HEPA filters are high-efficiency filters that typically capture more than 99.5% of all particulate contamination. They are made of plastic (PP+PET) or fiberglass and can capture things like pollen, viruses, bacteria, mold and PM2. Air purifiers evolved in response to people's reactions to allergens such as pollen, animal dander, dust, and mold spores. Reactions (sneezing, runny nose, irritated eyes, and even more serious consequences, such as asthma attacks) are the result of antigens found in the home. These antigens are the main triggers of asthma, and there are more than 17 million asthmatics in the United States alone. Air purifiers remove some of these particles, reducing allergic-type responses.
Due to their extremely small size, allergens can pass through a standard vacuum bag and redistribute into the air, where they stay for days. Even a single microgram of cat allergens is enough to provoke an allergic response in most of the six to 10 million Americans who are allergic to cats. Other airborne particles, such as bacteria and viruses, can cause diseases, some of which are fatal. There are many reasons why millions of air purifiers are sold in the United States each year for allergies, asthma, and deadly diseases. HEPA filters are made of very fine glass wires with a diameter of less than 1 micron (one micron is 0.00004 in., 0.001 mm).
In comparison, a human hair has a diameter of approximately 75 microns (0.003 in, 0.07 mm). The fine glass strands are entangled and compressed to form a filter mat. Because the individual yarns are so microscopic, most of the carpet is composed of air. Mat openings are very small, generally less than 0.5 micron (0.00002 inch, 0.0005 mm). HEPA filters will collect particles up to 0.3 microns (0.0003 mm) in diameter.
Even if the filter is only 2.5 mm (0.10 inch) wide, it would consist of 2500 layers of glass strands. Electrostatic precipitators rely on electrostatic forces to remove particles from the air. They work by creating a cloud of free electrons through which dust particles are forced to pass. As dust particles pass through the plasma, they are charged, making it easy to collect. Electrostatic precipitators can collect particles up to 0.01 micron (0.00001 mm) in diameter. Air purity has been a concern ever since humans have lived in groups.
One of the reasons hunter-gatherers are nomads is that they periodically need to get away from their garbage cans and latrines. In A, D. In 1306, King Edward I of England banned the burning of coal in London because of the heavy pollutants left in the air. The industrial revolution of the 18th and 19th centuries only worsened the problem. The burning of coal to produce electricity and fuel trains produced a dark cloud of smoke over all the world's major industrial centers and covered entire cities with soot.
To address this problem, engineers built taller smoke stacks to keep debris in the air away from the source. Regardless of the height of the piles, downwind people complained that ash and acid gases from the combustion of coal (the source of acid rain) destroyed their crops. Air pollution worsened again after World War II, when cars became the main mode of transport in the industrialized world. Car smog has provided Los Angeles with the worst air quality in the world. HEPA filters are designed based on the size of the particles to be removed and the required air flow rate. The finer the pores of the HEPA material, the finer the particles removed from the air.
However, collecting finer particles means that the filter material will become clogged sooner and will need to be replaced more frequently. The designer will specify the diameter of the glass fibers and the density of the mesh of the filter cloth that sets the pore size of the filter. HEPA filters may contain binders that provide additional strength, but this also results in a filter that clogs up sooner. By-products of manufacturing include non-carbonaceous materials that are distilled from activated carbon production; specification filter material; and excess material that must be discarded during HEPA filter production. Most other manufacturing waste such as plastic channels from injection machines and excess sheet metal can be recycled. Additional debris occurs during operation of air filters; ions produced by electrostatic precipitators interact with oxygen in the air to produce ozone at high concentrations which is poisonous. Ozone levels produced in a domestic electrostatic precipitator are unlikely to reach dangerous levels but some people may be sensitive even to low ozone levels; collector plates should be cleaned periodically. HEPA filters have a limited lifespan depending on how much air is being filtered through them and how much particulate matter is present in it; most manufacturers recommend replacing them every few years. Used filters cannot be recycled so they end up in landfills; activated carbon can be recycled but handling small amounts found in household air purifiers would be too costly so it usually ends up in landfills after it is fully used. As scientists learn more about environmental pollutants and their impact on human health, providing cleaner air indoors will become increasingly important; current generation HEPA filters can only remove particles up to 0.3 microns (0.0003 mm) while it is believed that particles up to 0.1 microns (0.0001 mm) can cause mechanical damage to lung tissue; viruses can have a diameter as small as 0.02 microns (0.00002 mm).The new standard for filtration is ULPA (Ultra-Low Penetration Air) which requires removing particles up to 0.12 microns (0.00012 mm), approximately one-third what HEPA filters can remove; fiberglass, polyester felt or PTFE may be used for this purpose.
