Wearing surgical masks in public could help slow COVID-19 pandemic's advance

Surgical masks may help prevent infected people from making others sick with seasonal viruses, including coronaviruses, according to new research that could help settle a fierce debate spanning clinical and cultural norms.

In laboratory experiments, the masks significantly reduced the amounts of various airborne viruses coming from infected patients, measured using the breath-capturing "Gesundheit II machine" developed by Dr. Don Milton, a professor of applied environmental health and a senior author of the study published April 3 in the journal Nature Medicine.

Milton has already conferred with federal and White House health officials on the findings, which closely follow statements this week from the head of the Centers for Disease Control and Prevention saying the agency was reconsidering oft-stated advice that surgical masks aren't a useful precaution outside of medical settings. (The debate takes place at a time when clinicians themselves face dangerously inadequate supplies of masks -- a shortfall other UMD researchers are scrambling to help solve.)

The question of masks has roiled society as well, with some retailers refusing to let employees wear them for fear of sending negative signals to customers, and cases of slurs and even physical attacks in the United States and elsewhere against Asians or Asian Americans who were wearing masks, a measure some consider a necessity during a disease outbreak.

The study, conducted prior to the current pandemic with a student of Milton's colleagues on the Faculty of Medicine at the University of Hong Kong, does not address the question of whether surgical masks protect wearers from infection. It does suggest that masks may limit how much the infected -- who in the case of the novel coronavirus often don't have symptoms -- spread diseases including influenza, rhinoviruses and coronaviruses.

Milton, who runs the Public Health Aerobiology, Virology, and Exhaled Biomarker Laboratory in the School of Public Health, demonstrated in a 2013 study that surgical masks could help limit flu transmission. However, he cautions that the effect may not be as great outside of controlled settings.

Nevertheless, he said, the chance they could help justifies taking a new look at whether all people should be encouraged to wear them when they venture out of their houses to stores or other populated locations during the current COVID-19 lockdown.

"In normal times we'd say that if it wasn't shown statistically significant or the effective in real-world studies, we don't recommend it," he said. "But in the middle of a pandemic, we're desperate. The thinking is that even if it cuts down transmission a little bit, it's worth trying."

Previous studies have shown that coronavirus and other respiratory infections are mostly spread during close contact, which has been interpreted by some infectious disease specialists to mean that the disease could spread only through contact and large droplets, such as from a cough or sneeze -- a message that has often been shared with the public.

"What they don't understand is that is merely a hypothesis," Milton said. The current study (along with earlier ones) shows, by contrast, that tiny, aerosolized droplets can indeed diffuse through the air. That means it may be possible to contract COVID-19 not only by being coughed on, but by simply inhaling the breath of someone nearby who has it, whether they have symptoms or not. Surgical masks, however, catch a lot of the aerosolized virus as it's exhaled, he said.

The study was conducted at the University of Hong Kong as part of the dissertation research of the lead author, Dr. Nancy Leung, who, under the supervision of the co-senior authors Drs. Cowling and Milton, recruited 246 people with suspected respiratory viral infections. Milton's Gesundheit machine compared how much virus they exhaled with and without a surgical mask.

"In 111 people infected by either coronavirus, influenza virus or rhinovirus, masks reduced detectable virus in respiratory droplets and aerosols for seasonal coronaviruses, and in respiratory droplets for influenza virus," Leung said. "In contrast, masks did not reduce the emission of rhinoviruses."

Although the experiment took place before the current pandemic, COVID-19 and seasonal coronaviruses are closely related and may be of similar particle size. The report's other senior author, Professor Benjamin Cowling, division head of epidemiology and biostatistics, School of Public Health, HKUMed, and co-director of the World Health Organization Collaborating Centre for Infectious Disease Epidemiology and Control, said, "The ability of surgical masks to reduce seasonal coronavirus in respiratory droplets and aerosols implies that such masks can contribute to slowing the spread of (COVID-19) when worn by infected people."

Milton pointed to other measures his research has found is even more effective than masks, such as improving ventilation in public places like grocery stores, or installing UV-C lights near the ceiling that works in conjunction with ceiling fans to pull air upwards and destroy viruses and bacteria.

"Personal protective equipment like N95 masks are not our first line of defense," Milton said. "They are our last desperate thing that we do."

Hong Kong University contributed to this report.

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Story Source:

Materials provided by University of Maryland. Original written by Chris Carroll. Note: Content may be edited for style and length.

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Journal Reference:

1. Nancy H. L. Leung, Daniel K. W. Chu, Eunice Y. C. Shiu, Kwok-Hung Chan, James J. McDevitt, Benien J. P. Hau, Hui-Ling Yen, Yuguo Li, Dennis K. M. Ip, J. S. Malik Peiris, Wing-Hong Seto, Gabriel M. Leung, Donald K. Milton, Benjamin J. Cowling. Respiratory virus shedding in exhaled breath and efficacy of face masks. Nature Medicine, 2020; DOI: 10.1038/s41591-020-0843-2

Even animals benefit from social distance to prevent disease, research shows

Microorganisms living inside and on our body play a crucial role in both the maintenance of our health and the development of disease. Now researchers at UTSA have uncovered evidence about the importance of maintaining physical distance to minimize the spread of microbes among individuals.

The scientists observed monkeys in the wild to understand what role genetics, diet, social groupings and distance in a social network play when it comes to the microbes found inside an animal's gut.

"Social microbial transmission among monkeys can help inform us about how diseases spread. This has parallels to our current situation in which we are trying to understand how social distancing during the COVID 19 pandemic and future disease outbreaks may influence disease transmission," said Eva Wikberg, an assistant professor in UTSA's Department of Anthropology who studies the interaction between ecology, behavior and genetics in primates.

The gut microbiome refers to all the microorganisms inhabiting the digestive tract, starting with the stomach and ending with the colon. Over the past decade the microbiome has come under more scientific focus because it's believed that an unhealthy gut microbiome can lead to obesity, impaired immune function, weakened parasite resistance and even behavioral changes.

However, researching microbiomes is difficult because of the variation in microbial composition between individuals. One long-standing question is whether this variation is driven by genetic makeup, diets or social environments.

This research inquiry has been especially hard in wild populations because of the lack of detailed data necessary to tease apart the myriad factors that shape the microbiome.

To find an answer, Wikberg and fellow researchers studied the fecal matter of 45 female colobus monkeys that congregated in eight different social groups in a small forest by the villages of Boabeng and Fiema in Ghana. The scientists saw major differences among social groups' gut microbiomes.

However, individuals from different groups that were more closely connected in the population's social network had more similar gut microbiomes. This discovery indicates that microbes may be transmitted during occasional encounters with members of other social groups.

A similar setting may be when people come into one-meter proximity of each other at a store. Being in close proximity or accidentally brushing up against someone else may be all it takes to transmit certain microbes.

This study suggests that microbes transmitted this way help the colobus monkeys digest the leaves in their diet. However, further research is needed to investigate whether this type of transmission yields health benefits, which could explain why different social groups occasionally have friendly between-group encounters.

"Studies of wild animals can teach us a lot about the importance of using interventions, such as social distancing, to ensure a safer community during this pandemic," said Wikberg.

The study's findings are reported in the May issue of the journal Animal Behaviour.

Wikberg's other research collaborators are Diana Christie and Nelson Ting, of the Institute of Ecology and Evolution at the University of Oregon, and Pascale Sicotte, of the Department of Anthropology and Archaeology at the University of Calgary.

Story Source:

Materials provided by University of Texas at San Antonio. Original written by Milady Nazir. Note: Content may be edited for style and length.

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Journal Reference:

1. Eva C. Wikberg, Diana Christie, Pascale Sicotte, Nelson Ting. Interactions between social groups of colobus monkeys (Colobus vellerosus) explain similarities in their gut microbiomes. Animal Behaviour, 2020; 163: 17 DOI: 10.1016/j.anbehav.2020.02.011

New AI diagnostic can predict COVID-19 without testing

New AI diagnostic can predict COVID-19 without testing

Researchers at King's College London, Massachusetts General Hospital and health science company ZOE have developed an artificial intelligence diagnostic that can predict whether someone is likely to have COVID-19 based on their symptoms. Their findings are published today in Nature Medicine.

The AI model uses data from the COVID Symptom Study app to predict COVID-19 infection, by comparing people's symptoms and the results of traditional COVID tests. Researchers say this may provide help for populations where access to testing is limited. Two clinical trials in the UK and the US are due to start shortly.

More than 3.3 million people globally have downloaded the app and are using it to report daily on their health status, whether they feel well or have any new symptoms such as persistent cough, fever, fatigue and loss of taste or smell (anosmia).

In this study, the researchers analysed data gathered from just under 2.5 million people in the UK and US who had been regularly logging their health status in the app, around a third of whom had logged symptoms associated with COVID-19. Of these, 18,374 reported having had a test for coronavirus, with 7,178 people testing positive.

The research team investigated which symptoms known to be associated with COVID-19 were most likely to be associated with a positive test. They found a wide range of symptoms compared to cold and flu, and warn against focusing only on fever and cough. Indeed, they found loss of taste and smell (anosmia) was particularly striking, with two thirds of users testing positive for coronavirus infection reporting this symptom compared with just over a fifth of the participants who tested negative. The findings suggest that anosmia is a stronger predictor of COVID-19 than fever, supporting anecdotal reports of loss of smell and taste as a common symptom of the disease.

The researchers then created a mathematical model that predicted with nearly 80% accuracy whether an individual is likely to have COVID-19 based on their age, sex and a combination of four key symptoms: loss of smell or taste, severe or persistent cough, fatigue and skipping meals. Applying this model to the entire group of over 800,000 app users experiencing symptoms predicted that just under a fifth of those who were unwell (17.42%) were likely to have COVID-19 at that time.

Researchers suggest that combining this AI prediction with widespread adoption of the app could help to identify those who are likely to be infectious as soon as the earliest symptoms start to appear, focusing tracking and testing efforts where they are most needed.

Professor Tim Spector from King's College London said: "Our results suggest that loss of taste or smell is a key early warning sign of COVID-19 infection and should be included in routine screening for the disease. We strongly urge governments and health authorities everywhere to make this information more widely known, and advise anyone experiencing sudden loss of smell or taste to assume that they are infected and follow local self-isolation guidelines."

Story Source:

Materials provided by King's College London. Note: Content may be edited for style and length.

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Journal Reference:

1. Cristina Menni, Ana M. Valdes, Maxim B. Freidin, Carole H. Sudre, Long H. Nguyen, David A. Drew, Sajaysurya Ganesh, Thomas Varsavsky, M. Jorge Cardoso, Julia S. El-Sayed Moustafa, Alessia Visconti, Pirro Hysi, Ruth C. E. Bowyer, Massimo Mangino, Mario Falchi, Jonathan Wolf, Sebastien Ourselin, Andrew T. Chan, Claire J. Steves, Tim D. Spector. Real-time tracking of self-reported symptoms to predict potential COVID-19. Nature Medicine, 2020; DOI: 10.1038/s41591-020-0916-2

Potentially fatal combinations of humidity and heat are emerging across the globe

High temperature on thermometer (stock image).

Most everyone knows that humid heat is harder to handle than the "dry" kind. And recently, some scientists have projected that later in the century, in parts of the tropics and subtropics, warming climate could cause combined heat and humidity to reach levels rarely if ever experienced before by humans. Such conditions would ravage economies, and possibly even surpass the physiological limits of human survival.

According to a new study, the projections are wrong: such conditions are already appearing. The study identifies thousands of previously rare or unprecedented bouts of extreme heat and humidity in Asia, Africa, Australia, South America and North America, including in the U.S. Gulf Coast region. Along the Persian Gulf, researchers spotted more than a dozen recent brief outbreaks surpassing the theoretical human survivability limit. The outbreaks have so far been confined to localized areas and lasted just hours, but they are increasing in frequency and intensity, say the authors. The study appears this week in the journal Science Advances.

"Previous studies projected that this would happen several decades from now, but this shows it's happening right now," said lead author Colin Raymond, who did the research as a PhD. student at Columbia University's Lamont-Doherty Earth Observatory. "The times these events last will increase, and the areas they affect will grow in direct correlation with global warming."

Analyzing data from weather stations from 1979 to 2017, the authors found that extreme heat/humidity combinations doubled over the study period. Repeated incidents appeared in much of India, Bangladesh and Pakistan; northwestern Australia; and along the coasts of the Red Sea and Mexico's Gulf of California. The highest, potentially fatal, readings, were spotted 14 times in the cities of Dhahran/Damman, Saudi Arabia; Doha, Qatar; and Ras Al Khaimah, United Arab Emirates, which have combined populations of over 3 million. Parts of southeast Asia, southern China, subtropical Africa and the Caribbean were also hit.

The southeastern United States saw extreme conditions dozens of times, mainly near the Gulf Coast in east Texas, Louisiana, Mississippi, Alabama and the Florida Panhandle. The worst spots: New Orleans and Biloxi, Miss. Such conditions also reached inland into Arkansas and along the southeastern coastal plain.

Not surprisingly, incidents tended to cluster on coastlines along confined seas, gulfs and straits, where evaporating seawater provides abundant moisture to be sucked up by hot air. In some areas further inland, moisture-laden monsoon winds or wide areas of crop irrigation appear to play the same role.

Prior climate studies failed to recognize most past incidents because climate researchers usually look at averages of heat and humidity measured over large areas and over several hours at a time. Raymond and his colleagues instead drilled directly into hourly data from 7,877 individual weather stations, allowing them to pinpoint shorter-lived bouts affecting smaller areas.

Humidity worsens the effects of heat because humans cool their bodies by sweating; water expelled through the skin removes excess body heat, and when it evaporates, it carries that heat away. The process works nicely in deserts, but less well in humid regions, where the air is already too laden with moisture to take on much more. Evaporation of sweat slows. In the most extreme instances, it could stop. In that case, unless one can retreat to an air-conditioned room, the body's core heats beyond its narrow survivable range, and organs begin to fail. Even a strong, physically fit person resting in the shade with no clothes and unlimited access to drinking water would die within hours.

Meteorologists measure the heat/humidity effect on the so-called "wet bulb" Centigrade scale; in the United States, these readings are often translated into "heat index" or "real-feel" Fahrenheit readings. Prior studies suggest that even the strongest, best-adapted people cannot carry out normal outdoor activities when the wet bulb hits 32 C, equivalent to a heat index of 132 F. Most others would crumble well before that. A reading of 35 -- the peak briefly reached in the Persian Gulf cities -- is considered the theoretical survivability limit. That translates roughly to a heat index of 160 F. (The heat index actually ends at 127 F, so these readings are literally off the charts.) "It's hard to exaggerate the effects of anything that gets into the 30s," said Raymond.

The study found that worldwide, wet-bulb readings approaching or exceeding 30C on the wet bulb have doubled since 1979. The number of readings of 31 -- previously believed to occur only rarely -- totaled around 1,000. Readings of 33 -- previously thought to be almost nonexistent -- totaled around 80.

A heat wave that struck much of the United States last July maxed out at about 30C on the wet bulb, translating into heat indexes approaching 115 F in places; the highest was 122 F, in Baltimore, Md., and a similar wave hit in August. The waves paralyzed communities and led to at least a half-dozen deaths, including those of an air-conditioning technician in Phoenix, Az., and former National Football League lineman Mitch Petrus, who died in Arkansas while working outside.

It was a modest toll; heat-related illnesses already kill more U.S. residents than any other weather-related hazard including cold, hurricanes or floods. An investigation last year by the website InsideClimate News revealed that cases of heat stroke or heat exhaustion among U.S. troops on domestic bases grew 60 percent from 2008 to 2018. Seventeen soldiers died, almost all in the muggy U.S. Southeast. High-humidity heat waves in Russia and Europe, where far fewer people have air conditioning, have killed tens of thousands.

"We may be closer to a real tipping point on this than we think," said Radley Horton, a Lamont-Doherty research scientist and coauthor of the paper. Horton coauthored a 2017 paper projecting that such conditions would not take hold until later in the century.

While air conditioning may blunt the effects in the United States and some other wealthy countries, there are limits. Before the new study, one of the previously highest heat/humidity events ever reported was in the Iranian city of Bandar Mahshahr, which almost reached a 35C wet-bulb reading on July 31, 2015. There were no known deaths; residents reported staying inside air-conditioned vehicles and buildings, and showering after brief sojourns outside. But Horton points out that if people are increasingly forced indoors for longer periods, farming, commerce and other activities could potentially grind to a halt, even in rich nations-a lesson already brought home by the collapse of economies in the face of the novel coronavirus.

In any case, many people in the poorer countries most at risk do not have electricity, never mind air conditioning. There, many rely on subsistence farming requiring daily outdoor heavy labor. These facts could make some of the most affected areas basically uninhabitable, says Horton.

Kristina Dahl, a climatologist at the Union of Concerned Scientists who led a study last year warning of increasing future heat and humidity in the United States, said the new paper shows "how close communities around the world are to the limits." She added that some localities may already be seeing conditions worse than the study suggests, because weather stations do not necessarily pick up hot spots in dense city neighborhoods built with heat-trapping concrete and pavement.

Steven Sherwood, a climatologist at the Australia's University of New South Wales, said, "These measurements imply that some areas of Earth are much closer than expected to attaining sustained intolerable heat. It was previously believed we had a much larger margin of safety."

The study was coauthored by Tom Matthews, a lecturer in climate science at Loughborough University in the United Kingdom. Colin Raymond is now a postdoctoral researcher at NASA's Jet Propulsion Laboratory.

Story Source:

Materials provided by Earth Institute at Columbia University. Original written by Kevin Krajick. Note: Content may be edited for style and length.

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Journal Reference:

1. Colin Raymond, Tom Matthews, Radley M. Horton. The emergence of heat and humidity too severe for human tolerance. Science Advances, 2020; 6 (19): eaaw1838 DOI: 10.1126/sciadv.aaw1838

Vitamin D levels appear to play role in COVID-19 mortality rates

Doctor, vitamin D concept (stock image).

After studying global data from the novel coronavirus (COVID-19) pandemic, researchers have discovered a strong correlation between severe vitamin D deficiency and mortality rates.

Led by Northwestern University, the research team conducted a statistical analysis of data from hospitals and clinics across China, France, Germany, Italy, Iran, South Korea, Spain, Switzerland, the United Kingdom (UK) and the United States.

The researchers noted that patients from countries with high COVID-19 mortality rates, such as Italy, Spain and the UK, had lower levels of vitamin D compared to patients in countries that were not as severely affected.

This does not mean that everyone -- especially those without a known deficiency -- needs to start hoarding supplements, the researchers caution.

"While I think it is important for people to know that vitamin D deficiency might play a role in mortality, we don't need to push vitamin D on everybody," said Northwestern's Vadim Backman, who led the research. "This needs further study, and I hope our work will stimulate interest in this area. The data also may illuminate the mechanism of mortality, which, if proven, could lead to new therapeutic targets."

The research is available on medRxiv, a preprint server for health sciences.

Backman is the Walter Dill Scott Professor of Biomedical Engineering at Northwestern's McCormick School of Engineering. Ali Daneshkhah, a postdoctoral research associate in Backman's laboratory, is the paper's first author.

Backman and his team were inspired to examine vitamin D levels after noticing unexplained differences in COVID-19 mortality rates from country to country. Some people hypothesized that differences in healthcare quality, age distributions in population, testing rates or different strains of the coronavirus might be responsible. But Backman remained skeptical.

"None of these factors appears to play a significant role," Backman said. "The healthcare system in northern Italy is one of the best in the world. Differences in mortality exist even if one looks across the same age group. And, while the restrictions on testing do indeed vary, the disparities in mortality still exist even when we looked at countries or populations for which similar testing rates apply.

"Instead, we saw a significant correlation with vitamin D deficiency," he said.

By analyzing publicly available patient data from around the globe, Backman and his team discovered a strong correlation between vitamin D levels and cytokine storm -- a hyperinflammatory condition caused by an overactive immune system -- as well as a correlation between vitamin D deficiency and mortality.

"Cytokine storm can severely damage lungs and lead to acute respiratory distress syndrome and death in patients," Daneshkhah said. "This is what seems to kill a majority of COVID-19 patients, not the destruction of the lungs by the virus itself. It is the complications from the misdirected fire from the immune system."

This is exactly where Backman believes vitamin D plays a major role. Not only does vitamin D enhance our innate immune systems, it also prevents our immune systems from becoming dangerously overactive. This means that having healthy levels of vitamin D could protect patients against severe complications, including death, from COVID-19.

"Our analysis shows that it might be as high as cutting the mortality rate in half," Backman said. "It will not prevent a patient from contracting the virus, but it may reduce complications and prevent death in those who are infected."

Backman said this correlation might help explain the many mysteries surrounding COVID-19, such as why children are less likely to die. Children do not yet have a fully developed acquired immune system, which is the immune system's second line of defense and more likely to overreact.

"Children primarily rely on their innate immune system," Backman said. "This may explain why their mortality rate is lower."

Backman is careful to note that people should not take excessive doses of vitamin D, which might come with negative side effects. He said the subject needs much more research to know how vitamin D could be used most effectively to protect against COVID-19 complications.

"It is hard to say which dose is most beneficial for COVID-19," Backman said. "However, it is clear that vitamin D deficiency is harmful, and it can be easily addressed with appropriate supplementation. This might be another key to helping protect vulnerable populations, such as African-American and elderly patients, who have a prevalence of vitamin D deficiency."

Backman is the director of Northwestern's Center for Physical Genomics and Engineering and the associate director for Research Technology and Infrastructure at the Robert H. Lurie Comprehensive Cancer Center at Northwestern University.

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Story Source:

Materials provided by Northwestern University. Original written by Amanda Morris. Note: Content may be edited for style and length.

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Journal Reference:

1. Ali Daneshkhah, Vasundhara Agrawal, Adam Eshein, Hariharan Subramanian, Hemant Kumar Roy, Vadim Backman. The Possible Role of Vitamin D in Suppressing Cytokine Storm and Associated Mortality in COVID-19 Patients. medRxiv, Posted April 30, 2020; [link]

Potentially fatal combinations of humidity and heat are emerging across the globe

Most everyone knows that humid heat is harder to handle than the "dry" kind. And recently, some scientists have projected that later in the century, in parts of the tropics and subtropics, warming climate could cause combined heat and humidity to reach levels rarely if ever experienced before by humans. Such conditions would ravage economies, and possibly even surpass the physiological limits of human survival.

According to a new study, the projections are wrong: such conditions are already appearing. The study identifies thousands of previously rare or unprecedented bouts of extreme heat and humidity in Asia, Africa, Australia, South America and North America, including in the U.S. Gulf Coast region. Along the Persian Gulf, researchers spotted more than a dozen recent brief outbreaks surpassing the theoretical human survivability limit. The outbreaks have so far been confined to localized areas and lasted just hours, but they are increasing in frequency and intensity, say the authors. The study appears this week in the journal Science Advances.

"Previous studies projected that this would happen several decades from now, but this shows it's happening right now," said lead author Colin Raymond, who did the research as a PhD. student at Columbia University's Lamont-Doherty Earth Observatory. "The times these events last will increase, and the areas they affect will grow in direct correlation with global warming."

Analyzing data from weather stations from 1979 to 2017, the authors found that extreme heat/humidity combinations doubled over the study period. Repeated incidents appeared in much of India, Bangladesh and Pakistan; northwestern Australia; and along the coasts of the Red Sea and Mexico's Gulf of California. The highest, potentially fatal, readings, were spotted 14 times in the cities of Dhahran/Damman, Saudi Arabia; Doha, Qatar; and Ras Al Khaimah, United Arab Emirates, which have combined populations of over 3 million. Parts of southeast Asia, southern China, subtropical Africa and the Caribbean were also hit.

The southeastern United States saw extreme conditions dozens of times, mainly near the Gulf Coast in east Texas, Louisiana, Mississippi, Alabama and the Florida Panhandle. The worst spots: New Orleans and Biloxi, Miss. Such conditions also reached inland into Arkansas and along the southeastern coastal plain.

Not surprisingly, incidents tended to cluster on coastlines along confined seas, gulfs and straits, where evaporating seawater provides abundant moisture to be sucked up by hot air. In some areas further inland, moisture-laden monsoon winds or wide areas of crop irrigation appear to play the same role.

Prior climate studies failed to recognize most past incidents because climate researchers usually look at averages of heat and humidity measured over large areas and over several hours at a time. Raymond and his colleagues instead drilled directly into hourly data from 7,877 individual weather stations, allowing them to pinpoint shorter-lived bouts affecting smaller areas.

Humidity worsens the effects of heat because humans cool their bodies by sweating; water expelled through the skin removes excess body heat, and when it evaporates, it carries that heat away. The process works nicely in deserts, but less well in humid regions, where the air is already too laden with moisture to take on much more. Evaporation of sweat slows. In the most extreme instances, it could stop. In that case, unless one can retreat to an air-conditioned room, the body's core heats beyond its narrow survivable range, and organs begin to fail. Even a strong, physically fit person resting in the shade with no clothes and unlimited access to drinking water would die within hours.

Meteorologists measure the heat/humidity effect on the so-called "wet bulb" Centigrade scale; in the United States, these readings are often translated into "heat index" or "real-feel" Fahrenheit readings. Prior studies suggest that even the strongest, best-adapted people cannot carry out normal outdoor activities when the wet bulb hits 32 C, equivalent to a heat index of 132 F. Most others would crumble well before that. A reading of 35 -- the peak briefly reached in the Persian Gulf cities -- is considered the theoretical survivability limit. That translates roughly to a heat index of 160 F. (The heat index actually ends at 127 F, so these readings are literally off the charts.) "It's hard to exaggerate the effects of anything that gets into the 30s," said Raymond.

The study found that worldwide, wet-bulb readings approaching or exceeding 30C on the wet bulb have doubled since 1979. The number of readings of 31 -- previously believed to occur only rarely -- totaled around 1,000. Readings of 33 -- previously thought to be almost nonexistent -- totaled around 80.

A heat wave that struck much of the United States last July maxed out at about 30C on the wet bulb, translating into heat indexes approaching 115 F in places; the highest was 122 F, in Baltimore, Md., and a similar wave hit in August. The waves paralyzed communities and led to at least a half-dozen deaths, including those of an air-conditioning technician in Phoenix, Az., and former National Football League lineman Mitch Petrus, who died in Arkansas while working outside.

It was a modest toll; heat-related illnesses already kill more U.S. residents than any other weather-related hazard including cold, hurricanes or floods. An investigation last year by the website InsideClimate News revealed that cases of heat stroke or heat exhaustion among U.S. troops on domestic bases grew 60 percent from 2008 to 2018. Seventeen soldiers died, almost all in the muggy U.S. Southeast. High-humidity heat waves in Russia and Europe, where far fewer people have air conditioning, have killed tens of thousands.

"We may be closer to a real tipping point on this than we think," said Radley Horton, a Lamont-Doherty research scientist and coauthor of the paper. Horton coauthored a 2017 paper projecting that such conditions would not take hold until later in the century.

While air conditioning may blunt the effects in the United States and some other wealthy countries, there are limits. Before the new study, one of the previously highest heat/humidity events ever reported was in the Iranian city of Bandar Mahshahr, which almost reached a 35C wet-bulb reading on July 31, 2015. There were no known deaths; residents reported staying inside air-conditioned vehicles and buildings, and showering after brief sojourns outside. But Horton points out that if people are increasingly forced indoors for longer periods, farming, commerce and other activities could potentially grind to a halt, even in rich nations-a lesson already brought home by the collapse of economies in the face of the novel coronavirus.

In any case, many people in the poorer countries most at risk do not have electricity, never mind air conditioning. There, many rely on subsistence farming requiring daily outdoor heavy labor. These facts could make some of the most affected areas basically uninhabitable, says Horton.

Kristina Dahl, a climatologist at the Union of Concerned Scientists who led a study last year warning of increasing future heat and humidity in the United States, said the new paper shows "how close communities around the world are to the limits." She added that some localities may already be seeing conditions worse than the study suggests, because weather stations do not necessarily pick up hot spots in dense city neighborhoods built with heat-trapping concrete and pavement.

Steven Sherwood, a climatologist at the Australia's University of New South Wales, said, "These measurements imply that some areas of Earth are much closer than expected to attaining sustained intolerable heat. It was previously believed we had a much larger margin of safety."

The study was coauthored by Tom Matthews, a lecturer in climate science at Loughborough University in the United Kingdom. Colin Raymond is now a postdoctoral researcher at NASA's Jet Propulsion Laboratory.

Potentially fatal combinations of humidity and heat are emerging across the globe