3-Layer Filter This Filter Replacement Kit is made of 2 Pre-filter, 1 Middle filter, 1 Main filter, which can effectively adsorb fine particles Pre Filter Captures visible smoke and dust. Our pre-filter is 25mm thick and has 2.5 times longer lifespan than others on the market Middle Filter Effectively absorbs and filters harmful particles down to 0.3µm with 99.97% purification rate Middle filter: Effectively absorbs and filters harmful particles down to 0.3µm with 99.97% purification rate Note: There are three products in each package