How do you select filter medium?


I am a senior, have a 35 year old home. Heat/ac vents are in the ceiling. It is not currently in my budget to have the duct work professionally vacuumed/cleaned. I was wondering if it is possible to put some sort of medium in the individual room vents to help w/dust and allergies? I don't want to use anything that will diminish air flow thus increasing workload on my geothermal unit. Please advise.

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