ZHENGZHOU, Dec. 21 (Xinhua) -- Zhengzhou, capital of central China's Henan Province, issued a red alert for heavy air pollution late Thursday night, forecasting the pollution to persist for several days.
The red alert is issued when the air quality index (AQI) will surpass 200 for four consecutive days, or surpass 300 for two days, or the daily AQI to reach 500, in the city.
Local authorities have activated the highest level-I emergency response since Thursday night. Industrial enterprises were ordered to halt or reduce production, and all construction projects and major outdoor events were suspended.
As of Thursday night, a total of 18 cities in Henan had issued alerts for heavy air pollution, according to the provincial department of ecology and environment.
Under China's four-tier warning system, red is the most severe, followed by orange, yellow and blue.
