At present, there are several kinds of air purifiers in domestic and foreign markets: 1. electrostatic dust collector; 2. electronic dust collector; 3. ozone type; 4. negative ion type; 5. filter element type; 6. activated carbon adsorption type; 7. ultraviolet sterilization type; 8. comprehensive type. 1. Electrostatic dust collection type: through plastic fibers or fine metal mesh with static charge, to absorb particulate matter polluted by filter mesh, to achieve the purpose of improving particulate matter polluted in indoor air. But the filter must be cleaned regularly, otherwise the purification efficiency will decrease with the increase of using time. 2. Electronic dust collector: The polluted particulate matter is charged by high pressure. When the polluted particulate matter is charged through the collecting plate with positive and negative electrodes crossing, it is adsorbed to the collecting plate, so as to improve the polluted particulate matter in indoor air. However, the collection board needs to be cleaned regularly, otherwise its purification efficiency will decrease with the increase of using time. 3. Ozone type: ozone, chemical formula O 3, for the normal existence of the atmosphere, ozone in nature has played a role in protecting the atmosphere, ozone has a strong bactericidal effect, often used in hospital disinfection of certain specific environment. Ozone air purifier uses the principle of electronic discharge to produce appropriate ozone, and releases ozone into the surrounding air to achieve the purpose of sterilization and disinfection. However, because the concentration of ozone release is difficult to control, excessive ozone in the air will damage human alveolar tissue and endanger human health (the concentration in the internationally permitted air is 0.05 micromole/mole). Therefore, the United States Environmental Protection Agency (EPA) does not recommend the use of ozone air purifiers. 4. Negative ions: There are a lot of negative ions produced by nature in forests, waterfalls, lakes and seashores. A large number of negative ions will absorb dust and polluted particles in the air, and aggregate into larger particles, which will settle to the surface, thus refreshing the natural environment air. The negative ion air purifier uses the negative ions produced by high voltage discharge to release into indoor air, absorbs the dust and polluted particles in the air, gathers into larger particles, and then settles on the surface of the ground and objects, so as to achieve the purpose of refreshing indoor air. Negative ion air purifier, the release of negative ions must reach a certain concentration, must be equipped with dust collector, in order to really play the role of purifying indoor air, otherwise, large particles of pollutants deposited on the ground and surface of objects will also bring secondary pollution. It is necessary to add that ozone is also produced while negative ions are produced by high voltage discharge. 5. Filter type: The filter type air purifier uses forced indoor air through various filters to filter and absorb polluted particles and harmful microorganisms in the air, so as to achieve the purpose of purifying indoor air. At present, the filter element air purifiers at home and abroad generally use the following kinds of filter element materials: (1) ordinary filter element: mainly made of plastic fibers, fine mesh metal mesh and non-woven materials, which can filter large dust and polluted particles in indoor air. HEPA filter element (high efficiency air particulate filter element): is currently recognized as a high standard filter material by the medical profession, laboratory and other professional environment. The diameter of the filter hole of the HEPA filter material with high medical precision can be as small as 0.3 micron. The three-dimensional filter element made of folded HEPA filter material increases the filtering area. It not only effectively filters and absorbs fine particles and bacteria in the air, but also prolongs the service life. (3) Other special filter elements, such as medical anti-microbial filter materials, which can effectively inhibit microbial reproduction and regeneration. 6. Activated carbon adsorption type: Activated carbon is a kind of black microcrystalline carbon material with well-developed internal micro-pore structure (1 gram of activated carbon has a micro-pore expansion area of nearly 300-1000 square meters). Activated carbon is an excellent adsorbent, catalyst and catalyst carrier. It is widely used in almost all fields such as modern industry, science and technology, medical treatment, military affairs and daily life. Activated carbon can be divided into coal activated carbon, lignin activated carbon, organic activated carbon, regenerated activated carbon, fruit shell activated carbon and coconut shell activated carbon. Among them, activated carbon from mesoporous coal is often used to purify air and make industrial gas masks for workers in toxic gas environment. After adding appropriate amount of natural zeolite or potassium iodide, activated carbon can increase the range of adsorbing toxic gases in air. Activated carbon specially treated by adjusting pore size is also used as biochemical weapon. Military gas mask, filter sarin gas and anthrax bacillus, etc. Air purifier is the principle of using activated carbon to adsorb poisonous gases in the air with high adsorption capacity (1 gram of activated carbon for air purification has a microporous area of nearly 1000 square meters, which can absorb 60% of the chemical poisonous gases and odors of its own weight). By forcing indoor air through the active carbon filter layer inside the purifier, it can adsorb poisonous gases and odors effectively. To achieve the purpose of purifying indoor toxic gases. 7. Ultraviolet sterilizing type: Ultraviolet sterilizing type air purifying disinfector uses the same way of forcing the flow of indoor air, so that air passes through the isolation container with ultraviolet disinfection lamp which does not directly irradiate human body, so as to kill all kinds of bacteria, viruses and fungi in indoor air. Ultraviolet radiation is divided into A wave, B wave, C wave and vacuum ultraviolet radiation. Ultraviolet radiation used in disinfection and sterilization should be in the C band. Its wavelength range is 200-275 nm (nanometer), and the strongest band is 250-275 nm (nanometer). Used to kill bacteria