What are the options for filtering fresh air entering a building (especially outdoor air with wildfire smoke)?
Electrostatic air filters are awesome I had a large set n my last house on the air handler intake. Even when we could not see the mountain across the street the air quality inside the house was excellent.
I can say this as one of my granddaughters has severe asthma we normally leave the fan running 24/7 that system has a small make up air intake that enters the duct several feet prior to the grid. So a small amount of fresh air is being drawn in the home and filtered, during the bad smoke I had to clean the filter under a month because it was filthy but the entire time my grand daughter was inside she had no problems They stayed for over a month because prior to that she had been hospitalized.
I have seen portable electrostatic fans that do the same thing and these are less expensive than hepa filters because the filter needs to be changed and you can not just put a hepa filter in your air handler because of the pressure drop.
I have not used the passive filters Ecnerwal comments on but know customers that like them but they also need to be cleaned regularly and do not have alarms like the powered ones do when dirty (some start snapping some set off a buzzer) but the passive ones are much cheaper than a hepa or hardwired/ ducted electrostatic filter.
I would verify you have make up air on your air handler it is usually a small duct less than 4” on the intake side of the air handler some times located in the crawl space or attic and it can be as small as a 1” hole I would guess mine was dampened down to aprox 1” because we ran the fan 24/7.
@jxramos answer looks like a good one but since I did not see electrostatic I felt this should be covered the portable fan with electrostatic grid would be my suggestion for constant filtering or try the electrostatic if you keep your fan running we found it worked the best for our granddaughter 24/7.