1,000,000 ultrafine particles (0.1 microns wide) weigh the same as 1 coarse particle or 64 fine particles (2.5 microns wide). From: Ultrafine particles: unique physicochemical properties relevant to health and disease
Ultrafine particles (diameter of 0.1 microns or less) are considered more harmful to human health than larger particles.
Their microscopic size, smaller than any bacteria, allows these invisible particles to penetrate the deepest lung passageways, sometimes carrying toxic metals or organic compounds that trigger inflammation and disease.
These ultrafines can hang in the air for hours and even days.
However, air quality monitors measure the weight of the particles collected from the air (per cubic metre), not the actual number of particles. And wood stoves are certified according to how many grams of fine particulates they can put out per hour, not by the number of fine or ultrafine particles they emit.
Yet 1 gram of ultrafine particles has over 15,000 times the number of particles than 1 gram of particles 2.5 microns in size (the largest of the ‘fine’ particulates). And more particles means a lot more surface area for toxins to attach to & enter our bodies.
While new wood stoves may emit fewer grams of particles (i.e. less weight), numerically they may put out many more ultrafines than older stoves according to this review: Nanoparticle emissions from residential wood combustion: A critical literature review, characterization, and recommendations.