The latest data from weather state bureau PAGASA showed that a dangerous level of heat index may hit nine areas in the country on Monday.

Six areas are expected to have an ‘init factor’ of 42° Celsius: Tuguegarao City in Cagayan, San Jose in Occidental Mindoro, Puerto Princesa City and Aborlan in Palawan, Central Bicol State University of Agriculture in Pili, Camarines Sur, and Roxas City in Capiz.

Meanwhile, Aparri in Cagayan is projected to have a heat index of 43°C, and Cotabato City in Maguindanao has a temperature index of 44°C.

The highest heat index is projected to be felt in Daet in Camarines Norte, with its temperature forecast to peak at 46 °C.

Baguio City and Benguet State University in La Trinidad, Benguet, have the lowest heat index expected to reach 27 °C.

The heat index, as defined by PAGASA, is the human discomfort index that gives the "apparent" temperature, or what humans perceive or feel as the temperature affecting their body.

The effect-based danger classification ranges from 42°C to 51°C, and its effects on the body are that heat cramps and exhaustion are likely, and heat stroke is probable with continued exposure.

To prevent heat-related illnesses, PAGASA encouraged the public to limit their time spent outdoors; drink plenty of water; avoid tea, coffee, soda, and liquor; wear umbrellas, hats, and sleeved clothing outdoors; and schedule heavy work activities in the beginning or at the end of the day when it is cooler.