Safety inserts of the primary air filter are often not replaced to save some extra money. This can be a rather costly saving. Safety inserts are a second stage filter with less filtration efficiency than the primary air filter. Germany filtration specialist Hengst has some valuable tips for anyone handling airfilters:
The safety insert is placed inside your primary and functions simply as a back up to protect your engine…
• …during the replacement process of your primary filter. The engine remains covered by the safety insert.
• …if your primary filter is damaged and has a bypass. Remember, damages of air filters are easy to happen during the cleaning process (not recommended) for a second use.
• …if the primary air filter is cleaned whereby dust particles likely settle on the inside (clean side) of the filter and would be sucked into the engine immediately after start.
• …from possible bypasses of the primary due to installation faults. Important to know is that the safety insert is not a spare filter, it is simply second life-line for your engine, if the primary air filter fails to perform to get you to the next service station.

De-dusting filters cause particles to resettle on the clean side which will be sucked into the engine immediately at first start. Safety Inserts can assist in protecting your engine.
Pictured left: E1013L Air filter, Right: E1013LS safety insert. “L” stands for “Luft” which is the German word for “Air’. “S” stands for “Sicherheitseinsatz” which is “safety insert” in English. Products shown are Hengst filter and filter insert.