The best way to know when to change a HEPA filter is to use a PM2.5 air quality monitor, also called a particle counter. An air quality monitor can be used to ensure that the air in your home is at safe levels. The recommended time to replace the HEPA filter is between 3 and 6 months, which represents a very large average. If you see a visible accumulation of dust or particles on a HEPA filter (it may look gray or dirty) or you see reduced airflow through your portable air purifier, you can assume that this is an indication that the HEPA filter must be cleaned.
However, experts say that HEPA filters should always be replaced rather than cleaned. HEPA filters can also be found in your home's air treatment system, and some vacuums have them too. In reality, a filter must be changed when its ability to filter a high degree of particulate matter is reduced. Filters that work with high efficiency, resulting in a higher percentage of contaminants they remove from the air, will need to be replaced more often than an inefficient filter that allows more particles to circulate through your home's ventilation grilles or commercial facility.
Washable. These types of filters are made of a metallic material or fabric that can be washed, which means that they don't need to be changed as often as other types of filters. HEPA filters are very good at capturing particles of all sizes and, in theory, remove at least 99.97 percent of particles with a size of 0.3 microns. Most people take too much care to carry out this simple inspection, but the important thing to remember is that not only do you have to look for dust and dirt, but you also have to make sure that the filter is not wet, bent or damaged in any way, as this will affect the efficiency of the air filter.
Increase in allergies and respiratory problems If you notice a sudden increase in allergies or respiratory illnesses among your household residents or facility workers, this is usually a strong sign that air filters are clogged and cannot filter out particles that aggravate these conditions. A HEPA filter, or high-efficiency particulate air filter, is a type of pleated mechanical air filter that, in theory, can remove at least 99.97% of dust, pollen, mold, bacteria and any airborne particles with a size of 0.3 microns (µm). There are many factors that determine the lifespan of a HEPA filter (location, speed of use, time of use, etc.). If you're still using a filter that's only 50% efficient, that means it will take about twice as long to purify the air in the same room compared to a filter with 99% efficiency. The filter efficiency, airflow, and overall filtration performance of an air purifier decrease significantly when the filter medium becomes clogged due to prolonged use.
I found it difficult to find data-backed explanations for the HEPA filter replacement recommendations from many of the big brand-name air purifier companies. Most manufacturers also offer a recommended replacement interval (which is usually about six to 12 months), and some portable air purifiers have an indicator light that tells you when to replace the HEPA filter.