, including lung images and medical information collected during daily medical care.
    Doctors have waited a long time for an early means of diagnosing idiopathic pulmonary fibrosis, a potentially fatal disease that can scar a person’s lungs. Except for drugs that can delay the disease’s progression, established therapies do not exist. Since doctors face many difficulties diagnosing the disease, they often have to request a specialist diagnosis. In addition, many of the diagnostic techniques, such as lung biopsy, are highly invasive. These investigative measures may exacerbate the disease, increasing a patient’s risk of dying.
    Taiki Furukawa, Assistant Professor of the Nagoya University Hospital, in collaboration with RIKEN and Tosei General Hospital, has developed a new technology to diagnose idiopathic pulmonary fibrosis. Using artificial intelligence (AI), the group analyzed medical data from patients in Tosei General Hospital’s interstitial pneumonia treatment facility, collected during normal care. They found that their AI diagnosed idiopathic pulmonary fibrosis with a similar level of accuracy as a human specialist. They published their results in the journal Respirology.
    Despite finding that their AI performed just as well as experts, the team stress that they do not see it as replacing medical professionals. Instead, they hope that specialists will use AI in medical treatment to ensure that they do not miss opportunities for early treatment. Its use would also avoid invasive procedures, such as lung biopsies, which could save lives.
    “Idiopathic pulmonary fibrosis has a very poor prognosis among lung diseases,” Furukawa says. “It has been difficult to diagnose even for general respiratory physicians. The diagnostic AI developed in this study would allow any hospital to get a diagnosis equivalent to that of a specialist. For idiopathic pulmonary fibrosis, the developed diagnostic AI is useful as a screening tool and may lead to personalized medicine by collaborating with medical specialists.”
    Furukawa is excited about the possibilities: “The practical application of diagnostic AI and collaborative diagnosis with specialists may lead to a more accurate diagnosis and treatment. We expect it to revolutionize medical care.”
    This study was supported by JSPS KAKENHI, Grant/Award Number: JP19110253; The Hori Science and Arts Foundation; The Japanese Respiratory Foundation.
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    Magnetic quantum material broadens platform for probing next-gen information technologies

    Scientists at the Department of Energy’s Oak Ridge National Laboratory used neutron scattering to determine whether a specific material’s atomic structure could host a novel state of matter called a spiral spin liquid. By tracking tiny magnetic moments known as “spins” on the honeycomb lattice of a layered iron trichloride magnet, the team found the first 2D system to host a spiral spin liquid.
    The discovery provides a test bed for future studies of physics phenomena that may drive next-generation information technologies. These include fractons, or collective quantized vibrations that may prove promising in quantum computing, and skyrmions, or novel magnetic spin textures that could advance high-density data storage.
    “Materials hosting spiral spin liquids are particularly exciting due to their potential to be used to generate quantum spin liquids, spin textures and fracton excitations,” said ORNL’s Shang Gao, who led the study published in Physical Review Letters.
    A long-held theory predicted that the honeycomb lattice can host a spiral spin liquid — a novel phase of matter in which spins form fluctuating corkscrew-like structures.
    Yet, until the present study, experimental evidence of this phase in a 2D system had been lacking. A 2D system comprises a layered crystalline material in which interactions are stronger in the planar than in the stacking direction.
    Gao identified iron trichloride as a promising platform for testing the theory, which was proposed more than a decade ago. He and co-author Andrew Christianson of ORNL approached Michael McGuire, also of ORNL, who has worked extensively on growing and studying 2D materials, asking if he would synthesize and characterize a sample of iron trichloride for neutron diffraction measurements. Like 2D graphene layers exist in bulk graphite as honeycomb lattices of pure carbon, 2D iron layers exist in bulk iron trichloride as 2D honeycomb layers. “Previous reports hinted that this interesting honeycomb material could show complex magnetic behavior at low temperatures,” McGuire said. More

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    Researchers 3D print sensors for satellites

    MIT scientists have created the first completely digitally manufactured plasma sensors for orbiting spacecraft. These plasma sensors, also known as retarding potential analyzers (RPAs), are used by satellites to determine the chemical composition and ion energy distribution of the atmosphere.
    The 3D-printed and laser-cut hardware performed as well as state-of-the-art semiconductor plasma sensors that are manufactured in a cleanroom, which makes them expensive and requires weeks of intricate fabrication. By contrast, the 3D-printed sensors can be produced for tens of dollars in a matter of days.
    Due to their low cost and speedy production, the sensors are ideal for CubeSats. These inexpensive, low-power, and lightweight satellites are often used for communication and environmental monitoring in Earth’s upper atmosphere.
    The researchers developed RPAs using a glass-ceramic material that is more durable than traditional sensor materials like silicon and thin-film coatings. By using the glass-ceramic in a fabrication process that was developed for 3D printing with plastics, there were able to create sensors with complex shapes that can withstand the wide temperature swings a spacecraft would encounter in lower Earth orbit.
    “Additive manufacturing can make a big difference in the future of space hardware. Some people think that when you 3D-print something, you have to concede less performance. But we’ve shown that is not always the case. Sometimes there is nothing to trade off,” says Luis Fernando Velásquez-García, a principal scientist in MIT’s Microsystems Technology Laboratories (MTL) and senior author of a paper presenting the plasma sensors.
    Joining Velásquez-García on the paper are lead author and MTL postdoc Javier Izquierdo-Reyes; graduate student Zoey Bigelow; and postdoc Nicholas K. Lubinsky. The research is published in Additive Manufacturing. More

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    A key role for quantum entanglement

    A method known as quantum key distribution has long held the promise of communication security unattainable in conventional cryptography. An international team of scientists has now demonstrated experimentally, for the first time, an approach to quantum key distribution that is based on high-quality quantum entanglement — offering much broader security guarantees than previous schemes.
    The art of cryptography is to skillfully transform messages so that they become meaningless to everyone but the intended recipients. Modern cryptographic schemes, such as those underpinning digital commerce, prevent adversaries from illegitimately deciphering messages — say, credit-card information — by requiring them to perform mathematical operations that consume a prohibitively large amount of computational power. Starting from the 1980s, however, ingenious theoretical concepts have been introduced in which security does not depend on the eavesdropper’s finite number-crunching capabilities. Instead, basic laws of quantum physics limit how much information, if any, an adversary can ultimately intercept. In one such concept, security can be guaranteed with only a few general assumptions about the physical apparatus used. Implementations of such ‘device-independent’ schemes have long been sought after, but remained out of reach. Until now, that is. Writing in Nature, an international team of researchers from the University of Oxford, EPFL, ETH Zurich, the University of Geneva and CEA report the first demonstration of this sort of protocol — taking a decisive step towards practical devices offering such exquisite security.
    The key is a secret
    Secure communication is all about keeping information private. It might be surprising, therefore, that in real-world applications large parts of the transactions between legitimate users are played out in public. The key is that sender and receiver do not have to keep their entire communication hidden. In essence, they only have to share one ‘secret’; in practice, this secret is string of bits, known as a cryptographic key, that enables everyone in its possession to turn coded messages into meaningful information. Once the legitimate parties have ensured for a given round of communication that they, and only they, share such a key, pretty much all the other communication can happen in plain view, for everyone to see. The question, then, is how to ensure that only the legitimate parties share a secret key. The process of accomplishing this is known as ‘key distribution’.
    In the cryptographic algorithms underlying, for instance, RSA — one of the most widely used cryptographic systems — key distribution is based on the (unproven) conjecture that certain mathematical functions are easy to compute but hard to revert. More specifically, RSA relies on the fact that for today’s computers it is hard to find the prime factors of a large number, whereas it is easy for them to multiply known prime factors to obtain that number. Secrecy is therefore ensured by mathematical difficulty. But what is impossibly difficult today might be easy tomorrow. Famously, quantum computers can find prime factors significantly more efficiently than classical computers. Once quantum computers with a sufficiently large number of qubits become available, RSA encoding is destined to become penetrable.
    But quantum theory provides the basis not only for cracking the cryptosystems at the heart of digital commerce, but also for a potential solution to the problem: a way entirely different from RSA for distributing cryptographic keys — one that has nothing to do with the hardness of performing mathematical operations, but with fundamental physical laws. Enter quantum key distribution, or QKD for short.
    Quantum-certified security
    In 1991, the Polish-British physicist Artur Ekert showed in a seminal paper that the security of the key-distribution process can be guaranteed by directly exploiting a property that is unique to quantum systems, with no equivalent in classical physics: quantum entanglement. Quantum entanglement refers to certain types of correlations in the outcomes of measurements performed on separate quantum systems. Importantly, quantum entanglement between two systems is exclusive, in that nothing else can be correlated to these systems. In the context of cryptography this means that sender and receiver can produce between them shared outcomes through entangled quantum systems, without a third party being able to secretly gain knowledge about these outcomes. Any eavesdropping leaves traces that clearly flag the intrusion. In short: the legitimate parties can interact with one another in ways that are — thanks to quantum theory — fundamentally beyond any adversary’s control. In classical cryptography, an equivalent security guarantee is provably impossible.
    Over the years, it was realized that QKD schemes based on the ideas introduced by Ekert can have a further remarkable benefit: users have to make only very general assumptions regarding the devices employed in the process. By contrast, earlier forms of QKD based on other basic principles require detailed knowledge about the inner workings of the devices used. The novel form of QKD is now generally known as ‘device-independent QKD’ (DIQKD), and an experimental implementation thereof became a major goal in the field. Hence the excitement as such a breakthrough experiment has now finally been achieved.
    Culmination of years of work
    The scale of the challenge is reflected in the breadth of the team, which combines leading experts in theory and experiment. The experiment involved two single ions — one for the sender and one for the receiver — confined in separate traps that were connected with an optical-fibre link. In this basic quantum network, entanglement between the ions was generated with record-high fidelity over millions of runs. Without such a sustained source of high-quality entanglement, the protocol could not have been run in a practically meaningful manner. Equally important was to certify that the entanglement is suitably exploited, which is done by showing that conditions known as Bell inequalities are violated. Moreover, for the analysis of the data and an efficient extraction of the cryptographic key, significant advances in the theory were needed.
    In the experiment, the ‘legitimate parties’ — the ions — were located in one and the same laboratory. But there is a clear route to extending the distance between them to kilometres and beyond. With that perspective, together with further recent progress made in related experiments in Germany and China, there is now a real prospect of turning the theoretical concept of Ekert into practical technology. More

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    Quantum cryptography: Hacking is futile

    The Internet is teeming with highly sensitive information. Sophisticated encryption techniques generally ensure that such content cannot be intercepted and read. But in the future high-performance quantum computers could crack these keys in a matter of seconds. It is just as well, then, that quantum mechanical techniques not only enable new, much faster algorithms, but also exceedingly effective cryptography.
    Quantum key distribution (QKD) — as the jargon has it — is secure against attacks on the communication channel, but not against attacks on or manipulations of the devices themselves. The devices could therefore output a key which the manufacturer had previously saved and might conceivably have forwarded to a hacker. With device- independent QKD (abbreviated to DIQKD), it is a different story. Here, the cryptographic protocol is independent of the device used. Theoretically known since the 1990s, this method has now been experimentally realized for the first time, by an international research group led by LMU physicist Harald Weinfurter and Charles Lim from the National University of Singapore (NUS).
    For exchanging quantum mechanical keys, there are different approaches available. Either light signals are sent by the transmitter to the receiver, or entangled quantum systems are used. In the present experiment, the physicists used two quantum mechanically entangled rubidium atoms, situated in two laboratories located 400 meters from each other on the LMU campus. The two locations are connected via a fiber optic cable 700 meters in length, which runs beneath Geschwister Scholl Square in front of the main building.
    To create an entanglement, first the scientists excite each of the atoms with a laser pulse. After this, the atoms spontaneously fall back into their ground state, each thereby emitting a photon. Due to the conservation of angular momentum, the spin of the atom is entangled with the polarization of its emitted photon. The two light particles travel along the fiber optic cable to a receiver station, where a joint measurement of the photons indicates an entanglement of the atomic quantum memories.
    To exchange a key, Alice und Bob — as the two parties are usually dubbed by cryptographers — measure the quantum states of their respective atom. In each case, this is done randomly in two or four directions. If the directions correspond, the measurement results are identical on account of entanglement and can be used to generate a secret key. With the other measurement results, a so-called Bell inequality can be evaluated. Physicist John Stewart Bell originally developed these inequalities to test whether nature can be described with hidden variables. “It turned out that it cannot,” says Weinfurter. In DIQKD, the test is used “specifically to ensure that there are no manipulations at the devices — that is to say, for example, that hidden measurement results have not been saved in the devices beforehand,” explains Weinfurter.
    In contrast to earlier approaches, the implemented protocol, which was developed by researchers at NUS, uses two measurement settings for key generation instead of one: “By introducing the additional setting for key generation, it becomes more difficult to intercept information, and therefore the protocol can tolerate more noise and generate secret keys even for lower-quality entangled states,” says Charles Lim.
    With conventional QKD methods, by contrast, security is guaranteed only when the quantum devices used have been characterized sufficiently well. “And so, users of such protocols have to rely on the specifications furnished by the QKD providers and trust that the device will not switch into another operating mode during the key distribution,” explains Tim van Leent, one of the four lead authors of the paper alongside Wei Zhang and Kai Redeker. It has been known for at least a decade that older QKD devices could easily be hacked from outside, continues van Leent.
    “With our method, we can now generate secret keys with uncharacterized and potentially untrustworthy devices,” explains Weinfurter. In fact, he had his doubts initially whether the experiment would work. But his team proved his misgivings were unfounded and significantly improved the quality of the experiment, as he happily admits. Alongside the cooperation project between LMU and NUS, another research group from the University of Oxford demonstrated the device-independent key distribution. To do this, the researchers used a system comprising two entangled ions in the same laboratory. “These two projects lay the foundation for future quantum networks, in which absolutely secure communication is possible between far distant locations,” says Charles Lim.
    One of the next goals is to expand the system to incorporate several entangled atom pairs. “This would allow many more entanglement states to be generated, which increases the data rate and ultimately the key security,” says van Leent. In addition, the researchers would like to increase the range. In the present set-up, it was limited by the loss of around half the photons in the fiber between the laboratories. In other experiments, the researchers were able to transform the wavelength of the photons into a low-loss region suitable for telecommunications. In this way, for just a little extra noise, they managed to increase the range of the quantum network connection to 33 kilometers. More

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    Breakthrough quantum algorithm

    City College of New York physicist Pouyan Ghaemi and his research team are claiming significant progress in using quantum computers to study and predict how the state of a large number of interacting quantum particles evolves over time. This was done by developing a quantum algorithm that they run on an IBM quantum computer. “To the best of our knowledge, such particular quantum algorithm which can simulate how interacting quantum particles evolve over time has not been implemented before,” said Ghaemi, associate professor in CCNY’s Division of Science.
    Entitled “Probing geometric excitations of fractional quantum Hall states on quantum computers,” the study appears in the journal of Physical Review Letters.
    “Quantum mechanics is known to be the underlying mechanism governing the properties of elementary particles such as electrons,” said Ghaemi. “But unfortunately there is no easy way to use equations of quantum mechanics when we want to study the properties of large number of electrons that are also exerting force on each other due to their electric charge.
    His team’s discovery, however, changes this and raises other exciting possibilities.
    “On the other front, recently, there has been extensive technological developments in building the so-called quantum computers. These new class of computers utilize the law of quantum mechanics to preform calculations which are not possible with classical computers.”
    We know that when electrons in material interact with each other strongly, interesting properties such as high-temperature superconductivity could emerge,” Ghaemi noted. “Our quantum computing algorithm opens a new avenue to study the properties of materials resulting from strong electron-electron interactions. As a result it can potentially guide the search for useful materials such as high temperature superconductors.”
    He added that based on their results, they can now potentially look at using quantum computers to study many other phenomena that result from strong interaction between electrons in solids. “There are many experimentally observed phenomena that could be potentially understood using the development of quantum algorithms similar to the one we developed.”
    The research was done at CCNY — and involved an interdisciplinary team from the physics and electrical engineering departments — in collaboration with experts from Western Washington University, Leeds University in the UK; and Schlumberger-Doll Research Center in Cambridge, Massachusetts. The research was funded by the National Science Foundation and Britain’s Engineering and Science Research Council.
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    Quantum entanglement makes quantum communication even more secure

    Stealthy communication just got more secure, thanks to quantum entanglement.

    Quantum physics provides a way to share secret information that’s mathematically proven to be safe from the prying eyes of spies. But until now, demonstrations of the technique, called quantum key distribution, rested on an assumption: The devices used to create and measure quantum particles have to be known to be flawless. Hidden defects could allow a stealthy snoop to penetrate the security unnoticed.

    Now, three teams of researchers have demonstrated the ability to perform secure quantum communication without prior confirmation that the devices are foolproof. Called device-independent quantum key distribution, the method is based on quantum entanglement, a mysterious relationship between particles that links their properties even when separated over long distances.

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    In everyday communication, such as the transmission of credit card numbers over the internet, a secret code, or key, is used to garble the information, so that it can be read only by someone else with the key. But there’s a quandary: How can a distant sender and receiver share that key with one another while ensuring that no one else has intercepted it along the way?

    Quantum physics provides a way to share keys by transmitting a series of quantum particles, such as particles of light called photons, and performing measurements on them. By comparing notes, the users can be sure that no one else has intercepted the key. Those secret keys, once established, can then be used to encrypt the sensitive intel (SN: 12/13/17). By comparison, standard internet security rests on a relatively shaky foundation of math problems that are difficult for today’s computers to solve, which could be vulnerable to new technology, namely quantum computers (SN: 6/29/17).

    But quantum communication typically has a catch. “There cannot be any glitch that is unforeseen,” says quantum physicist Valerio Scarani of the National University of Singapore. For example, he says, imagine that your device is supposed to emit one photon but unknown to you, it emits two photons. Any such flaws would mean that the mathematical proof of security no longer holds up. A hacker could sniff out your secret key, even though the transmission seems secure.

    Device-independent quantum key distribution can rule out such flaws. The method builds off of a quantum technique known as a Bell test, which involves measurements of entangled particles. Such tests can prove that quantum mechanics really does have “spooky” properties, namely nonlocality, the idea that measurements of one particle can be correlated with those of a distant particle. In 2015, researchers performed the first “loophole-free” Bell tests, which certified beyond a doubt that quantum physics’ counterintuitive nature is real (SN: 12/15/15).

    “The Bell test basically acts as a guarantee,” says Jean-Daniel Bancal of CEA Saclay in France. A faulty device would fail the test, so “we can infer that the device is working properly.”

    In their study, Bancal and colleagues used entangled, electrically charged strontium atoms separated by about two meters. Measurements of those ions certified that their devices were behaving properly, and the researchers generated a secret key, the team reports in the July 28 Nature.

    Typically, quantum communication is meant for long-distance dispatches. (To share a secret with someone two meters away, it would be easier to simply walk across the room.) So Scarani and colleagues studied entangled rubidium atoms 400 meters apart. The setup had what it took to produce a secret key, the researchers report in the same issue of Nature. But the team didn’t follow the process all the way through: The extra distance meant that producing a key would have taken months.

    In the third study, published in the July 29 Physical Review Letters, researchers wrangled entangled photons rather than atoms or ions. Physicist Wen-Zhao Liu of the University of Science and Technology of China in Hefei and colleagues also demonstrated the capability to generate keys, at distances up to 220 meters. This is particularly challenging to do with photons, Liu says, because photons are often lost in the process of transmission and detection.

    Loophole-free Bell tests are already no easy feat, and these techniques are even more challenging, says physicist Krister Shalm of the National Institute of Standards and Technology in Boulder, Colo. “The requirements for this experiment are so absurdly high that it’s just an impressive achievement to be able to demonstrate some of these capabilities,” says Shalm, who wrote a perspective in the same issue of Nature.

    That means that the technique won’t see practical use anytime soon, says physicist Nicolas Gisin of the University of Geneva, who was not involved with the research.

    Still, device-independent quantum key distribution is “a totally fascinating idea,” Gisin says. Bell tests were designed to answer a philosophical question about the nature of reality — whether quantum physics really is as weird as it seems. “To see that this now becomes a tool that enables something else,” he says, “this is the beauty.” More

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    Humans may not be able to handle as much heat as scientists thought

    More than 2,000 people dead from extreme heat and wildfires raging in Portugal and Spain. High temperature records shattered from England to Japan. Overnights that fail to cool.

    Brutal heat waves are quickly becoming the hallmark of the summer of 2022.

    And even as climate change continues to crank up the temperature, scientists are working fast to understand the limits of humans’ resilience to heat extremes. Recent research suggests that heat stress tolerance in people may be lower than previously thought. If true, millions more people could be at risk of succumbing to dangerous temperatures sooner than expected.

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    “Bodies are capable of acclimating over a period of time” to temperature changes, says Vivek Shandas, an environmental planning and climate adaptation researcher at Portland State University in Oregon. Over geologic time, there have been many climate shifts that humans have weathered, Shandas says. “[But] we’re in a time when these shifts are happening much more quickly.”

    Just halfway through 2022, heat waves have already ravaged many countries. The heat arrived early in southern Asia: In April, Wardha, India, saw a high of 45° Celsius (113° Fahrenheit); in Nawabshah, Pakistan, in May recorded temperatures rose to 49.5° C (121.1° F).

    Extreme heat alerts blared across Europe beginning in June and continuing through July, the rising temperatures exacerbating drought and sparking wildfires. The United Kingdom shattered its hottest-ever record July 19 when temperatures reached 40.3° C in the English village of Coningsby. The heat fueled fires in France, forcing thousands to evacuate from their homes. 

    And the litany goes on: Japan experienced its worst June heat wave since record-keeping began in 1875, leading to the country’s highest-ever recorded June temperature of 40.2° C.  China’s coastal megacities, from Shanghai to Chengdu, were hammered by heat waves in July as temperatures in the region also rose above 40° C. And in the United States, a series of heat waves gripped the Midwest, the South and the West in June and July. Temperatures soared to 42° C in North Platte, Neb., and to 45.6° C in Phoenix.

    The current global rate of warming on Earth is unprecedented (SN: 7/24/19). And scientists have long predicted that human-caused climate change will increase the occurrence of heat waves. Globally, humans’ exposure to extreme heat tripled from 1983 to 2016, particularly in South Asia.

    The heat already is taking an increasing toll on human health. It can cause heat cramps, heat exhaustion and heat stroke, which is often fatal. Dehydration can lead to kidney and heart disease. Extreme heat can even change how we behave, increasing aggression and decreasing our ability to focus (SN: 8/18/21).

    Staying cool

    The human body has various ways to shed excess heat and keep the core of the body at an optimal temperature of about 37° C (98.6° F). The heart pumps faster, speeding up blood flow that carries heat to the skin (SN: 4/3/18). Air passing over the skin can wick away some of that heat. Evaporative cooling — sweating — also helps.

    But there’s a limit to how much heat humans can endure. In 2010, scientists estimated that theoretical heat stress limit to be at a “wet bulb” temperature of 35° C. Wet bulb temperatures depend on a combination of humidity and “dry bulb” air temperature measured by a thermometer. Those variables mean a place could hit a wet bulb temperature of 35° C in different ways — for instance, if the air is that temperature and there’s 100 percent humidity, or if the air temperature is 45° C and there’s 50 percent humidity. The difference is due to evaporative cooling.

    When water evaporates from the skin or another surface, it steals away energy in the form of heat, briefly cooling that surface. That means that in drier regions, the wet bulb temperature — where that ephemeral cooling effect happens readily — will be lower than the actual air temperature. In humid regions, however, wet and dry bulb temperatures are similar, because the air is so moist it’s difficult for sweat to evaporate quickly.

    So when thinking about heat stress on the body, scientists use wet bulb temperatures because they are a measure of how much cooling through evaporation is possible in a given climate, says Daniel Vecellio, a climate scientist at Penn State.

    “Both hot/dry and warm/humid environments can be equally dangerous,” Vecellio says — and this is where the body’s different cooling strategies come into play. In hot, dry areas, where the outside temperature may be much hotter than skin temperature, human bodies rely entirely on sweating to cool down, he says. In warm, humid areas, where the air temperature may actually be cooler than skin temperatures (but the humidity makes it seem warmer than it is), the body can’t sweat as efficiently. Instead, the cooler air passing over the skin can draw away the heat.

    How hot is too hot?

    Given the complexity of the body’s cooling system, and the diversity of human bodies, there isn’t really a one-size-fits-all threshold temperature for heat stress for everybody. “No one’s body runs at 100 percent efficiency,” Vecellio says. Different body sizes, the ability to sweat, age and acclimation to a regional climate all have a role.

    Still, for the last decade, that theoretical wet bulb 35° C number has been considered to be the point beyond which humans can no longer regulate their bodies’ temperatures. But recent laboratory-based research by Vecellio and his colleagues suggests that a general, real-world threshold for human heat stress is much lower, even for young and healthy adults.

    The researchers tracked heat stress in two dozen subjects ranging in age from 18 to 34, under a variety of controlled climates. In the series of experiments, the team varied humidity and temperature conditions within an environmental chamber, sometimes holding temperature constant while varying the humidity, and sometimes vice versa.

    The subjects exerted themselves within the chamber just enough to simulate minimal outdoor activity, walking on a treadmill or pedaling slowly on a bike with no resistance. During these experiments, which lasted for 1.5 to two hours, the researchers measured the subjects’ skin temperatures using wireless probes and assessed their core temperatures using a small telemetry pill that the subjects swallowed.

    In warm and humid conditions, the subjects in the study were unable to tolerate heat stress at wet bulb temperatures closer to 30° or 31° C, the team estimates. In hot and dry conditions, that wet bulb temperature was even lower, ranging from 25° to 28° C, the researchers reported in the February Journal of Applied Physiology. For context, in a very dry environment at about 10 percent humidity, a wet bulb temperature of 25° C would correspond to an air temperature of about 50° C (122° F).

    These results suggest that there is much more work to be done to understand what humans can endure under real-world heat and humidity conditions, but that the threshold may be much lower than thought, Vecellio says. The 2010 study’s theoretical finding of 35° C may still be “the upper limit,” he adds. “We’re showing the floor.”

    And that’s for young, healthy adults doing minimal activity. Thresholds for heat stress are expected to be lower for outdoor workers required to exert themselves, or for the elderly or children. Assessing laboratory limits for more at-risk people is the subject of ongoing work for Vecellio and his colleagues.

    A worker wipes away sweat in Toulouse, France, on July 13. An intense heat wave swept across Europe in mid-July, engulfing Spain, Portugal, France, England and other countries.VALENTINE CHAPUIS/AFP via Getty Images

    If the human body’s tolerance for heat stress is generally lower than scientists have realized, that could mean millions more people will be at risk from the deadliest heat sooner than scientists have realized. As of 2020, there were few reports of wet bulb temperatures around the world reaching 35° C, but climate simulations project that limit could be regularly exceeded in parts of South Asia and the Middle East by the middle of the century.

    Some of the deadliest heat waves in the last two decades were at lower wet bulb temperatures: Neither the 2003 European heat wave, which caused an estimated 30,000 deaths, nor the 2010 Russian heat wave, which killed over 55,000 people, exceeded wet bulb temperatures of 28° C.

    Protecting people

    How best to inform the public about heat risk is “the part that I find to be tricky,” says Shandas, who wasn’t involved in Vecellio’s research. Shandas developed the scientific protocol for the National Integrated Heat Health Information System’s Urban Heat Island mapping campaign in the United States.

    It’s very useful to have this physiological data from a controlled, precise study, Shandas says, because it allows us to better understand the science behind humans’ heat stress tolerance. But physiological and environmental variability still make it difficult to know how best to apply these findings to public health messaging, such as extreme heat warnings, he says. “There are so many microconsiderations that show up when we’re talking about a body’s ability to manage [its] internal temperature.”

    One of those considerations is the ability of the body to quickly acclimate to a temperature extreme. Regions that aren’t used to extreme heat may experience greater mortality, even at lower temperatures, simply because people there aren’t used to the heat. The 2021 heat wave in the Pacific Northwest wasn’t just extremely hot — it was extremely hot for that part of the world at that time of year, which makes it more difficult for the body to adapt, Shandas says (SN: 6/29/21).

    Heat that arrives unusually early and right on the heels of a cool period can also be more deadly, says Larry Kalkstein, a climatologist at the University of Miami and the chief heat science advisor for the Washington, D.C.–based nonprofit Adrienne Arsht-Rockefeller Foundation Resilience Center. “Often early season heat waves in May and June are more dangerous than those in August and September.”

    One way to improve communities’ resilience to the heat may be to treat heat waves like other natural disasters — including give them names and severity rankings (SN: 8/14/20). As developed by an international coalition known as the Extreme Heat Resilience Alliance, those rankings form the basis for a new type of heat wave warning that explicitly considers the factors that impact heat stress, such as wet bulb temperature and acclimation, rather than just temperature extremes.

    The rankings also consider factors such as cloud cover, wind and how hot the temperatures are overnight. “If it’s relatively cool overnight, there’s not as much negative health outcome,” says Kalkstein, who created the system. But overnight temperatures aren’t getting as low as they used to in many places. In the United States, for example, the average minimum temperatures at nighttime are now about 0.8° C warmer than they were during the first half of the 20th century, according to the country’s Fourth National Climate Assessment, released in 2018 (SN: 11/28/18).

    By naming heat waves like hurricanes, officials hope to increase citizens’ awareness of the dangers of extreme heat. Heat wave rankings could also help cities tailor their interventions to the severity of the event. Six cities are currently testing the system’s effectiveness: four in the United States and in Athens, Greece, and Seville, Spain. On July 24, with temperatures heading toward 42° C, Seville became the first city in the world to officially name a heat wave, sounding the alarm for Heat Wave Zoe.

    As 2022 continues to smash temperature records around the globe, such warnings may come not a moment too soon. More