Systemair CarboAir Pre Filter protects the CarboAir Carbon Filter from dust, dirt and airborne particles. Preventing the carbon filter from becoming blocked with particles and prolonging it's life. The premium Pre-filter socks need to changed when they look dirty. They are ultra-efficient at removing dust particles and being thicker but less dense than some pre-filters mean less restriction of air flow. The life of the pre-filter depends on the environment, they can last months or weeks depending on the volume of particles they are removing from the air.