It is difficult to determine an average because the situation varies between countries,
but one considers that the particulate pollutant emissions (PM10 and PM2.5, mostly) usually exceed 1 g and can even reach 100 g for 1 kilowatt hour produced. How many particles does an air purifier with HEPA filter eliminate when it consumes 1 kWh?
Let's take the best equipment using this technology, released in 2015, from the most reputable brand on the market.
This unit will treat approximately 7 m3 of air per Wh consumed. Thus, for an "average" particle pollution of 50 µg per m3, 0.35 g of particles are elminated per kWh.
And this efficiency goes down as the filter gets clogged, as the device will require more energy to treat the same volume of air!
One understands that the "particle balance" is negative: even an efficient air purifier with HEPA filter eliminates, at best, the same quantity of particles as those it requires, at worst, 500 times less particles !