The world’s leading health-related organizations consider PM10, PM2,5 and PM1 fine dust fractions as the most important and dangerous for humans. Their official documentation to the public always refers to these PM levels. It is therefore logical that filter test methods and classifications follow this approach to demonstrate filtration performance towards the most harmful fine dusts.
The lighter and smaller a particle is, the longer it stays in the air. Particles smaller than 1 micron contribute only a few % to the mass, at the same time contributing to over 90% of the numbers.