EXPLAINER: How hot is too hot? Understanding the heat index

As the northeast monsoon, or amihan, season draws to a close, the public is being urged to prepare for rising temperatures and to closely monitor the heat index, according to the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA).

The weather bureau said hotter conditions are expected in April and May, when heat index levels typically peak across many parts of the country.

What is the heat index?

The heat index is not just the air temperature.

It reflects how hot it actually feels to the human body by combining air temperature and relative humidity. High humidity slows the evaporation of sweat, making it harder for the body to cool itself.

PAGASA noted that the heat index can increase by as much as 9°C when a person is directly exposed to sunlight.

When does it become dangerous?

PAGASA classifies heat index levels into four categories:

• 27°C to 32°C (Caution): Fatigue is possible with prolonged exposure
• 33°C to 41°C (Extreme Caution): Heat cramps and heat exhaustion may occur
• 42°C to 51°C (Danger): Heat exhaustion is likely, and prolonged exposure may lead to heat stroke
• 52°C and above (Extreme Danger): Heat stroke is highly likely and considered a medical emergency

During last year’s dry season, Dagupan City recorded a heat index of 52°C—among the highest in the country.

How does extreme heat affect the body?

When temperatures are too high, the body struggles to regulate its internal temperature.

This can lead to a rise in body heat and strain on vital organs such as the heart and kidneys. If not addressed, it may result in heat exhaustion or heat stroke.

According to the World Health Organization, extreme heat can also worsen existing health conditions, including cardiovascular diseases, respiratory illnesses, and diabetes.

Heat stroke occurs when the body can no longer cool itself, potentially causing confusion, loss of consciousness, and even death if left untreated.

Who are most at risk?

Those more vulnerable to heat-related illnesses include:

• Older adults and young children
• Pregnant individuals
• People with chronic medical conditions
• Outdoor workers, such as construction and delivery personnel

Urban areas may also experience higher temperatures due to the “urban heat effect,” where concrete structures and limited vegetation trap heat.

How can you stay safe?

Authorities recommend the following precautions:

• Drink plenty of water, even if not thirsty
• Avoid prolonged exposure to direct sunlight, especially at midday
• Use umbrellas, hats, or light clothing
• Stay in shaded or well-ventilated areas
• Learn basic first aid for heat-related emergencies

The public is also advised to regularly check daily heat index forecasts from PAGASA.

Its official website features an interactive heat index map, or iHeatMap, where users can view location-specific data, including hourly temperatures and corresponding heat index levels.—With research and information from GMA Integrated News Feed/MCG, GMA Integrated News