Shanghai may have experienced its longest period of June pollution over the past few days since it launched its air quality index in 1997.

The city experienced more than 60 hours of pollution, caused by PM2.5 particles. According to the Shanghai Environmental Monitoring Center’s PM2.5 measurements, fine particles exceeded the national standard and even reached a peak of three times the standard on Friday night.

via Heat and pollution make it a weekend to forget – China.org.cn.

What does PM stand for?