sentences:  In the first batch of 74 cities that implemented the 2012 Environmental Air Quality Standards, the average concentration of PM2.5 and sulfur dioxide dropped by 42 percent and 68 percent, respectively, between 2013 and 2018.Zhong Nanshan, the president of the China Medical Association, warned in 2012 that air pollution could become China's biggest health threat.Measurements by Beijing municipal government in January 2013 showed that highest recorded level of PM2.5 (particulate matter smaller than 2.5 micrometers in size), was at nearly 1,000 μg per cubic meter.