|A man smokes at Zhuzhou Railway Station in Zhuzhou city, central China's Hunan Province, Dec. 10, 2013. (Xinhua/Bai Yu)|
Particulate matters or PM, is the term for particles found in the air, including dust, dirt, soot, smoke, and liquid droplets. PM2.5 particles are air pollutants with a diameter of 2.5 micrometers or less, small enough to invade even the smallest airways.
Particles less than 10 micrometers in diameter (PM10) pose a health concern because they can be inhaled into and accumulate in the respiratory system. Particles less than 2.5 micrometers in diameter (PM2.5) are referred to as "fine" particles and are believed to pose the greatest health risks. Because of their small size (approximately 1/30th the average width of a human hair), fine particles can lodge deeply into the lungs.
Studies show that in addition to natural sources, these particles mainly come from human factors including vehicle exhaust, industrial emissions, construction and road dust, waste incineration, fireworks, cooking and barbeques, and smoking.
At present, 20 cities in China have started the source apportionment work of PM 2.5. Before the end of June 2014, earliest participants in the research including Beijing, Tianjin and Hebei province will get the preliminary analytical results. By the end of 2014, the three major regions (Beijing, Tianjin and Hebei province; Yangtze River Delta; Pearl River Delta) will complete the source apportionment work of PM 2.5, and release the results to the public.