A woman wearing a mask walks at the Bund along the Huangpu River on a hazy day in Shanghai
A woman wearing a mask walks at the Bund along the Huangpu River on a hazy day in Shanghai May 3, 2011. As a strong sandstorm hits parts of China's northwest, Shanghai experienced its worst air quality in more than four years. The sandstorm significantly increased the level of respirable particles in the air and pushed Shanghai's air pollution level to "severe", according to the Shanghai Environmental Monitoring Center (SEMC). REUTERS/Carlos Barria (CHINA - Tags: ENVIRONMENT)