When film drying cabinets are in frequent use, Arkay recommends checking the air filter every 30 days. A clogged air filter will restrict air flow and reduce efficiency, resulting in longer drying times. The easiest way to determine if the air filter needs replacement is to remove the old filter and compare it to a new one. When there are obvious signs of discoloration, it's time to replace the filter. Keep a supply of replacement filters on hand with this convenient package of 4 filters, sized specifically for Arkay CD-40, CD-20, and CD-10 drying cabinets. Arkay part #602542. The larger size filters for CD-80 cabinets are also available - please see the separate listing.