A high-efficiency particulate arrester (HEPA) filter refers to a refined filtration system designed to clean air. HEPA filters are usually found in vacuum cleaners and central air units, where they work to remove small particles from air, including spores, dust and pet dander. These filters usually have the ability to trap particles around 1 to 0.5 microns in diameter, which means that a true HEPA filter should be able to filter out 99.97 per cent of unwanted particles.
HEPA filters are made from tiny synthetic fibres that only let microscopic streams of air through. This makes the filters look like paper or cloth products. A real HEPA filter cannot be washed. You can wash another type of filter--such as an electrostatic version--using a slightly damp cloth to clean off the dust. HEPA filters, however, are much more advanced and are made from delicate materials. Using water on such a filter will compromise the fibres and make it less effective at filtering the air.
HEPA filters also work to trap dust in their fibres at such a small level that even if you could wash the filter properly without damaging it, the water would not do much good. Even a powerful stream of water would probably not wash away many of the particulates trapped in the filter and would probably do more harm than good.
Your only recourse with a dirty HEPA filter is to throw it away. HEPA filters, although expensive, are designed to be replaced after a certain time frame has passed. Brushing the filter, knocking out some of the dust or wiping it down may remove a few outside particles and slightly improve performance for a short period of time, but the interior particles will stay in place and block the airways, decreasing the overall efficiency of your central air system.
True HEPA filters, with their composition and high rate of filtration, should not be washed. There are less-efficient filters that are still useful, however, that may use the word "HEPA" in their description. You are able to wash them and retain their usefulness over an extended period of time.