Data from Weather Online, a U.K.-based meteorological services firm, shows the index climbing to a 'very high' level of 10 on Thursday. It is forecast to rise further to around 12 for several days from Friday, an extremely dangerous level that can cause eye damage, overheating and dehydration, especially to children and babies.

A UV Index of between 0 and 3 is considered ‘low’, and above 11 is deemed ‘extreme,’ with radiation that could burn skin and damage eyes within 30 minutes. Such high UV levels were also reported in Saigon in mid-February and late March.

The level never exceeds 8 in the U.K. In fact it only rises to 7 in the two weeks around the summer solstice.

Trinh Ngo Binh, a HCMC dermatologist, warned that such high UV levels could accelerate skin aging and pose more severe threats like skin cancer.

He advised people to avoid direct exposure to sunlight between 10 a.m. and 3 p.m. and to wear sunscreens, sunglasses and brimmed hats if they have to go out under the sun.

Le Thai Van Thanh, a lecturer at the HCMC University of Medicine and Pharmacy, said exposure to ultraviolet rays is a leading cause of skin-related diseases, especially hyperpigmentation and skin aging.

Since children always engage in outdoor activities they need special protection especially because their skin is immature and vulnerable and the risk of skin diseases is many times higher than for adults, she said.

Major hospitals in the city each has been admitting hundreds of children every day who suffered from respiratory and digestive problems related to the scorching heat.

The city has been hit by a severe heat wave, with temperatures soaring to 34-37 degrees Celsius. But the RealFeel temperature, which indicates how hot it feels outdoors, was around 40 degrees Celsius, or 104 degrees Fahrenheit. The dry season is expected to last at least until mid May.

A study published in the journal Climatic Change last September said Southeast Asian countries like the Philippines and Vietnam would be most affected by heat-related mortality along with countries in Southern Europe and South America.