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Does your weather report app show you “wet bulb temperature”? How does human body regulates its core temperature ?


Extreme levels of heat stress have more than doubled in the last 4 decades, with significant implications for human health. That trend is expected to continue. As Earth’s climate warms, heat waves are becoming more frequent and severe. The health dangers of extreme heat have scientists and medical experts increasingly concerned. We see weather report on our mobile apps, weather forecasters use different tools to assess the potential for heat stress. In addition to forecasts of temperature and relative humidity, you’ll see something called the heat index, or in some mobile apps they mention feels like. The heat index is a measure of what the air temperature feels like to our bodies when relative humidity is factored in.


There is another measure of heat stress called wet-bulb temperature.


Wet-bulb temperature is the lowest temperature to which an object can cool down when moisture evaporates from it. The lower the wet-bulb temperature, the easier it is for us to cool down. It measures how well our bodies cool down by sweating when it’s hot and humid, and tells us if conditions may be harmful to our health, or even deadly. Originally, wet-bulb temperature was measured by wrapping a wet cloth around the bulb of a thermometer and exposing it to air. As water evaporated from the cloth, the thermometer recorded the drop in temperature. The higher the relative humidity, the less moisture evaporated before the bulb and the surrounding air are the same temperature. Today, wet-bulb temperature is typically calculated using measurements from electronic instruments at weather stations.


The highest wet-bulb temperature that humans can survive when exposed to the elements for at least six hours is about 95 degrees Fahrenheit (35 degrees Celsius). Wet-bulb temperatures are on the rise around the world, and Earth’s climate has begun to exceed this limit. The results have important implications. The hotter it is, the more strain our bodies feel, and the more we need to sweat to cool down. But humid air has less capacity to hold additional moisture, so water evaporates more slowly in humid conditions.

Think about when you step out of a hot shower. Water evaporates off your body and you feel cooler. But if it’s hot or humid (or both) in the room, it’s harder to feel cool. This feeling relates directly to what the wet-bulb temperature is measuring.


As long as the wet-bulb temperature is well below your skin temperature, your body can release heat to your surroundings through radiation and sweating. But as the wet-bulb temperature approaches your core temperature, you lose the ability to cool down. This triggers changes in your body. You dehydrate. Your organs become stressed, especially your heart. Blood rushes to your skin to try to release heat, starving your internal organs. The results can be deadly.


“Once the wet-bulb temperature exceeds 35 degrees Celsius (95 degrees Fahrenheit), no amount of sweating or other adaptive behavior is enough to lower your body to a safe operating temperature,” “Most of the time it’s not a problem, because the wet-bulb temperature is usually 5-to-10 degrees Celsius below body temperature, even in hot, humid places.”


Indeed, if humidity is low, extreme temperatures are tolerable. “If you’re sitting in the shade, conditions may not be pleasant, but they’re survivable. “But in humid regions, once you approach wet-bulb temperatures of 34-to-36 degrees Celsius (93-to-97 degrees Fahrenheit), it doesn’t matter what you’re doing. You can’t survive for extended periods of time.”


Those most susceptible to high wet-bulb temperatures include the elderly, people who work outside, and those with underlying health issues. People without access to air conditioning are also vulnerable. Air conditioning removes humidity from the air and is the best solution when wet-bulb temperatures get too high. Fans can help sweat evaporate more efficiently, but they’re less effective.


Source: NASA

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