Data from Weather Online, a U.K.-based meteorological services firm, indicated that places like Ho Chi Minh City and Da Nang would have, from Wednesday to Friday, levels exceeding 12, which are capable of causing eye damage, overheating and dehydration, especially to children and babies.

Such high UV levels were also reported in HCMC in mid-February and late March.

Besides, popular tourist destinations such as Da Lat in the Central Highlands province of Lam Dong, Sa Pa in the northern highlands province of Lao Cai and Phu Quoc Island in the southern province of Kien Giang also recorded 12 last Thursday and Friday, posing health risks to holiday-goers engaging in outdoor activities. The three spots are usually thought of as cool summer escapes.

The UV Index is an international standard measurement of the strength of ultraviolet radiation from the sun. Between 0 and 3 is considered low and above 11 is deemed extreme with radiation that could burn skin and damage eyes within 20-30 minutes.

"The current baking weather has resulted in the increasing number of people with cerebrovascular accident," a health expert said.

Older people should avoid going out and limit outdoor activities during peak hours, doctors said.

Dr Nguyen Van Chi of Hanoi’s Bach Mai Hospital said seniors and children are most vulnerable to heat strokes.

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 engage a great deal 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 southern city have been admitting hundreds of children every day with respiratory and digestive problems related to the scorching heat.

Huong Khe District in the central province of Ha Tinh reported a national record high temperature of 43.4 degrees Celsius (110 degrees Fahrenheit) last Saturday.

Many parts of the country have been bracing for what could be the hottest summer in history. The weather office has warned of more heat waves in the coming months.

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.