Ever stepped out to be hit by a blistering wave of heat? When the surroundings felt like a furnace and the body seemed to give up from exhaustion?
This is how it was a week ago, when in many parts of India, including Delhi, the mercury touched 40 degrees Celsius, before sudden rainfall drastically lowered temperatures. The high humidity pushed the ‘feel-like temperature’ to almost 50 degrees Celsius, and the heat wave disrupted daily life.
Now, imagine a long battle with unrelenting heat for weeks or months, stifling communities and threatening livelihoods. Heat waves and chronic heat stress may seem similar at first glance and are often spoken about interchangeably, but have vastly different consequences. This explainer highlights the causes and consequences of these phenomena and looks at possible mitigation measures.
Heat wave versus heat stress
A heat wave is a condition of air temperature that becomes fatal to the human body when exposed to it, says the India Meteorological Department (IMD). Experts say it occurs when temperatures go around 5 degrees Celsius above normal across a region.
According to the World Health Organization, heat stress occurs because of extended periods of high day and night temperatures, which increases the risk of heat-related illnesses and exacerbates underlying medical conditions.
Meanwhile, the heat index is the combination of air temperature and relative humidity; it measures how hot it feels when relative humidity is factored in with the actual air temperature.
“There is a bigger problem than ‘maximum temperature.’ Temperature and humidity together have become a deadly combination,” says Prabhakaran Veeraarasu, environmental engineer, Poovulagin Nanbargal, a voluntary organisation working on socio-environmental issues in Tamil Nadu.
“Heat stress is about how we feel and how our bodies react to the heat discomfort. Humidity becomes a very important factor here,” says Raj Bhagat Palanichamy, geoanalytics expert.
How heat waves are measured
As per a study by Environment International, an estimate of 1,116 deaths annually were attributed to heat waves in India. Heat wave decisions are made according to two criteria in India based on a departure from the normal temperature and an absolute temperature threshold.
At the start of each season and month, the IMD issues weather outlooks covering one to three months. Every Thursday, it releases an extended range forecast predicting heat wave risks for the next two weeks. “Additionally, daily heat wave warnings are given for the upcoming seven days. Based on these forecasts, relevant agencies should take necessary actions to mitigate the impact of extreme heat,” says Akhil Srivastava, scientist at IMD.
“IMD also issues ‘severe heat wave’ warnings, where the threshold criteria remain the same. However, the departure from the normal temperature should be more than 6.4 degrees Celsius, or the absolute maximum temperature should be 47 degrees Celsius and higher,” adds Akhil.
Humidity and heat
Sweating helps cool the body, but when humidity is high, sweat doesn’t evaporate easily, trapping heat and increasing the ‘feel-like’ temperature. Wet bulb temperature (WBT) measures how much moisture the air can hold, affecting cooling. Online calculators help understand how temperature and humidity combine to create extreme heat conditions.
“This is exactly why in Chennai, even when the air temperature is relatively low, the heat is still hazardous as they have high WBT,” says Vivek Gilani, founder and director of Fair Conditioning Foundation.
Factors that influence heat stress
Heat stress is a rather wide concept that explores the impact of heat on people. There are several factors which affect heat stress response in humans:
- Physical fitness
- Physical exertion level (could be occupation based like sports, construction work, gig work, driving etc)
- Humidity
- Hydration level
- Time exposed
- Air temperatures
- Radiant heat source
- Obesity (body mass index ≥ 30 kg/m2)
- Chronic diseases (diabetes, high blood pressure, heart disease, kidney issues)
- Medications (diuretics, blood pressure medications, etc.)
- Illicit drug use (opioids, methamphetamine, cocaine, etc.)
- Age (both infants and elderly)
- Wind
- Altitude
- Socioeconomic inequalities (people living in informal settlements, having no resources to have cool roofs, air conditioning/cooler machines).
Read more: Heat insurance: The missing piece in India’s heat action plans
Health impact: Acute and chronic
It’s crucial to differentiate the health impact of a heat wave and chronic heat stress because the mitigation measures vary depending on the exposure and impact. While heat wave impact requires immediate medical attention, heat stress impact is more chronic, hence less-studied and less attributed to heat.
Heat stress is the leading cause of weather-related deaths and can exacerbate underlying illnesses, including cardiovascular disease, diabetes, mental health, asthma, and can increase the risk of accidents and transmission of some infectious diseases.
Heat strokes can last a few weeks and if undiagnosed can cause chronic damage to other organs like heart, lungs, brain, liver, kidneys, making it fatal very quickly. The strain put on the body as it tries to cool itself also stresses the heart and kidneys. As a result, heat extremes can cause and worsen chronic conditions (cardiovascular, renal, mental, respiratory and diabetes related conditions).
Chronic heat stress puts socio-economically, occupationally and physiologically vulnerable people at a higher risk for chronic damage along with heat-related illnesses (HRI). People aged over 60 years are at increased risk of HRIs during heat waves because of physiological impairments. Outdoor and manual workers, athletes and civil protection employees are exposed to excess heat because of their work and are susceptible to exertional heat stress.
Urban and rural poor are often disproportionately exposed to overheating due to low-quality housing and a lack of access to cooling.
Economic impact of heat stress
Heat-related income loss accounted for over 5% of the country’s GDP in India in 2021. Additionally, due to increased warm nights, people, especially in informal settlements, do not get adequate rest, increasing their health risks and productivity loss.
“It is extremely difficult to measure the economic impact of heat as we cannot identify every pay-compromise and productivity levels impacting the overall GDP. We can only get rough estimates. It’s a similar picture for health. Heat cannot go as a cause of death in most cases, even cases with heatstrokes can be recorded differently when there are comorbidities,” points out Raj.
What mitigation measures can be adopted?
Experts opine that in many cases, we do not need specific heat-tagged policy changes, but must continue implementing existing policies to beat the heat effectively. “Planting and space for trees should be in the design of urban roads, which can serve the heat purpose if implemented properly,” says Raj.
Vivek adds, “Heat mitigation is an architectural issue as well, so we should enhance the curriculum of our architects to study the basics of heat impact to ensure better-prepared cities are built.”
“The government should provide a standard operating procedure (SOP) to tackle heat-emergiences. People don’t know what to do during heat exhaustion and heat strokes. People should be trained for first-aid to prevent fatalities,” adds Prabhakaran.
As per the NDMA public health advisory, these are some basic do’s and don’t during peak-hot weather
Do’s
- Stay hydrated, consume ORS.
- Stay covered in thin loose garments
- Stay alert for IMD warnings
- Stay indoors/in well-ventilated, cool places
Don’ts
- Avoid getting out in the sun, especially between 12 noon and 3 pm
- Avoid strenuous activities when outside in the afternoon
- Do not go out barefoot
- Avoid cooking during peak summer hours. Open doors and windows to ventilate cooking area adequately
- Avoid alcohol, tea, coffee and carbonated soft drinks or drinks with large amount of sugar- as these actually, lead to loss of more body fluid or may cause stomach cramps
- Avoid high-protein food and do not eat stale food
- Do not leave children or pets in parked vehicles.