| Earlier this summer, officials from the U.N. weather agency admitted they're still figuring out how to tell 8 billion people around the world what constitutes deadly heat. The warning came as the Earth's temperature sets records. July 3 was the Earth's hottest day in modern records — and that was shattered multiple times in following days. Experts forecast record heat over the coming years. Yet there are more than 300 heat stress indexes around the world to explain the risk this poses to people. Each one, from the National Weather Service's heat index to local warning systems to Canada's Humidex to New York City's scale, embraces its own standards. Even for the world's top scientists, this tower of babble is confusing. "All indexes give you numbers," says David Romps, a climate scientist at the University of California at Berkeley. "But it's not clear what you should do with those numbers." It's never been more important to communicate what to do amid brutal heat waves. Tens of thousands of people around the globe are dying of heat-related causes. I asked several experts for help with figuring out the best ways to stay safe, and what's a better heat scale. The heat index You've probably heard of it. Originally called an "assessment of sultriness," it's now referred to as "apparent temperature," "feels-like temperature" or "real-feel temperature." It translates how two factors — temperature and relative humidity — affect humans' ability to regulate their internal temperature. High humidity makes our natural cooling mechanism — sweating — less efficient at wicking away heat. The problem? "The heat index is super conservative in predicting bad health outcomes," says Romps. Wet bulb globe temperature (WBGT) Another alternative method to measure heat stress is wet bulb globe temperature. It uses three types of sensors to offer a more effective indicator of heat stress if you're exercising in the sun — heat index assumes you're resting in the shade. It accounts for wind, evaporative cooling, solar intensity and temperature. Unfortunately, the WBGT is not intuitive — and most places don't offer forecasts. The readings are measured in Fahrenheit but bare little resemblance to our daily experience. For example, the scale jumps from "low" risk at 80 to 85 degrees to "extreme" risk at just above 90, conditions that could lead to convulsions or death. Still, ignoring it can be deadly, even for the healthiest among us. Over the past 25 years, at least 50 high school football players in the United States have died of heatstroke. One heat scale to rule them all? The scientists I interviewed said there will probably never be a universal heat risk index. We must grapple with the unique interactions of geography, physics and physiology to assess risks around the world. Until agencies standardize their language, the best way to use heat risk indexes is to study the categories of risk, rather than temperature readings of any particular scale: 95 degrees on a standard thermometer is safe, but the same reading on the WBGT could be deadly. Then know yourself: What are your risk factors? Can you find cooling if the power goes out? Think about your neighbors as well. Adjust for your — or their — personal level of risk. Here's a quick guide to stay safe in the heat. - Daily life: Use the heat index to gauge the heat stress you might encounter over the course of an average day.
- Outside activities: Use the WBGT if you're working outside in the sun because it better estimates the risk of heatstroke.
- Vulnerable populations: Play it safe on every scale since these heat scales reflect the effect of heat on a healthy adult. Any risk factors, from age to dehydration to cardiovascular disease, elevate the chances of injury or death.
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