Heat Waves In India: What Is The Criterion For Declaring Heat Wave And Impact Of Heat On Health

Heavy showers in the past few days over parts of the country have brought much-needed relief from heatwaves. Despite that, temperatures in parts of central and north India are touching 40 degrees Celsius.

Scientists estimate climate change has made extreme heat 30 times more likely in India and the World Bank has flagged India is likely to be one of the first places in the world where heat waves breach the human survivability threshold, a Bloomberg report said.

Last week, Health Minister Mansukh Mandaviya chaired a high-level meeting to review the public health preparedness for management of heat-related illnesses and virtually interacted with state health ministers, state disaster management ministers and principal secretaries/additional chief secretaries and information commissioners of seven states (Uttar Pradesh, Bihar, Odisha, Jharkhand, Andhra Pradesh, Chhattisgarh and Telangana) which were experiencing severe heatwave conditions.

According to PTI, the Health Minister urged the states to implement the State Action Plan, based on the National Action Plan, at the ground levels with a timely warning to the people, and ensure preventive preparedness to reduce the severe impact of heatwave.  

Let's take a detailed look at how heat waves are declared in India and its impact on health.

According to the India Meteorological Department, qualitatively, a heat wave is a condition of air temperature which becomes fatal to human body when exposed.

Quantitatively, it is defined based on the temperature thresholds over a region in terms of actual temperature or its departure from normal.

Heat wave is considered if the maximum temperature of a station reaches at least 40 degrees Celsius or more for Plains and at least 30 degrees Celsius or more for Hilly regions.

Based on Departure from Normal:

• Heat Wave: Departure from normal is 4.50°C to 6.40°C

• Severe Heat Wave: Departure from normal is >6.40degree C

Based on Actual Maximum Temperature:

• Heat Wave: When actual maximum temperature ≥ 45°C

• Severe Heat Wave: When actual maximum temperature ≥47°C

Criterion for describing Heat Wave for coastal stations?

When maximum temperature departure is 4.5°C or more from normal, Heat Wave may be described provided actual maximum temperature is 37°C or more.

According to IMD, the health impacts of Heat Waves typically involve dehydration, heat cramps, heat exhaustion and/or heat stroke.

The signs and symptoms are as follows:

• Heat Cramps: Ederna (swelling) and Syncope (Fainting) generally accompanied by fever below 39 degrees Celsius i.e.102 degrees Fahrenheit. 

• Heat Exhaustion: Fatigue, weakness, dizziness, headache, nausea, vomiting, muscle cramps and sweating. 

• Heat Stoke: Body temperatures of 40 degrees Celsius i.e. 104 degrees Fahrenheit or more along with delirium, seizures or coma. This is a potential fatal condition.

Heat wave is occurring mainly during March to June and in some rare cases even in July. The peak month of the heat wave over India is May.