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I’ve spent a decade studying the health impacts of extreme heat. In the communities I’ve studied, people don’t care to debate the origins of climate change or whether it’s even real. They care about how many times they have to choose between buying food or medicine and running their air conditioner. They think about paying their water bill or their power bill, because they can’t afford both.

Despite these very real consequences of extreme heat, the U.S. has lagged behind other nations in both recognizing the risk of extreme heat and organizing the necessary resources and capacity to address it.


Heat exposure has killed more people in the U.S. than any other weather-related event over the past 30 years. We don’t know exactly how many, because heat morbidity and mortality are vastly undercounted. One thing is clear, though: The risk associated with heat is grossly misunderstood. This is largely because heat’s impact is rather quiet, quite literally. It doesn’t collide with the human senses like hurricanes, floods, or tornadoes can. The view out my window is the same at 85 degrees and 105 degrees. Yet the lack of visual and auditory impact doesn’t make heat any less deadly.

We need effective heat warning systems to properly inform people at risk when they should take caution. The first step is to revisit and redesign our current systems, which are deeply inadequate. Health problems from heat occur at temperatures significantly lower than when warnings are issued. This is because, for the most part, weather warnings for heat are issued based on the probability and magnitude of the heat event itself, rather than health outcomes. For example, local National Weather Service offices may issue a heat warning for a heat event that falls in the 98th percentile of heat events for that area, when health impacts are seen at the 95th percentile or even lower. This means that health impacts are experienced long before any warning is issued.

Additionally, current heat warning systems often rely upon simple indicators of dangerous conditions, such as daytime maximum temperatures. But more robust measures exist. For instance, health impacts are closely linked to persistently high overnight temperatures. This is particularly the case among our most vulnerable, such as pregnant women, who experience increased risk of preterm birth when overnight temperatures remain high.


Wet bulb globe temperature is also an effective measure for exertional heat stress that can affect student athletes or outdoor workers, more so than simple daytime maximum temperatures. The wet bulb globe temperature accounts for wind speed and solar radiation, which play a significant role in heat-health outcomes, in addition to air temperature and humidity. Even heat index, which combines both temperature and humidity and is commonly used by decision-makers, would allow for a more robust metric for a formal warning system.

Different groups of people are vulnerable to heat at different thresholds, which further complicates the warning system challenges. Aging populations and pregnant women are susceptible to heat at different thresholds than working-age people, who are susceptible at different thresholds than student athletes. We need to establish a commission of scientists who can evaluate these factors, establish exposure thresholds, and make recommendations for a warning system that reaches the populations at risk, when they are most at risk.

Most importantly, any successful warning system would have to come with ways to mitigate heat exposure. Warning without action is not helpful. However, the recommended cooling strategies can’t be reliant upon air-conditioned environments and must recognize non-AC interventions, such as electric fans, evaporative coolers, cool showers, or foot immersion. While it would be wonderful if every home was equipped with an air conditioner and people could afford to run it, this isn’t the reality for a great number of households — the very same households who are highest at risk for heat injury, illness, or death.

Those who rely upon good information for planning and preparedness would also benefit greatly from an early warning system that can forecast approaching days of high heat risk. For example, a wet bulb global temperature forecast would help coaches know whether conditions later in the week are expected to be too dangerous. School officials could alter practice schedules and activities in response. It is a big challenge for parents to learn just as school releases that practice is canceled, forcing them to make last-minute arrangements to get their child home from school. Farmers and others who manage outdoor workers would be able change work schedules in response to forecasted high-risk days, avoiding costly labor loss. We have the technology to create forecasts for high heat risk days based on multiple metrics, such as wet bulb globe temperature or persistently high overnight temperatures. But we need to do better at delivering this information quickly to the people who need it.

We are certain to see more severe heat seasons in the coming decade(s) with greater numbers of people affected and with greater economic impact. Heat-related illness and death are 100% preventable. Key to prevention is to first recognize the risk. Second is education and awareness of practical prevention measures. And third: action. It is time we merge science with technology to deploy effective policies around early warning for heat.

Ashley Ward is a senior policy associate at Duke University’s Nicholas Institute for Energy, Environment & Sustainability.

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