Or just use (at least) MERV13, and ideally MERV16, filters in your fresh air intake ducts: * https://www.greenbuildingadvisor.com/article/understanding-filter-

Or just use (at least) MERV13, and ideally MERV16, filters in your fresh air intake ducts: * https://www.greenbuildingadvisor.com/article/understanding-filter-ratings-merv-fpr-and-mpr https://www.greenbuildingadvisor.com/article/understanding-f... * https://www.researchgate.net/figure/Mean-PM-25-removal-efficiency-for-HVAC-filters-listed-by-MERV-classification_fig1_280321614 https://www.researchgate.net/figure/Mean-PM-25-removal-effic... You'll take care of >70% of PM2.5 particles.