How can I improve ventilation in my room with a window fan while filtering pollen from a nearby tree?
You'd be better off with a box fan as it has a large, square area that will support your filter media properly. It will need to be powerful enough to develop airflow despite the next addition.



Not sure if you can find it, but we have here available electrostatic filter media by the roll. Cut several pieces large enough to cover the inlet side of the box fan. Mount one to the grill, keep an eye on it for pollen buildup, swap in a spare and wash the clogged one.



It's not as efficient as a HEPA filter system, but then the cost of the filter material isn't going to put you in the poor house like the cost of expensive HEPA microfilters that will quickly clog even though you keep the outer prefilter cleaned will.