Life-threatening heat can occur even below the commonly cited 35°C wet-bulb limit, which accounts for both temperature and humidity. Dry heat, too, can be just as dangerous as humid conditions.
Extreme heat is no longer a distant threat.
A new study warns that “non-survivable” heat conditions have already been recorded in several parts of the world, including South Asia, leading to thousands of deaths and pushing beyond what scientists once considered safe limits.
Researchers examined six major heatwaves between 2003 and 2024, including events in India and Pakistan (2015), Europe (2003), and Southeast Asia (2024).
Their findings, published in Nature Communications, reveal that life-threatening heat can occur even below the widely accepted 35°C wet-bulb threshold, which accounts for both temperature and humidity.
Hazardous heat can occur at lower temperatures than previously believed, particularly in extremely hot and dry conditions.
Exposure to direct sunlight and advancing age further increase the risk.
What is meant by “non-survivable heat”?
It describes conditions in which the human body is unable to cool itself effectively, even with rest and adequate hydration.
As a result, body temperature rises to dangerous levels, potentially leading to severe overheating and, in extreme cases, death.
Understanding the range of heat conditions
Researchers explain that heat-related illness develops gradually rather than all at once, progressing through distinct stages:
Heat stress is the earliest phase, when the body begins to struggle to regulate its temperature. Symptoms may include fatigue, sweating, thirst, and dizziness.
Heat exhaustion is more severe, marked by heavy sweating, weakness, nausea, headaches, and fainting, as the body loses excessive fluids and salts.
Heatstroke is a medical emergency, with body temperature rising above 40°C. Sweating may stop, and symptoms such as confusion, seizures, or unconsciousness can occur. Without immediate treatment, it can be fatal.
The study highlights that this progression can occur more rapidly than previously expected, even in dry heat conditions.
Dry heat can be equally deadly
Traditionally, humid heat has been viewed as more dangerous.
However, the study found that extremely hot and dry conditions – such as those during the 2023 Phoenix heatwave – can be just as deadly.
During the 2015 heatwave in India and Pakistan, temperatures reached 46.6°C, resulting in an estimated 2,500 deaths in India alone.
Older adults face the highest risk
The study highlights that people aged 65 and above are particularly vulnerable, often reaching life-threatening conditions much sooner than expected.
As the body ages, its ability to regulate temperature – primarily through sweating – declines, making it harder to cope with rising heat levels.
As a result, older adults can hit dangerous heat thresholds at lower temperatures, especially when exposed to direct sunlight.
During major heatwaves, many were found to be exposed to such conditions for several consecutive days, significantly increasing the risk of heatstroke and death.
Overall, what may seem like manageable heat for others can quickly escalate into a serious, life-threatening situation for the elderly.
The silent heat crisis in India
Researchers also note that many heat-related deaths – particularly in countries like India – may go underreported.
This is due to gaps in data systems, high population density, and limited access to cooling facilities.