Periodic replacement of the air filter is recommended for optimum performance of the ionizer. Compress outer sleeving away from the filter housing. Examine the filter for any evidence of contamination. The filter will turn red if there is any oil contamination. If there has been moisture build-up, there will be a change in air volume or a brownish color to the filter. If either of these conditions exist, you should replace the filter by unsnapping connectors. Depress air line connectors allowing removal of filter. The filter replacement is sold as Desco Item #60355 (two filters to a package). Be sure to observe the flow direction on filter when installing a replacement.