High-efficiency particulate air (HEPA) filters are components used inside vacuums, air purifiers and other devices to help remove particles from the air. To be considered a HEPA filter, a filter must conform to specific standards ensuring it can remove a certain percentage of particles from the air. All certified HEPA filters will operate nearly identically, because they meet the same standard, making the best one the filter that correctly fits the appliance in which it is used and the one that is the most cost-effective. Some filters include extra features, such as a layer of activated charcoal or the ability to be washed and reused several times, although the effectiveness of these features is not regulated in the same way as the main HEPA filter components. There are some HEPA-like filters that are less expensive than a certified HEPA filter, although they usually do not meet the same stringent guidelines that a certified HEPA filter does and might not be able to capture very small particles.
To carry the name "HEPA filter", a filter is required to clear the air of 99.97 percent of all particles that are 0.3 microns or more. When choosing a HEPA filter, this information usually is stated on the packaging. Additionally, when a filter has been certified as HEPA-compliant, it is assigned a specific serial number that also may be printed on the packaging. Some filters claim to remove close to the same amount of particles but are not certified. These filters generally are referred to as HEPA-type filters and might be considerably less expensive than a certified version, although verifying the stated effectiveness of such a filter might be difficult.
Beyond certification, the next consideration for choosing the best HEPA filter is the size, shape and manufacturer of the filter. Some filters are boxes, some are cylinders, and others are thin panels. If one chooses a HEPA filter that is not manufactured by a company associated with the appliance's manufacturer, then checking the exact dimensions is very important. Two filters that both look like cylinders might have slight variations that make them different enough to be non-interchangeable.
A HEPA filter can have additional features beyond those specifically required. One common feature is a layer of activated charcoal that removes odors and some chemicals the HEPA layers do not. Another feature is the ability to wash the filter so it can be reused several times before being replaced, although the effectiveness of the filter might degrade with each washing. A few filters, especially those designed for some industrial environments, might have proprietary filtering materials that increase the percentage of particles that are removed from the air, although these types of filters might not always work properly in machines not designed for the extra air resistance they create.