Air Quality Index - What in the world is that?

摘要

The photos are startling: large cities like Beijing and New Delhi enveloped in a gray haze; people wearing face masks in an attempt to protect their lungs; empty playgrounds on days when children must be kept inside because pollution levels are so high. The smog red alerts in mainland China have forced factories, schools, and roadways to shut down. These red alerts indicate that the air is "hazardous," but what exactly does that mean? How are these levels calculated and how do pollution levels around the world compare with levels in the United States? The answer is a bit more complicated than comparing the red alert level of one country to another.