EXPLAINER: What is the heat index?

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Metro Manila (CNN Philippines, April 24) — The Philippines is currently experiencing scorching heat, with the state weather bureau logging on April 21 the highest computed heat index this year at 48 degrees Celsius — almost half of the boiling point of water.

Butuan City, Agusan Del Norte got the highest heat index this year followed by Legazpi City, Albay and San Jose, Occidental Mindoro with 47 °C.

But what is the heat index and why does it matter?

The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) defined the heat index as the "measure of the contribution that high humidity makes with abnormally high temperatures in reducing the body's ability to cool itself."

It is the discomfort that your body feels when relative humidity, combined with the air temperature increases.

According to the National Weather Service, when the body temperature gets too hot, it begins to perspire or sweat to cool itself off. But the body will not be able to regulate its temperature if perspiration does not evaporate quickly.

Humid conditions make sweating and evaporation less efficient. In other words, the heat index can be tolerable if the relative humidity is low.

Here's the Heat Index Chart as provided by the local state bureau:


27 to 32 °C - Caution

-Fatigue is possible with prolonged exposure and activity.

- Continuing activity could lead to heat cramps.

33 to 41 °C - Extreme Caution

-Heat cramps and heat exhaustion are possible.

- Continuing activity could lead to heat stroke.

42 to 51 °C - Danger

- Heat cramps and heat exhaustion are likely to occur.

- Heat stroke is probable with continued exposure.

52 °C and beyond - Extreme danger

- Heat stroke is imminent.

In its five-day forecast heat index issued on Sunday, PAGASA said Tacloban City may reach a maximum of 47 °C heat index on Aril 25 and 48 °C on April 26.

The highest computed heat index on Sunday was in Legazpi City, Albay at 46 °C.

READ: PAGASA: Heat reaches dangerous levels around the country

Climate change

Shifts in extreme heat events are due to the increases in the overall temperature of the earth associated with climate change, according to the Centers for Disease Control and Prevention (CDC).

It said burning of fossil fuels, such as coal and gas, causes the earth's temperature to rise, like a blanket traps in heat.

"This extra trapped heat disrupts many of the interconnected systems in our environment," the CDC added.

In the Philippines, PAGASA noted there had been an average of 0.0108 °C per year-increase in the temperature due to climate change.

What to do?

The World Health Organization said countries with high temperatures are likely to increase the risk of heat-related deaths and illnesses, as well as cardiovascular and respiratory disorders.

Here are some ways you can protect yourself from extreme heat according to PAGASA:

Limit the time spent outdoors.

- Drink plenty of water.

- Avoid tea, coffee, soda, and liquor.

- Use umbrellas and wear hats and sleeved clothing outdoors.

- Schedule heavy activities for the beginning or end of the day, when it's cooler.

Meanwhile, the WHO also advised:

- Move to the coolest room in the home, especially at night.

- If it is not possible to keep your home cool, spend 2 to 3 hours of the day in a cool place (such as an air-conditioned public building).

- Avoid going outside during the hottest time of the day.

-Avoid strenuous physical activity if you can. If you must do strenuous activity, do it during the coolest part of the day, which is usually in the morning between 4 a.m. and 7 a.m.

- Stay in the shade.

- Do not leave children or animals in parked vehicles.