Thailand’s 52°C heat index warning signals a public health threat

In early 2025, the Thai Meteorological Department and the Department of Health under the Ministry of Public Health issued a warning that the heat index in some parts of Thailand could soar to 52°C or higher, which is classified as “extremely dangerous”, or the “red zone”, under the four-level warning system jointly developed by the two agencies.

The heat index is a value calculated from two main variables, air temperature and relative humidity, and converted into a figure showing how hot it really “feels” to the human body at that moment.

It can often be much higher than the actual air temperature.

For example, during Thailand’s hot season, between March and mid-May, relative humidity is usually around 60-75%, meaning an air temperature of just 36°C can produce a heat index of more than 50°C.

That is why the Department of Health has stressed the need to monitor the heat index, not just the reading on a thermometer.

For the four-level heat index warning system jointly used by the Department of Health and the Thai Meteorological Department, the levels are as follows:

Observation level (green): a heat index of 27-32.9°C.

At this level, prolonged exposure to direct sunlight may cause fatigue, headaches or rashes.

Watch level (yellow): 33-41.9°C.

The risk of fainting and heat cramps begins to rise.

Danger level (orange): 42-51.9°C.

Prolonged outdoor activity at this level carries a high risk of heat stroke.

Extremely dangerous level (red): 52°C and above.

The Department of Health says it may be life-threatening even without heavy activity.

Between March 5-14, 2025, heat index levels in many provinces, including Bangkok, were in the orange zone at 42-51.9°C, while red-level warnings were issued in some areas.

For the daily heat index on Wednesday, March 25, 2026, Bangkok was found to be at a dangerous level.

What does 52°C feel like

Many people may find it hard to imagine what 52°C would actually feel like.

Try thinking about opening the door of a car that has been parked in the sun at midday, before the air conditioning starts working.

That is the temperature inside a closed vehicle, which may rise to 60-70°C.

But a heat index of 52°C outside the car feels closer to standing in front of an oven set to 50°C.

But unlike standing by an oven, people are not just staying still.

They are walking, riding motorbikes, delivering parcels, selling goods, and working on construction sites.

Those activities generate another layer of heat inside the body.

There is another clear comparison.

A typical dry sauna is often set at 70-90°C, but the humidity is very low, so the body can still evaporate some sweat.

Outdoor air on a day when the heat index reaches 52°C combines both heat and high humidity, meaning the body can evaporate very little sweat.

In terms of health effects, it can be even more dangerous than a sauna.

More worrying still, this level is not simply “hot but bearable”.

It is the point at which the body’s cooling system begins to fail.

What signals does the body send? From Heat Cramps to Heat Stroke

The classification of these stages of heat-related illness is based on the system used by the US Centers for Disease Control and Prevention (CDC), the National Oceanic and Atmospheric Administration (NOAA), and information from the National Center for Biotechnology Information (NCBI), which explains that heat-related illness does not happen all at once, but develops as a group of symptoms that progress and become more severe over time.

Stage 1 Heat Cramps

The first stage is heat cramps.

These occur when the body loses large amounts of sweat and electrolytes, causing muscles, especially in the legs and abdomen, to contract painfully.

The CDC explains that this condition is common among people who exercise or work hard in hot conditions and drink plain water without replacing electrolytes.

Symptoms to watch for include painful muscle cramps, especially in the legs and abdomen, heavy sweating and fatigue.

At this stage, the body can usually recover if the person stops to rest and drinks water and electrolytes immediately.

But if they force themselves to continue, they may progress more quickly to the next stage.

Stage 2 Heat Exhaustion

Heat exhaustion is the body’s response to losing large amounts of water and electrolytes through sweating.

It usually occurs in conditions of extreme heat and high humidity, and if not treated immediately, it may develop into heat stroke.

The symptoms of heat exhaustion include several warning signs: severe headache, nausea, vomiting, dizziness, light-headedness, weakness, a rapid heartbeat, pale, cool skin soaked in sweat, extreme fatigue, as well as reduced and dark-coloured urine.

What makes this frightening is that, according to the NCBI, the transition from heat exhaustion to heat stroke can happen within a very short time if the person remains in the sun and does not receive care.

Waiting in the hope that “it will probably pass” may be a seriously dangerous mistake.

Stage 3 Heat Stroke A life-threatening emergency

According to the CDC, heat stroke is the most severe heat-related illness.

It occurs when the body can no longer control its core temperature.

The cooling mechanism fails, including the sweating system.

Body temperature may surge to 41-42°C or higher within just 10-15 minutes, and without immediate treatment, it may cause death or permanent disability.

Key signs of heat stroke that require urgent attention include a body temperature above 40°C, hot, red, dry skin with no sweating, severe headache, confusion, slurred speech, changes in behaviour, a strong and rapid heartbeat, rapid breathing, and, most importantly, possible loss of consciousness.

Thailand’s Department of Health stresses that when someone shows signs of heat stroke, 1669 must be called immediately.

While waiting for an ambulance, the patient should be moved into the shade, unnecessary clothing removed, and the body cooled with cold water, especially around the back of the neck, the armpits and the groin, while a fan is used at the same time to help release heat from the body as quickly as possible.

What should not be done when someone has heat stroke is equally important: do not give water if the patient is unconscious or semi-conscious, as this may cause choking, and do not use blankets or anything else to cover the body.

Who is at greatest risk

Heat does not affect everyone equally.

The Department of Health says some groups face significantly higher risks than the general public and require special care during periods of high heat index.

They include:

Older people aged 65 and above are the first group that must be watched closely because their bodies adjust more slowly to temperature changes, and they become dehydrated more easily.

In 2024, the Department of Disease Control said older people were the group with the highest number of heat stroke deaths.

Young children and infants are another highly vulnerable group because their body cooling systems are not yet fully developed.

Children cannot always say whether they feel hot or unwell, and danger may develop more quickly than in adults.

The Department of Health has specifically warned that children must never be left in a switched-off car, because the temperature inside can rise sharply within minutes.

People with chronic illnesses such as heart disease, high blood pressure, diabetes, kidney disease and obesity are also at higher risk because these conditions affect blood circulation and the body’s ability to release heat.

In addition, some medicines used by these patients, such as diuretics, blood pressure medication, psychiatric drugs and some antihistamines, can also make it harder for the body to cool itself.

Outdoor workers are a group often overlooked when discussing the effects of extreme heat.

This includes construction workers, food and parcel delivery riders, traffic police, security guards, vendors, farmers and workers in the manufacturing sector.

These people have no choice over whether to go outdoors and often have to work for several continuous hours during the hottest period of the day.

People living in homes or rooms with poor ventilation and no air conditioning are also at risk.

In crowded communities, row houses or old urban buildings, indoor temperatures at night may be higher than outside because concrete absorbs heat during the day and releases it slowly at night, leaving no real break from the heat.

According to surveillance data on heat stroke illness and deaths between 2019 and 2023, there were 131 cumulative deaths, or an average of around 26 a year, and the number of patients has been rising.

In 2024, in just the first two months of the hot season, March and April, there were already 30 deaths, compared with 37 in 2023 over four months from March to June.

That means the death rate has clearly accelerated.

This figure may still be lower than the reality, because many other patients enter the healthcare system with heat-related symptoms such as heart failure, kidney failure or other complications, but are recorded under another main diagnosis rather than Heat Stroke directly.

In the past, many Thais might have said, “Thailand’s hot season is just like this.”

But what has happened over the past few years is starting to look anything but normal.

Many countries around the world have begun introducing heat warning systems on a par with storm or flood alerts, because extreme heat is becoming a new form of public health disaster.