Temperatures hit "very dangerous" levels in Phuket on Wednesday and "dangerous" levels in Bangkok and 34 other provinces, and the heatwave was expected to continue into Thursday, a senior health official said.
People in vulnerable groups, including the elderly and ailing, were warned to stay out of the sun.
Dr Amporn Benjaponpitak, director-general of the department of health, said a “heat index” at the very dangerous red level could result in rashes, cramp, heat exhaustion, heatstroke or fainting, and even death.
Senior citizens, people who must work outdoors and those with affected by alcoholism, mental illness, obesity or chronic disease should take precautions.
“The heat index is the temperature that people feel they are experiencing,” Dr Amporn said. It took into account both the actual temperature and the relative humidity, which made it feel far hotter than it really was.
A heat index in the red, or very dangerous, level meant people could feel it was as hot as 52 degrees Celsius, or even more, when the actual temperature was lower, according to the Meteorological Department.
These conditions were forecast to extend into Thursday for Phuket.
Dr Amporn said that on Wednesday and Thursday the heat index would be at dangerous or orange levels in Bangkok and 34 provinces.
The provinces are Bueng Kan, Buri Ram, Chaiyaphum, Chanthaburi, Chon Buri, Chumphon, Kalasin, Krabi, Lampang, Lop Buri, Mae Hong Son, Maha Sarakham, Nakhon Ratchasima, Nakhon Sawan, Nakhon Si Thammarat, Nong Khai, Pattani, Phangnga, Phetchabun, Phetchaburi, Phichit, Prachin Buri, Prachuap Khiri Khan, Rayong, Roi Et, Sa Kaeo, Si Sa Ket, Sukhothai, Surat Thani, Surin, Tak, Trat, Ubon Ratchathani and Udon Thani.
When the heat index was in the dangerous or orange level, it felt like 42°C to 51.9°C. In that range, people could suffer cramp, heat exhaustion and heatstroke.
Dr Thiti Sawangtham, deputy director-general of the department of health, said that during very hot weather, people should drink plenty of water, not waiting until they feel thirsty, and refrain from alcohol and beverages with high sugar content, such as syrups and carbonated drinks. Clothing should ensure good ventilation.
People should seek medical aid if they develop red or hot skin, a fast pulse, headache, confusion, unstable consciousness, slow responses, speech problems or anxiety, or hallucinations, he said.
Treatment should prioritise applying cold water and ice cubes to parts of the body, especially the back of the neck and armpits, to quickly bring down the person's body temperature. They should then be rushed to hospital, Dr Thiti said.