Medical experts in Shanghai have warned people, especially the elderly, to stay out of the sun and remain cool, as high temperatures continue to claim lives.

At least 11 people have died of heat stroke in Shanghai so far this summer. The city's temperatures have been 39 C or above in recent days and the local meteorological department said this July is the hottest in decades.

A 59-year-old man surnamed Xia is in critical condition at a hospital in Nanjing, Jiangsu province, on Wednesday after suffering heat stroke. Nanjing is one of many cities in China that is experiencing sweltering heat and seeing a rising number of patients with heat stroke. Qin Huai / for China Daily

Leng Guangming, an official from the Shanghai Center for Disease Control and Prevention told China Daily on Wednesday: "In addition to those exposed to the sun, many people, especially the elderly, have gotten sick indoors because they don't have air-conditioning. It's important for people to pay more attention to heat stroke prevention."

Statistics from the center show that more than 10 people aged 20 to 60 have died of heat stroke in Shanghai this summer, more than 60 percent of them spending long periods outside in physically demanding activities.

Unlike other heat-related conditions, heat stroke is a medical emergency that is often fatal if not promptly and properly treated. It is a form of severe hyperthermia, an abnormally elevated body temperature, usually 40 C or higher, with accompanying physical symptoms, including changes in the functioning of the nervous system.

"In extremely hot and very humid weather, the body may not be able to dissipate the heat and evaporate sweat. Consequently, the body temperature rises," Leng said.

The latest victim was a 64-year-old merchant seaman from Taiwan who died on Wednesday morning, having arrived in Shanghai on July 13. He began to show symptoms of severe heat stroke on July 23 and fainted. He was taken to Shanghai Longhua Hospital, where he received emergency treatment, but died of organ failure several days later.