After an extensive review of current epidemiological literature, the US EPA guidance for air cleaners in the home was updated and reissued in July 2018. The revised guidance includes major changes in recommendations for central system filter efficiency, and new guidance for assessing the effectiveness of portable air filters. These relatively low-cost portable units turn out to be surprisingly effective at reducing small particle concentration when compared to the net annual effectiveness of central HVAC filtration. And portable air cleaners can compensate for the filtration shortcomings of increasingly popular wall-mounted mini-split units.