Heatwaves, the Heat Index and Preventing Heat Stroke

Every summer, Vietnamese weather bulletins warn of fierce heatwaves with temperatures topping 38-40°C. But there is another number that matters even more than the reading on the thermometer: the heat index. It tells you how hot your body actually "feels" and how high your risk of heat stroke really is.

When do heatwaves hit Vietnam?

Under Vietnam's meteorological standards, a day counts as a heatwave when the maximum temperature reaches 35°C or higher, intense heat from 37°C, and extreme heat from 39°C. When these conditions persist for several consecutive days over a wide area, it is officially declared a heatwave spell.

In the North and North-Central regions, heatwaves cluster between May and August, often driven by the dry, scorching foehn wind (locally called the "Lao wind") blowing from the west. The South and Central Highlands are hottest during the dry season, roughly March to May, when humidity is low. Big cities like Hanoi and Ho Chi Minh City suffer an added "urban heat island" effect, where concrete, asphalt and traffic push the felt temperature well above that of the surrounding countryside.

How the heat index differs from temperature

A thermometer only measures air temperature, but the human body cools itself mainly through sweat evaporation. When humidity is high, sweat struggles to evaporate, heat cannot escape, and you feel far hotter than the thermometer suggests.

The heat index combines temperature and humidity to reflect the true "feels-like" temperature. For example:

• 34°C at 40% humidity feels like about 34°C - fairly tolerable.
• The same 34°C at 70% humidity sends the heat index soaring toward 44°C - a dangerous level.

This is why on muggy, humid days in northern Vietnam, people tire quickly even when the temperature is not extreme. Once the heat index passes 41°C, the risk of cramps, exhaustion and heat stroke rises sharply; above 54°C is extremely dangerous.

What heat stroke looks like

Heat stroke is a medical emergency that occurs when the body's core temperature climbs above 40°C and its temperature-regulation system shuts down. It is the most severe stage in a chain of heat illnesses, following heat cramps and heat exhaustion.

Call for emergency help immediately if you notice:

• Skin that is hot and red, either dry or still sweating.
• A very high body temperature, rapid pulse and fast breathing.
• Severe headache, dizziness and nausea.
• Confusion, slurred speech, seizures or loss of consciousness - these are critical danger signs.

While waiting for the ambulance, move the person to shade, remove excess clothing, and cool them with wet cloths, fans and cold packs on the neck, armpits and groin. Never give fluids to someone who is drowsy or unconscious, as this can cause choking.

Who is most at risk

Some groups are far more vulnerable to heat and deserve extra attention:

• Older adults and people with heart disease, diabetes or high blood pressure.
• Young children and infants, whose temperature regulation is not yet fully developed.
• Outdoor workers: farmers, construction crews, delivery riders and street vendors.
• Athletes and anyone exercising intensely during the midday heat.

A crucial reminder: never leave a child or pet in a closed, parked car, even for a few minutes. The temperature inside can spike above 50°C within 10-15 minutes, which can be fatal.

Practical ways to prevent heat stroke

Prevention is always easier than treatment. On hot days, take charge with these steps:

• Stay hydrated with around 2-2.5 litres of water a day, sipping regularly even before you feel thirsty. Add electrolyte drinks when sweating heavily.
• Avoid going out during peak hours, from 10 a.m. to 4 p.m. If you must work outdoors, rest in the shade for 10-15 minutes every hour.
• Wear light-coloured, loose, breathable clothing; add a wide-brimmed hat, sunglasses and sunscreen.
• Cut back on alcohol and coffee, which worsen dehydration.
• Follow the weather forecast and the heat index so you can plan your day around the worst of the heat.

A heatwave is not just an inconvenience - it is a genuine health threat. Understanding the heat index and recognising the early signs of heat stroke can help you and your loved ones get through Vietnam's summers safely.