New measure for heat stress reframes climate challenge
The onset of climate change has fundamentally transformed the understanding of heat stress on humans. People in most parts of the world are not only experiencing warmer weather, but the rise in temperatures is also accompanied by changing moisture patterns. Humans are resilient to high temperatures, provided sweat can evaporate. However, when the air is saturated with moisture, sweat lingers on the skin, and the body’s thermoregulatory system is put under duress. The combined effect of the two variables on human health is not totally unknown to medical science. The wet bulb approach, for instance, attempts to recalibrate temperatures by accounting for heat and humidity. A 35°C wet-bulb temperature is generally considered the theoretical upper limit of human endurance. A new study published in Nature Communications draws on recent episodes of intense heat and humidity to conclude that this safe limit could be much lower. A 31°C wet-bulb temperature can be unforgiving for the elderly and people who work outdoors.
The new understanding reframes India’s climate challenge. Several cities routinely report heat indices that push human tolerance to its limits, even when wet-bulb temperatures remain below 35°C. The tragedy in NaviMumbai, three years ago, when sun stroke claimed 13 lives even when the temperature, according to the Met department, was about 35°C, underlined the need to redraw the metrics of well-being during summer. However, planning has been slow to factor in the new imperatives. Concrete-dense neighbourhoods, sparse tree cover, and inadequate ventilation create “urban heat islands” where temperatures can be several degrees higher than surrounding rural areas. In such environments, nighttime offers little space to recover from the heat stress of the day. Heat thresholds are also shaped by social conditions. For construction workers, farm labourers, street vendors, and those who work in the gig economy, heat and humidity are occupational hazards.
In recent years, Indian cities have tried to draw up heat action plans. However, these plans are often based on dry-heat thresholds, and not the more dangerous combination of heat and humidity. Most of them are also not attuned to local idiosyncrasies. Protecting people from heat, humidity and rainfall requires granular knowledge. Dealing with a new form of heat will require joining several dots. India’s cities do not have time to lose.
- 1The inadequacy of current Heat Action Plans (HAPs) reveals a significant governance challenge for urban local bodies and State Disaster Management Authorities. Guided by the National Disaster Management Authority (NDMA), these bodies must urgently update HAPs to incorporate wet-bulb temperature, a combined measure of heat and humidity. This policy evolution is crucial for effective disaster risk reduction and demonstrates the administrative response required to combat climate change impacts.
- 2The state's failure to mitigate extreme heat stress, particularly for outdoor workers, directly engages the Right to Life under Article 21, which encompasses the right to a safe environment and humane working conditions. Such negligence, as seen in the Navi Mumbai tragedy, could establish state liability and may even attract scrutiny under provisions related to causing death by negligence in the Bharatiya Nyaya Sanhita (BNS).
- 3Extreme heat poses a severe economic and social threat, disproportionately impacting vulnerable groups like construction workers, street vendors, and gig economy employees. For these populations, heat stress is an occupational hazard that reduces productivity, suppresses income, and increases health expenditure. This exacerbates urban poverty and inequality, necessitating targeted social security measures and adaptive labour regulations to ensure their well-being and livelihood.
- 4The scientific understanding of heat stress has shifted from dry-bulb temperature to the more critical wet-bulb temperature, which measures the combined effect of heat and humidity on human survivability. A recent study suggests the danger threshold could be a 31°C wet-bulb temperature, much lower than the previously assumed 35°C. This is crucial for addressing environmental issues like 'urban heat islands' created by poor urban planning.