Here is a brief explanation of a HEPA filter and how it works.
The HEPA filter is a high efficiency micro-fiber filter in a rigid metal frame. As air passes through the HEPA filter, small particles (such as smoke, dust, etc.), attach to the filter media where they are trapped. As it collects more and more particles, the HEPA filter actually increases in efficiency. However, as its efficiency increases, a reduction in air velocity occurs. In other words, as the HEPA filter continues to load, less air will be able to pass through the filter. The effectiveness of the system lessens, which is an indicator that it may be time to replace the HEPA filter. Proper maintenance of the HEPA filter is absolutely essential to the functionality of our air filtration systems as well as a vital part in continuing to provide a clean, healthy, pleasant indoor air environment.