ZHENGZHOU -- China's Henan Province issued a red alert for heavy air pollution Friday, 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, surpasses 300 for two days or the daily AQI reaches 500.

As of 4 pm on Friday, AQI surpassed 300 in seven cities including Zhengzhou, Kaifen, Xuchang and Anyang.

Local authorities have activated the highest level-I emergency response since Friday night in a total of 12 cities in the province, according to the provincial air pollution prevention and treatment office.

Under China's four-tier warning system, red is the most severe, followed by orange, yellow and blue.