Do I Use Hepa Filter For Meningitis How Can I Tell When The HEPA Filter In My Air Purifier Needs To Be Changed?

How can I tell when the HEPA filter in my air purifier needs to be changed? - do i use hepa filter for meningitis

I have a small air purifier for my room, I worked with since October. I thought just the tip of the device was to keep dust and allergens remain in the air. I noticed that even the plants collect dust. Does this mean that the ion filter should be changed?
When you open the back of the filter is not shown, dirty, and the front has only a thin layer of gray dust. What to see, as if it never changed.

Note:-The filter is the right size, because the size of rooms, windows and doors are left open occasionally.