Monday, November 29, 1999

How hot is too hot?

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Last week, just as the heat wave rolled back in and set us scrambling for the shade, researchers from the University of New South Wales in Australia and Purdue University in the US ominously pronounced that temperatures across the earth may rise by up to 12°C within just three centuries, making the earth practically uninhabitable.Which makes you wonder, just how hot is too hot? It's a question that bothered three scientists at the Royal Society of London over two centuries ago, so much so that chemist Charles Blagden, botanist Joseph Banks and naturalist Daniel Solander heated a tiny room with a red-hot metal stove all the way up to the boiling point of water—a perilous 100°C—and then cranked it up another 5°C, before stepping into the enclosure. Even as they could stay in the room for over eight minutes, an egg cooked in that heat in no time. What they had and the egg did not was sweat, which cooled their skin and the air it came into immediate contact with.In Delhi, at the Defence Institute of Physiology and Allied Sciences, under the Defence Research and Development Organisation,Dr T.P. Baburaj, who has been working in heat stress physiology for 16 years, recreates such experiments, albeit at much lower temperatures and for longer durations, in a walk-in facility, named, quite unnervingly, the 'Human Climatic Chamber'. "Our jawans have to brave temperatures of around 50°C—that's just ambient temperature, inside the tanks it's 10°C more—in Rajasthan and Orissa. We can simulate the Thar desert right here in this chamber, and subject volunteers from the armed forces to tolerable heat stress, all the while monitoring physiological changes such as the heart rate, body mean temperature and blood pressure," says Baburaj, who has carried out field studies at Mahajan Range, Rajasthan, and in Antarctica.Of course, temperature is just one of the variables responsible for heat stress. "Humidity is an important factor because it doesn't allow you to release body heat by sweating. Which is why it's easier to withstand higher temperatures in dry climes than much lower temperatures in coastal areas. If you don't sweat, you're likely to suffer an anhydrous heat stroke," he says. Other factors—solar radiation, wind velocity—also come into play and they are simulated at the chamber, so that heat stress is measured in terms of the Wet Bulb Globe Temperature (WBGT), a composite index of these variables."Humans have a core temperature—the temperature in the brain, approximately measured rectally or under the armpit—of about 36.6°C. The temperature of the periphery, or skin, has a gradient of about 1°C. But the core temperature itself cannot vary much. If it is slightly higher, it causes a condition called hyperthermia, and if lower, it induces hypothermia, which is characterised by shivering," Baburaj says.Heat stress doesn't come unannounced. "Syncope, or a faint feeling or brief loss of consciousness, is an indicator of mild heat stress. At higher temperatures, the capillaries expand and blood flows to the lower parts of the body. At this point, if you lie down for a few minutes—exertion increases chances of heat stroke—you can recover. If you go on, you're likely to suffer heat exhaustion, where the cardiac rate is high and physical response is delayed. This affects muscular activity," says Baburaj, adding, "Up to two per cent of body water evaporating as sweat is replenishable. Beyond that, you lose water from intracellular space. And that brings down bodily efficiency drastically. After you lose 4-5 per cent, there's cardiac drift, followed by multi-organ failure, and revival is difficult."At DIPAS, which houses the largest climate-simulating chambers in India, the focus is on keeping soldiers comfortable in high temperatures and preventing heat stroke. "Eight days of acclimatisation helps reduce the risk of heat stress. We're now working on developing a faster acclimatisation programme and optimum hydration strategies specific to areas of operation," Baburaj says. Dr G. Ilavazhagan, director, DIPAS, says, "We study the clinical parameters needed for man to survive. The food research lab is engaged in studies on Jains, who fast for days without any food. There's research on high-altitude and cold and hot weather survival. We have developed nutritional programmes, air-cooled garments and breathing-enabling apparatus."Baburaj has been responsible for developing several personal, micro cooling systems, including body suits embedded with phase change material that can turn from solid to liquid or liquid to vapour at a certain temperature to extract heat from the body. He is working on a micro-compressor-based cooling system and a thermo-electric cooling prototype. Also in the pipeline are a to-be-patented man-mounted air circulation system and a smart suit that can measure physiological variables.Double doors open into the chamber where a jawan waits, dressed in a white suit with tubes sewn all over inside, through which a continuous loop of 750 ml of compressor-cooled water at 21°C is circulated till it heats up to 26-27°C, cooling the body by heat transfer. (Though international norms allow experimental exposure only up to 34°C WBGT, DIPAS has conducted studies in higher temperatures on jawans wearing temperature-control suits.) The tank-mounted, water-based cooling system has been tested successfully in field trials, Baburaj says. The jawan walks into the inner chamber with a bed, an exercise bike, a bed and several fans. He's calm as he looks out of the window. He has seen worse on the line of duty.

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