Why can't the filter washed with water?

Please replace the filter instead of washing it

Large particles adhering to the filter can be brushed off up to a certain extent.
However, the dirt adhering to the activated carbon, which is wrapped in the filter, cannot be washed off with water.
Please replace the filter instead of washing it when it turns white with dirt including hardened flux.

If a filter package is left open, the activated carbon absorbs air-borne dust, and it will not absorb solder fume any more even if the filter has not yet been used.