BEIJING -- Beijing's air pollution emergency response office issued a yellow alert for smog Tuesday, forecasting the air pollution to persist until Wednesday night.

The yellow alert warned of an air quality index of more than 150 micrograms of harmful fine particulate matter per cubic meter of air for two consecutive days.

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

This round of smog is expected to hit Beijing, Tianjin, Hebei, Henan, Shandong, according to the Institute of Atmospheric Physics with the Chinese Academy of Sciences.