Protect your breath: Decoding the science and practice of air purification​​

At a time when haze frequently invades cities and indoor decoration pollution hides hidden dangers, fresh air has become a scarce resource. Air pollution is like an invisible health killer, threatening the quality of human life at all times. How to make air purification technology a "shield" to protect breathing? What scientific principles and innovative applications are behind this?
I. Air pollution: double warning from source to harm

(I) Invisible invasion of multiple pollution sources
Outdoor pollution is mainly caused by industrial emissions, traffic exhaust, and construction dust: sulfur dioxide (SO₂) and nitrogen oxides (NOx) emitted by thermal power plants form acid rain, and PM2.5 and volatile organic compounds (VOCs) in automobile exhaust directly penetrate into the alveoli; indoor pollution comes from formaldehyde and benzene released by decoration materials, polycyclic aromatic hydrocarbons in kitchen fumes, and ozone (O₃) released by printers. These pollutants enter the human body through the respiratory tract and become an important cause of lung cancer, asthma, and cardiovascular diseases. According to data from the World Health Organization, about 7 million people die prematurely from air pollution every year.
(II) Invisible health threats
PM2.5 can penetrate the respiratory mucosa and enter the blood because its particle size is less than 2.5 microns, carrying heavy metals and carcinogens; formaldehyde is a type of carcinogen, and long-term low-concentration exposure can cause nasopharyngeal cancer and leukemia; ozone causes irreversible lung damage by destroying the lipid membrane of lung cells. Air pollution can also accelerate skin aging and reduce immunity, and is particularly harmful to pregnant women and children.
II. Three-dimensional defense line of air purification: technical principles and practice
(I) Physical purification: "catcher" of particulate pollutants
The filtration technology HEPA (high-efficiency air filter) uses the van der Waals force, inertial collision and diffusion effect between fibers to capture particles larger than 0.3 microns, and the PM2.5 removal rate can reach 99.97%. It is commonly used in air purifiers and fresh air systems, and is the first line of defense against haze and dust.
Electrostatic dust removal charges particulate matter through high-voltage electrodes, and then uses electric field force to adsorb it on the dust collecting plate. This technology has low resistance and low energy consumption, and is suitable for handling high-concentration dust, but the electrodes need to be cleaned regularly to avoid secondary pollution.
(II) Chemical purification: "decomposer" of harmful gases
Adsorption and catalytic oxidation Activated carbon relies on porous structure to adsorb VOCs such as formaldehyde and benzene, but it has the defect of easy desorption after saturation. Upgraded technologies such as modified activated carbon loaded with manganese dioxide (MnO₂) can further oxidize the adsorbed pollutants into carbon dioxide and water through catalytic reactions. As a transition metal oxide, manganese dioxide has abundant active sites on its surface that can reduce the activation energy of the reaction and efficiently catalyze the decomposition of formaldehyde at room temperature: first, it adsorbs oxygen in the air to generate active oxygen species (such as ・O₂⁻), then undergoes redox reaction with formaldehyde molecules, and finally converts them into harmless CO₂ and H₂O. This "adsorption + catalysis" synergistic effect solves the secondary pollution problem of traditional adsorption materials, and is particularly suitable for the long-term treatment of low-concentration harmful gases in indoor environments.
Photocatalytic technology Titanium dioxide (TiO₂) generates electron-hole pairs under ultraviolet light, which excite hydroxyl radicals (・OH) to oxidize and decompose pollutants. However, due to its reliance on ultraviolet light sources and low quantum efficiency, its practical application is limited. The heterojunction material formed by the composite of manganese dioxide and TiO₂ can expand the light response range to the visible light region, improve the catalytic efficiency, and become a research hotspot for the next generation of photocatalytic materials.
(III) Biological purification: the synergy of nature and technology
Green plants such as green radish and ivy absorb pollutants through the stomata of their leaves, and root microorganisms further degrade harmful substances. Although the purification efficiency is limited, they have both the effect of beautifying the environment and psychological regulation. Biological enzyme preparations simulate the natural metabolic process, and the formaldehyde dehydrogenase released can specifically decompose pollutants and are often used for furniture deodorization.
III. Manganese dioxide: the "catalytic pioneer" of air purification
In the field of chemical purification, manganese dioxide has become a star material with its unique redox properties. In addition to the formaldehyde catalytic decomposition mentioned above, it can also efficiently treat nitrogen oxides (NOx): under low temperature conditions, the lattice oxygen on the surface of manganese dioxide reacts with NO to generate NO₂, which is further converted into nitrates through adsorbed oxygen, and finally generates nitrogen through reduction reaction. This low-temperature catalytic property makes it suitable for automobile exhaust treatment devices and industrial waste gas purification equipment, making up for the high cost and easy poisoning of traditional precious metal catalysts (such as platinum and palladium). In addition, manganese dioxide nanoparticles can be loaded on mask filter materials to catalyze the decomposition of ozone (O₃) in the air, reducing it to oxygen and protecting the wearer from oxidative damage.
Fourth, build a comprehensive purification system
Outdoor pollution control needs to be controlled from the source: promote clean energy to replace coal, optimize motor vehicle emission standards, and improve construction dust prevention technology; indoors, environmentally friendly decoration materials should be given priority, ventilation should be maintained, and efficient purification equipment should be used. It is worth noting that the fresh air system achieves "external prevention and internal treatment" by introducing outdoor fresh air and filtering and purifying it, becoming the preferred solution for confined spaces such as office buildings and schools.
From the London smog incident during the Industrial Revolution to today's global air quality management, human beings' pursuit of clean air has driven technological innovation. The application of materials such as manganese dioxide has enabled air purification to move from "passive protection" to "active degradation". When technology and nature work together, and when individual protection is combined with source management, every breath can truly become a gift of life, rather than a health hazard.