Filters are systems or elements used to remove substances such as dust or dirt, or electronic signals, etc., as they pass through filtering media or devices. Filters are available for filtering air or gases, fluids, as well as electrical and optical phenomena. Air filters are used for cleaning the air. Electronic filters clean electrical signals of unwanted frequencies in the audio and intermediate ranges while RF and microwave filters have the same function but in the radio and microwave frequency range. EMI and RFI filters minimize or eliminate electromagnetic interferences. Fluid filters remove debris from fluids. Fuel filters remove contaminants from fuels, while hydraulic filters remove contaminants from hydraulic fluids. Water filters clean water, not only for drinking and swimming but in water treatment plants as well. Optical filters pass light and filter light wavelengths.