The Meteorological Department expected heat indices to reach dangerous levels in five areas of the country on Sunday.
The highest "real feel" apparent regional temperatures expected on Sunday were:
• 51.2°C in Phetchabun in the North;
• 46°C in Kosum Phisai district, Maha Sarakham province, in the Northeast;
• 52.7°C in Bangkok's Bang Na area in the Central area;
• 53°C in Sattahip district, Chon Buri province, in the East; and
• 51.1°C in Phuket, in the South.
The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature.
Sunday's soaring temperatures caused 14 people, including voters and officials manning polling stations, to faint at Ramkhamhaeng University in Hua Mak area of Bang Kapi district, a polling station for advance voting in Bangkok.
Pairat Kasetsin, the Bang Kapi district chief, said many voters reportedly felt unwell and fainted when they turned up at their polling stations for advance voting around noon or in the afternoon because of the heat. They were provided with first-aid treatment in an area nearby.
The Bangkok Metropolitan Administration sent water trucks to the venue. Water was sprayed into the air to reduce the heat. More electric fans were installed around the area.
Elsewhere, three people were also reported to have lost consciousness because of the heat at Chan Kasem Rajabhat University, another polling station for advance voting in Bangkok.