Pedestrians wearing masks walk on a street in Beijing, capital of China, Oct. 18, 2016. A yellow alert for air pollution was issued on Tuesday in Beijing. (Xinhua/Ju Huanzong)

Photo taken on Oct. 18, 2016 shows the East Third Ring Road area shrouded in smog in Beijing, capital of China. A yellow alert for air pollution was issued on Tuesday in Beijing. (Xinhua/Ju Huanzong)

A pedestrian wearing mask walks on a street in Beijing, capital of China, Oct. 18, 2016. A yellow alert for air pollution was issued on Tuesday in Beijing. (Xinhua/Ju Huanzong)