The heat wave in eastern and southern areas of China over the past week is expected to continue for another 10 days.

The National Meteorological Center continued its heat alert on Wednesday, while authorities in Shanghai and Hubei province raised their heat alerts, as temperatures there were expected to exceed 40 C.

"The recent heat, caused by a subtropical high, is noted for its wide scope, high intensity and long duration," said Ma Xuekuan, the center's chief forecaster.

Shanghai municipality and provinces including Anhui, Hubei, Hunan, Jiangsu, Fujian and Zhejiang were forecast to see temperatures above 37 C in the coming days.

Temperatures in Shanghai's Xujiahui area reached 40.3 C, a record high this year and the third-highest in the city's history.

A heat alert map released by the center shows that nearly one-third of China would see temperatures above 35 C on Wednesday. The affected areas have a combined population of more than 700 million people.

The heat, which has lasted for days, has claimed the lives of at least four people in Hefei, Anhui province, 16 people in the neighboring Jiangsu province and one woman in Shanghai. Reports show that most victims were seniors and workers at construction sites.

Emergency medical centers in cities including Shanghai and Hefei said they have received more calls than ever from heat stroke patients in recent days.

"The death rate of heat stroke patients could reach above 30 percent," said Deng Xingqi, a doctor of the Fengxian District Central Hospital in Shanghai.

"If the temperature is forecast to reach above 40 C, all outdoor work should be suspended for the whole day, while on days with daily highs between 37 C and 40 C people should not work outdoors for longer than six hours a day," according to a notice released by the Hefei Commission of Urban-Rural Development.

Hefei authorities called on the city's public places to open their air-conditioned facilities to citizens.

Wang Zhenghua in Shanghai contributed to this story.

Contact the writer at zhulixin@chinadaily.com.cn