Beijing pollution

Authorities in Beijing, China, issued a 5-day air-quality red alert on Dec. 16 due to air pollution levels. At the time of the alert, the air quality index (AQI) of Beijing measured 500, the highest AQI level in 2016. 

The AQI measures the density in the air, per cubic meter, of particles sized less than 2.5 microns, known as PM2.5. The World Health Organization considers any AQI measurement over 25 to be unsafe. 

Beijing took steps to reduce traffic and factory production and to stop construction during the red alert period.

At least 21 other cities in China issued red alerts in December. Many rely on coal-burning power plants, which greatly increase air pollution levels.