At 8 p.m. on January 12, 2013, the American device registered 886 micrograms per cubic meter (μg/m3). To people unfamiliar with air pollution monitoring that is just a number, moreover one given in units not commonly used in the United States. Precisely for that reason, the U.S. Environmental Protection Agency developed an air quality index (AQI) that converts those measures into readily comprehensible numbers.
An AQI less than 50 means good air quality; 51–100 indicates moderately polluted air; 101–150 is considered unhealthy for adults with lung diseases; at “generally unhealthy” 151–200, everybody should reduce any prolonged outdoor exercise; at “very unhealthy” 201–300, adults with heart and lung diseases, the elderly, and children should avoid any outdoor physical activity; and when the index goes beyond 300 and all the way to 500, the best choice is to stay inside.
The peak concentration of 886 μg/m3 reached on January 12 translates to an AQI of 755, far beyond the defined scale. Beijing’s AQI rose above 500 by 2 a.m. on January 13 and remained above that level until 6 a.m. the next day.