PM is the acronym for particulate matter in English. The precise definition of PM2.5 is "the general name of solid particles or droplets with aerodynamic diameter less than or equal to 2.5 microns", also known as fine particles or particles entering the lungs, whose diameter is less than 1/20 of the thickness of human hair. However, compared with PM 10 and other atmospheric coarse particles, PM2.5 has a greater impact on human health and atmospheric environmental quality. Due to its small size and light weight, PM2.5 can stay in the air for a long time and be taken to far places through atmospheric circulation. The specific surface area of PM2.5 (the total surface area per unit mass of substances, usually the larger the value, the stronger the adsorption capacity) is larger than that of PM 100 and PM 10, and it can adsorb more bacteria, viruses and various pollutants harmful to human health. PM2.5 in the air can pass through the respiratory tract, enter the alveoli, accumulate in the alveoli, interfere with the gas exchange in the lungs, and cause various diseases. Therefore, PM2.5 is particularly harmful to health. According to the data of the World Health Organization, even in EU countries, the average life expectancy has been reduced by 8.6 months due to exposure to PM2.5 produced by human activities. At the same time, more than 2 million people worldwide die every year from inhaling fine particles from indoor and outdoor air pollution. 1982, the United States began a prospective study on PM2.5 At that time, it recruited1200,000 participants, followed by 16 years of follow-up. The paper was published in the Journal of the American Medical Association in 2002. This paper describes in detail the relationship between the increase of mortality and PM2.5-"When the concentration of PM2.5 in the air is higher than 10 μ g/m3 for a long time, the risk of death begins to rise. Every time the concentration increases 10 μ g/m3, the total death risk increases by 4%, the death risk of heart and lung diseases increases by 6%, and the death risk of lung cancer increases by 8%. " In 2003, the US Environmental Protection Agency made an estimate: "If PM2.5 reaches the standard, tens of thousands of people will die early every year, tens of thousands will go to hospitals, millions will miss work, and millions of children will get respiratory diseases." Therefore, in order to protect the environment and protect the environment, the cat has a medical mask imported from the United States, which has good filterability, not only preventing particles, but also preventing droplets. The filtration efficiency is 99.8% for particles as small as 0. 1 micron, and 99.9% for bacteria or virus carriers as small as 0.3 micron (common in droplets).