I have a twelve year old Mitsubishi unit in my bedroom. I also have allergic reactions to pollen, mold, etc. This year, for the first time, I wake up in the morning with red, burning eyes. My husband changes the filter every year and I've never had this reaction.
Changing the filter once a year sounds underwhelming to me, they need to be changed based on load , Or how much dust is trapped on n the filter, once a year is a long time. Another user suggested hydrogen peroxide, I like hydrogen peroxide for other forms of cleaning but not on evaporators, I use special non acid coil cleaners they pull the dust and mold out of the fins without damaging the fins. For example NU-calgon makes a coil cleaner that you mix with water and spray on the coils, it foams and pushes dust and mold out without harming the evaporator coils and fins. This product can be found on line under the name of Nu-Brite, I use this for my home systems and at industrial facilities I do not work for them just really like this product. I don’t know what type of filter you have but once a year? Probably not often enough. At 12 years old I am sure both the inside and outside coils could use a good cleaning (never use a pressure washer to clean coils high pressure can destroy the fins) a good cleaning will reduce energy usage also. A gallon of new-brute cost ~25$ but would be enough to do several cleans on a home system, there are also no rinse versions that are easier to use for the inside unit (I just use a shop vac to clean up the foam and rinse water inside). Since you haven’t had problems in the past I would clean the inside coil at a minimum with a fresh filter, or have your HVAC company do this it takes time and the chemicals so expect about $150 if you have it done +- depending on location. After re reading the question if this is a window unit the foam filter can be washed with warm soapy water an rinsed. The evaporator is tough to get to but the condenser coil on the outside is easy a good rinse of the system is probably needed.