The reason for this is the following：
HEPA filter is made of polypropylene or other composite materials and belongs to paper. Washing with water will destroy the filter structure. The purification principle of HEPA filter screen is not simply to filter out particles larger than the mesh like a sieve to purify the air, but to form adsorption through van der Waals force between fine particles and the filter screen.
Scrubbing with water also destroys the electrostatic adsorption capacity of the filter screen. Fine particles can easily pass through the filter screen that has lost its adsorption capacity, and the air purifier becomes an electric fan. Although the surface of the filter screen looks much cleaner after brushing, it also loses its purification ability.
Therefore, the HEPA filter really cannot be cleaned, and cleaning will damage the HEPA structure. The measured wind speed increases, and the filtration efficiency is greatly reduced. Cleaning the filter will seriously damage the structure of HEPA filter, resulting in incomplete filtration.
If you feel that the HEPA filter at home has become dirty after being used for a long time, you'd better replace it with a new one, and try to choose high-quality HEPA filter when replacing it.
Extended data:
The other part of HEPA filter is made of PET or PTFE, which is also the origin of many HEPA filters claimed to be washable online, but the purification efficiency of HEPA filter made of PET is very low. If you still care about the use cost of the filter air purifier, you can also consider the electrostatic precipitator air purifier.