Wet-bulb temperature is a vital metric that combines heat and humidity, representing the lowest temperature to which the human body can cool itself through sweating. This measure becomes particularly crucial when sweat evaporation ceases due to high moisture levels in the air, significantly elevating the risks of heatstroke and even fatalities. It serves as a more precise indicator of human heat stress compared to standard air temperature.
The globally recognized survival threshold for wet-bulb temperature is 35°C, which indicates that prolonged exposure beyond six hours can be fatal. At this temperature, even healthy individuals may struggle due to impaired thermoregulation.
Recent studies conducted by researchers at Harvard and Pennsylvania State University propose a revised tolerance threshold of around 31°C for wet-bulb temperature. This is particularly concerning as they found that serious symptoms of heat stress could manifest before reaching this level, especially during physical exertion.
In India, regions like the Indo-Gangetic plain and parts of central India frequently experience wet-bulb temperatures exceeding 30°C during heatwaves. Major cities such as Delhi, Nagpur, and Ahmedabad have reported spikes in hospital admissions and mortality rates during these extreme weather events. If the 31°C threshold is accurate, millions of Indians could face heightened health risks even before hitting the previously accepted limit of 35°C.
Outdoor laborers, including construction workers, street vendors, and agricultural laborers, are particularly vulnerable to extreme heat exposure. Most existing research has been conducted on populations from temperate regions, which may not accurately reflect the challenges faced by Indian workers, especially in the informal sector, who often lack access to hydration, shade, and adequate medical support.
While high air temperatures may be bearable under low humidity conditions, wet-bulb temperature becomes perilous when both heat and humidity levels are elevated. For instance, an air temperature of 40°C at 50% humidity can lead to dangerously high wet-bulb temperatures, potentially exceeding 31°C.
Reports from India indicate a troubling trend of fainting, dehydration, and sudden deaths in urban slums and agricultural fields during summer months, often linked to extreme wet-bulb conditions. A notable heatwave in March 2025 resulted in multiple rural fatalities, likely due to wet-bulb temperatures reaching critical levels.
To combat these risks, authorities should issue heat alerts based on wet-bulb temperature rather than just air temperature. Essential measures include improving access to drinking water, providing shaded areas at worksites, implementing staggered work hours, and adopting better urban planning practices, such as cool roofs and increased tree cover.
While Indians generally possess a degree of heat acclimatization due to long-term exposure, this should not be considered a foolproof defense. Acclimatization has its limits, and during crises like COVID-19, similar assumptions led to misjudgments about health risks. Heat stress must be addressed with the same urgency.
The Indian government has started responding to these concerns. The Union Environment Ministry has engaged in consultations regarding heat stress and adaptation strategies, with a focus on developing region-specific heat action plans. Cities like Ahmedabad have implemented some of India's first heat preparedness plans following the devastating heatwave of 2010.
Q1. What is wet-bulb temperature and why is it significant?
Answer: Wet-bulb temperature measures heat and humidity, indicating how effectively the body can cool itself. It is crucial for assessing heat stress risks, especially in high humidity conditions.
Q2. What is the survival threshold for wet-bulb temperature?
Answer: The widely accepted survival threshold for wet-bulb temperature is 35°C for six hours of exposure. Beyond this, heat stress becomes life-threatening.
Q3. What is the revised threshold suggested by researchers?
Answer: Recent research suggests that the actual threshold for human tolerance is around 31°C, particularly during physical activity, indicating higher risks than previously acknowledged.
Q4. How do heatwaves impact outdoor workers in India?
Answer: Outdoor workers face significant risks during heatwaves due to prolonged exposure. Lack of hydration and shade increases their vulnerability to heat stress.
Q5. What preventive actions should be taken during heatwaves?
Answer: Authorities should issue heat alerts based on wet-bulb temperature, improve access to hydration, and implement shaded rest areas at worksites to protect outdoor workers.
Question 1: What does wet-bulb temperature measure?
A) Heat only
B) Humidity only
C) Combination of heat and humidity
D) Air pressure
Correct Answer: C
Question 2: What is the accepted survival threshold for wet-bulb temperature?
A) 30°C
B) 32°C
C) 35°C
D) 40°C
Correct Answer: C
Question 3: At what wet-bulb temperature do serious heat stress symptoms begin?
A) 30°C
B) 31°C
C) 33°C
D) 35°C
Correct Answer: B
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