We bought the air purifier last year (and loved it), so we were due for a filter change; however, when I changed out the old filter to put in the new one, it's a DIFFERENT SIZE AND DOES NOT FIT!!! The new filter is slightly smaller and slightly thicker, so that the outer filters will no longer fit together. Perhaps there is another purifier that this filter fits, but it does NOT fit the XJ-3800.We will be returning this immediately and expect Amazon's great customer service to provide a full refund, including shipping, because either it is not the product it claims to be, or Amazon sent us the wrong product.