As India’s summer begins, understanding the heat and health conundrum

Anthropogenic Climate Change and Heat Hazards

Anthropogenic climate change is turning ambient heat, a relatively banal manifestation of the sun, into an inevitable environmental hazard. In 2023, with the atmospheric carbon dioxide level reaching new heights of 425 ppm, we witnessed the warmest decade on record spanning from 2014 to 2023. Until a few decades ago, hazards of high heat largely existed in confined, fire-based occupational settings or for people who exerted in hot weather for a long time, e.g. soldiers, athletes, and workers, and during occasional heatwaves.

Mentions of heat stroke have been found in literature since ancient times. With rising global temperatures, dangerous high heat has begun permeating our routine indoor spaces. This gradual expansion of the realm of extreme heat is potentially the gravest consequence of climate change for India.

India’s Increasing Vulnerability to Heat

India has observed a significant mean temperature increase of 0.15 degrees C per decade since 1950, according to a 2020 assessment by the Ministry of Earth Sciences. The observed warming is not occurring evenly across India. Warm days and warm nights have also increased at about seven and three days per decade, respectively, during the period 1951-2015. Currently, 23 States, mainly of plain and coastal regions, are considered more vulnerable to widespread heat impact. However, that doesn’t mean hilly states are safe. Although their maximum temperatures do not reach heatwave threshold levels of 45 degrees C, the population is experiencing higher temperatures compared to previous decades.

Heat Stress and Heat Stroke

Exposure to severe or continuous heat leads to heat stress. When uncompensated, heat stress manifests as heat-related illnesses. Such illnesses range from superficial/mild and manageable (e.g. prickly heat, heat-related swelling, heat cramps, heat exhaustion) to a medical emergency (i.e. heat stroke). Heat stroke is the most severe of heat-related illnesses: it presents with impaired brain function (i.e. stroke) due to uncontrolled body heating. It is a time-critical condition that often turns fatal if there is a delay or failure in reducing body temperature by rapid, active cooling. Besides neurological impairment, high core body temperature (at least 40 degrees C), or hot, dry skin are other heat stroke symptoms. These may be confused with fever. Clinically, a diagnosis of heat stroke poses a unique challenge as it requires the elimination of other causes of stroke, history of infection, and medication overdose.

There are two types of heat stroke. Exertional heat stroke is seen in young, fit individuals performing strenuous physical activities in hot or outdoor environments. It develops within one to two hours of exertion, especially when physical efforts are unmatched by physical fitness, even at moderately high temperatures. On the other hand, relatively slow-developing classic heat stroke is often difficult to fathom, challenging to identify instantly, and deadlier. Observed in individuals lacking the ability to regulate body temperature, e.g. children, the elderly, the sick, and the disabled, this heat stroke is more common during heatwaves. It contributes to about 9-37% of heat-related fatalities during heatwaves, and hence is also called epidemic heat stroke. Importantly, it occurs in hot environments in the absence of strenuous physical activity as individuals fail to perspire or cool off.

Pathophysiology of Heat

The heat-related illnesses mentioned above are not the only cause of emergency or mortality during hot summer days. Normal human body temperature stays within a narrow range of 36.3-37.3 degrees C. It maintains thermal balance through radiation (40%), evaporation (30%), convection (27%), and conduction (3%). Any external or internal condition that increases body temperature invokes various physiological responses changing cardiovascular, kidney, and metabolic systems, driven by increased blood flow to the skin and dehydration.

Heatwaves’ Compound Effects

Heatwaves are among the deadliest natural hazards. Apart from the aforementioned direct impacts, there are also multiple indirect health impacts due to disruption in services (power, water, transport), increase in food and water-borne diseases, increase in drowning and accidents, and due to overburdened/slowed health services. Globally, thousands of people die yearly and many suffer livelihood consequences due to heat. However, the full impact of a heatwave is often difficult to realize fully until weeks or months later.

That is not all. Heatwaves also have compound and cascading environmental impacts like concurrent drought, glacial lake outbursts causing flash floods, urban/wildfires, and increased air pollution from ground-level ozone and dust storms. Due to the potential to create widespread health and economic damage, heatwaves can be categorized as a disaster. The disaster risk triad framework delineated in the U.N. Intergovernmental Panel on Climate Change’s Fifth Assessment Reports (2014) can explain its health impacts in that context.

Understanding Heat Exposure

Multiple physical, socio-economical, environmental, and institutional factors determine health outcomes of heat exposure at the population level. In assessing it, using the disaster risk triad – hazard, exposure, and vulnerability– concept, the first and foremost challenge comes in estimating the exposure.

Conventionally, maximum daily temperatures have served the purpose. For example, thresholds for announcing heatwave and severe heatwave warnings for plain areas of India are at maximum temperatures of 45 degrees C and 47 degrees C, respectively. Although the role of high humidity in amplifying heat stress has been recognized, most available heat indices are not adequate for estimating our population’s risk. One widely used heat index (maximum temperature and humidity) equation was formed decades ago, before climate models existed, with assumptions and perceptions of different levels of temperatures among healthy, western individuals. It is also found to be underestimating the severity of heatwaves. The wet bulb globe, which mirrors how the human body cools itself with sweat in outdoor conditions, has been a promising measure. The quest for finding and validating a suitable index for the Indian population is very much on.

Heating up the Puzzle

Because of the multifaceted dimensions of heat impacts, assembling pieces of the population heat exposure puzzle is complicated especially as warming continues. However, understanding existing vulnerabilities and taking action to prevent extreme heat’s impact is very much in our hands. And importantly, heat-related illnesses are preventable. Assessing patterns of socio-economic, demographic, and environmental factors, access to basic services, disease distribution, existing institutional mechanisms, and preparedness helps to take stock of the situation and to prioritize resource allocation for vulnerable populations, and strengthen institutional responses.

Public Health Action

Adaptation measures to extreme heat

SDGs, Targets, and Indicators

1. Which SDGs are addressed or connected to the issues highlighted in the article?

• SDG 3: Good Health and Well-being
• SDG 11: Sustainable Cities and Communities
• SDG 13: Climate Action

2. What specific targets under those SDGs can be identified based on the article’s content?

• SDG 3.9: By 2030, substantially reduce the number of deaths and illnesses from hazardous chemicals and air, water, and soil pollution and contamination.
• SDG 11.5: By 2030, significantly reduce the number of deaths and the number of people affected and substantially decrease the direct economic losses relative to global gross domestic product caused by disasters, including water-related disasters, with a focus on protecting the poor and people in vulnerable situations.
• SDG 13.1: Strengthen resilience and adaptive capacity to climate-related hazards and natural disasters in all countries.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

• Number of heat-related illnesses and deaths
• Number of emergency admissions due to heat-related illnesses
• Number of cardiovascular and all-cause mortalities during heatwaves
• Increase in all-cause mortality with daily city temperature

Table: SDGs, Targets, and Indicators

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Source: thehindu.com

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