Not all filters are created equal. Peak performance is only realized when genuine Vornado True HEPA Filters are used. Over time, your HEPA filter begins to clog, reducing the amount of air that is filtered. To maintain true Whole Room Air Purification, it is imperative to change your filter every year using only genuine Vornado True HEPA filters. Others may claim to be compatible with Vornado units, but the Vornado Whole Room Air Purifier is only tested to achieve a 99.97% capture rate using a genuine Vornado True HEPA filter.