What is the purpose of a bathroom fan filter? What is the risk of not using a filter?
It's similar to the reasoning behind using a filter for your AC units. You want to filter the air to prevent clogging up the coils and dirtying up the ducts. There is a lot of moisture in a bathroom and if you mix that up with dust and lint from towels you've got a combination of particles that will stick to the blades of the fan and also clog up the shaft which will reduce or even stop the spinning of the fan and also settle in the duct work before reaching the outside. The filter will stop this from happening. Do yourself a favor and replace the filter. If the fan returns air back into the bathroom, a filter will clean the air for better breathing.