Han et al. (6) estimated that the global cost of using air purifiers, including the purchase price, filter replacement cost, and electricity consumption, to keep indoor PM2.5 concentrations below 5 μg/m3—the annual limit recommended by the WHO (8)—could exceed $4 trillion USD per year.

From: www.science.org/doi/10.11…

$4 trillion per year is $500 per person – planet wide.

That’s like buying everyone on the planet a last-gen iPhone, every year. It’s simply unreasonable.

If I get a moment, I’ll try and read the article to find the assumptions underlying this, but it doesn’t seem reasonable.