PM2.5 refers to particulate matter suspended in the air with a diameter finer than 1/30 of a hairline (less than or equal to 2.5 micrometers), known as fine particulate matter (FPM), also known as respirable pulmonary particulate matter (RPPM), which is a fine suspended particulate matter composed of secondary ions generated by combustion emissions as well as a variety of pollutants in the air through physicochemical interactions.The main sources of PM2.5 are coal combustion, motor vehicle exhaust emissions, dust, biomass The main sources of PM2.5 are coal combustion, motor vehicle emissions, dust, biomass burning, etc. Visibility and health advice: visibility in the 5-10 km, without special protection; in the 3-5 km should be appropriate to reduce outdoor activities; 2-3 km to reduce outdoor activities, stop the morning exercise, go out wearing a mask, respiratory disease patients try to minimize the outing; in less than 2 kilometers, it should be avoided outdoor activities, go out wearing a mask, respiratory disease patients try to avoid going out. PM2.5 can penetrate the nasal cilia and other defensive structures of the human respiratory system, deep into the respiratory tract until the lungs. PM2.5 surface will be adsorbed a large number of pollutants, such as sulfate and organic gas solution, such as carcinogens polycyclic aromatic hydrocarbons, etc., which is one of the reasons why the fine particulate matter is harmful to health. Susceptible people: PM2.5 is harmful to human respiratory, blood, cardiovascular and reproductive systems. It is more dangerous to the following groups: people who have previously suffered from respiratory and cardiovascular system diseases, the elderly, children, infants and newborns who are not in good health. Respiratory system: PM2.5 is easy to be deposited in the alveolar region, integrated into the blood, and act on the whole body; the insoluble part is deposited in the lungs, inducing or aggravating inflammation. China is the hardest hit by PM2.5 pollution globally, which is directly related to China’s over-reliance on coal for energy consumption. Close to 70% of China’s primary energy supply comes from burning coal, and coal consumption is close to half of the world’s. Preventive Measures: 1. Reduce Exercise Reduce outdoor physical activity on polluted days and at peak times of the day, thereby reducing the amount of pollutants inhaled. Air conditioners and filters Use air conditioners in the “internal circulation” position to minimize the entry of outdoor particles. 3, respiratory protection devices – masks N95 masks can be bought in large drugstores, N95 masks can be used many times, not washable. If the mask is damaged, contaminated or you feel resistance after wearing it, you should replace it. 4, the correct wear N95 masks should ensure that the fit face, completely cover the mouth and nose. Keep the mask sealed. After wearing the mask should be awakened exhalation and inhalation test to confirm that there is no air leakage. A band of paper masks and ordinary surgical masks have no protection against PM2.5.