Extreme heat is proving to be more deadly than what most cities and nations have prepared for, according to new research co-authored by Jennifer Vanos at Arizona State University. The study reconsidered the risk to human life posed by six hot-weather events that struck around the world from 2003 to 2024.

The researchers found that heat waves are bringing sustained periods of non-survivable heat stress and placing hundreds of millions of people at grave risk, in both humid and dry climates. In the incredibly hot weather that hit the Middle East, India and Pakistan, and South Asia, the researchers said their findings suggest that the true mortality rates were very likely much higher than reported.

Instead of using survival thresholds based solely on a simplified temperature-and-humidity measure applied to all people, Vanos and colleagues accounted for how effectively human bodies can use sweating to avoid overheating in dry and humid climates, as well as in sun or shade. The researchers accounted for the effects of increasing age, which decreases sweat capacity.

Unrelenting exposure to extreme conditions starts a chain of events that is almost impossible to reverse, even with expert medical care. That’s why it’s critically important to act in advance of heat waves to protect people, the researchers said. They reported their findings in the journal Nature Communications.

Vanos, an associate professor in the School of Sustainability, led development of the physiological model used to assess heat risk. ASU News talked with her about the latest study and the protective actions needed as extreme heat becomes a more common part of life.

Question: Why doesn’t measuring air temperature alone give an accurate picture of heat danger?

Answer: Air temperature ignores humidity, sunlight, wind and duration of exposure. Those are the key environmental factors of heat stress. We also need to understand what people are doing and wearing — also key factors affecting heat stress. So the same air temperature can be survivable in one setting and deadly in another, depending on these other variables that must be accounted for.

Q: Earlier studies talk about a 35 degrees Celsius “wet-bulb” threshold as the limit of human survival. What does your study change about that idea?

A: That threshold doesn’t fully account for how the human body responds to extreme heat physiologically, or the role of sun exposure. The body has limits on how much sweat it can produce and evaporate to avoid overheating. We know that we can evaporate much more of the sweat we produce in dry conditions, like Phoenix. By accounting for the simple physiological fact that we can only sweat so much, this new study and some of our past work show that extremely hot yet dry conditions are just as deadly as hot and humid conditions, which is missed if we just use a simplified wet-bulb threshold. So at very high temperatures, even dry air can overwhelm survivability limits, causing core body temperature to rise and cause danger without changing our circumstances.

Q: Why are older adults more vulnerable to heat, even under conditions that younger people can survive?

A: Aging reduces the body’s ability to regulate temperature. Older adults generally sweat less (so they lose less heat through evaporation) and thus dissipate heat less efficiently, which means they accumulate heat faster. The study shows that conditions survivable for younger adults can become life-threatening for people over 65.

Q: How much difference can shade, shelter or air conditioning really make when temperatures stay dangerously high for days or weeks?

A: Being indoors or out of direct sunlight dramatically reduced exposure to non-survivable heat. Shade lowers radiant heat, which is often the most important contributor to the heat load in hot weather. Adding active indoor cooling, like air conditioning, also helps the body shed heat more effectively when people have access to it.

Q: If deadly heat is already occurring at today’s level of global warming, what does that mean for the future?

A: It suggests that heat risks have been underestimated. Our study found lethal conditions occurring at about 1.1 degrees Celsius of global warming. As warming continues, such conditions are expected to become more frequent, longer-lasting and more widespread, so without increasing societal adaptive capacity, we would expect to see increasing heat deaths. Luckily, there’s a lot that people and global societies can do to protect themselves, whether staying healthier and more informed about how to behave in the heat (like not climbing a mountain in the middle of a hot day), or improving energy efficiency to support broader, more reliable indoor cooling.

Q: Does this study mean that heat deaths are being under-counted around the world?

A: Yes, very likely. Many heat-related deaths, especially in low-income regions, are not officially recorded as heat deaths. Our study only considered deaths from heatstrokeand notthe two other common types of heat-related deaths: cardiovascular collapse and renal failure. The actual number of deaths is probably much higher, but it’s also important to distinguish the type of heat-related death. Heatstroke deaths are directly caused by the heat, whereas other deaths, like cardiovascular or kidney-related heat deaths, occur at lower core temperatures. So these would happen far below the level at which the survivability threshold is reached in our model. We know from population-based studies that cardiovascular-related deaths are most common during heat waves, so understanding why from a heat strain standpoint, and how people can protect themselves, is critical to reducing heat-related deaths.

Q: What role does nighttime heat play in making heat waves more dangerous?

A: Nighttime heat is most dangerous for those without indoor cooling, as it prevents the body from recovering from daytime heat stress. Several events studied showed persistently high overnight temperatures, increasing cumulative heat strain — especially for people without air conditioning. We know that with urban growth, our nighttime temperatures do not drop as much as in non-urban areas. So, hotter nights in cities are a combined effect of climate change and urban growth.

Q: If shade and cooling can reduce risk so much, why do so many people still die during heat waves?

A: Many people lack access to cooling due to poverty, housing conditions, outdoor work, those experiencing homelessness, health or cultural factors. Many heat-related deaths happen to people who already may have a preexisting condition that puts them at high risk of a health event occurring during a heat wave, and the heat pushes their body a bit too much. So those deaths are more heat-associated rather than caused, but heat is still part of the equation — like the cardiovascular example I gave. Deaths from heat are often preventable, but only if proper actions and behaviors are taken, and protective resources are available and accessible — such as water and cooling for those most in need.

Q: How could this research change how governments and cities prepare for extreme heat?

A: From a science perspective, we need to advance more realistic, physiology-forward ways of assessing heat risk and stop assuming one-size-fits-all approaches for estimating or assessing heat risk to people. A person who is older and on specific medications is much more at risk than a young, healthy adult. We need to use that information to better protect the most vulnerable, message people at risk who may think they are not at risk, and promote personal health, fitness and safe acclimatization for an overall more heat-resilient society.

From a policy standpoint, it’s about prevention first, but also preparedness, supporting safe and accessible living conditions, reliable energy for cooling and clean water for hydration. These seem simple, but far too many people lack these basic needs. It’s also about designing cities for heat safety, and in a way that we can live in the heat, rather than just survive it. Finally, providing tailored early warnings, actionable education, and access to cooling centers.