When you compare air purifier efficiency the number of air changes per hour is very important. Here is an example Comparing a HEPA at 99.97% to an electrostatic at 97% in room that contains 32,000 ml of smoke, After the first cycle the HEPA leaves behind 40 ml. The Electrostatic leaves behind 41ml. On the second cycle .4 ml is left by the HEPA and .425 by the electrostatic. Most HEPA air purifiers only provide 2 cleanings per hour for the intended room size. A third cleaning by the electrostatic brings the remaining smoke down to .004 ml, a 4th leave so little you have to use scientific notation to express it.
While efficiency is very important, the number of times it cleans per hour is far more important. An EPA study comparing HEPA and Electrostatic found they worked as well on the smallest particles. The HEPA did remove more on each pass. But if the number of cleaning cycles were greater by the Electrostatic, the Electrostatic did a better over all job.