There is scientific consensus that 2023 was the hottest year ever in the planet’s recorded history. It will get progressively worse as this century sweats it out in a sauna of sweltering heat. Indeed, climate modellers forecasting the future say 2023 could be the coolest year in the memory of those born that year. As urbanisation increases, cities will grow at the expense of forests to become cauldrons of heat and humidity.
The consequences of climate change, mostly mediated through global warming, adversely affect human, animal and plant health in several ways: Direct effects of heat exposure on the body; extreme weather events; water scarcity; vector-borne and water-borne infections; non-communicable diseases (strokes; heart attacks; diabetes; respiratory disease; cancers); mental health disorders; food and nutrition insecurity due to reduced yield and nutrient quality of crops. Health systems, too, will face increased demand due to these and the movement of displaced climate refugees. They will also be affected by the adverse impact of extreme weather events on healthcare infrastructure, supply chains and personnel.
Of these myriad threats, heat waves are of immediate concern as summer advances across the Indian Subcontinent. While heat waves are defined both in terms of absolute temperatures recorded and relative rise in comparison to seasonal, “normal” values recorded in earlier years, it is not just the temperature that matters. A heat wave represents a combination of heat and humidity. At any level of heat, its impact on human body is amplified if humidity is high in the ambient air. The India Meteorological Department (IMD) says that such “moist heat” stress has increased by 30 per cent between 1980 and 2020.
Heat acts through dehydration, inability of the skin to cool the body through perspiration, dilatation of blood vessels and thickening of blood with increased risk of clot formation (thrombosis). Often, air pollution colludes with excess heat to assault the lungs and blood vessels. Infants and young children are more vulnerable than adults, since their bodies have higher fluid content than grownups. Among adults, vulnerability is high among the elderly, persons with disabilities and in those with serious comorbidities. Women are more vulnerable than men.
While the progressively severe effects of heat stress (heat exhaustion, heat stroke, circulatory failure and death) are well recognised, the effect of severe or prolonged heat exposure on non-communicable diseases (NCDs) is not often discussed in the public domain. Yet, these can severely damage health, through acute and chronic effects. Even as of now, NCDs contribute to 65 per cent of deaths in India — a majority of them in productive mid-life. The heat effects of climate change will only exacerbate the problem.
Heat increases the risk of brain strokes (paralytic attacks) due to thrombosis in blood vessels of the brain. Similarly, it can precipitate heart attacks and sudden death by triggering clot formation in the coronary arteries. Exercising vigorously in a hot environment can be dangerous. Clots formed in the leg veins can travel to the lungs suddenly causing catastrophic “pulmonary embolism”. By reducing the circulating blood volume, heat can damage the kidneys. The incidence of congenital cataracts in babies rises during heat waves. Even surgical wounds don’t heal well in excessively hot weather.
A study of 266 studies worldwide, published in 2022, revealed that heat waves were associated with a 11.7 per cent increase in mortality. The highest specific risk was for stroke and coronary heart disease. A high risk was also reported for fatal and non-fatal cardiac arrhythmias. As our population ages and cardiovascular risk factors (like high blood pressure, diabetes and obesity) rise in our population, every 1 degree centigrade rise in ambient temperature will compound the risk of serious cardiovascular events.
Wildfires triggered by excessive heat release particulate matter (PM 2.5) and toxic chemicals (ozone, carbon monoxide, polycyclic aromatic compounds and nitrogen dioxide) can cause extensive inflammation, increasing the risk of cardiovascular disease (heart attacks, strokes, heart rhythm abnormalities, pulmonary embolism, heart failure), respiratory disease, diabetes and pre-diabetes. Chemicals like benzene and formaldehyde (also present in wildfire emissions) increase the risk of cancer.
Less immediate, but with a long lasting impact on health, is the impact on food systems which threatens nutrition security. A combination of heat stress, water stress and increased carbon dioxide levels. Countries in South Asia and sub-Saharan Africa currently grow staples (like rice and wheat) at the highest levels of heat tolerance. A further increase of 1 degree centigrade will lower their yield by 10 per cent. The crops will also be more deficient in zinc, protein and iron. The Data Sciences Centre of Columbia University has estimated that if global warming continues unabated, India of 2050 will have 49 million more zinc deficient persons and 38.2 million new protein deficient persons, while 106.1 million children and 396 million women would be iron deficient. Protective foods like fruit, vegetables and fish would be depleted. These foods reduce the risk of cardiovascular disease and diabetes. Rising ocean temperatures will flood coastal agriculture while reducing fish yields. Loss of biodiversity will reduce access to nutritious “wild foods”. Heat will kill us not only through its effect on our skin but also our stomachs.
Even as we must try energetically to mitigate the drivers of climate change and global warming, we must prepare adaptation plans. These will include heat action plans designed specifically for urban and rural areas, climate smart and climate resilient food and healthcare systems, education of public and healthcare providers and anticipatory planning to meet the increased healthcare demands that heatwaves will bring in their wake. Heat shelters, water stations, heat reflective roofs and wall paints for housing, well ventilated homes and an increase in green spaces must become part of urban planning. Personal protection too will be needed — light coloured, loose fitting clothes, umbrellas or caps are helpful. The most important personal protection measure is to drink lots of water.
The writer is Distinguished Professor of Public Health, PHFI. Views are personal
