3 ms·

Presumably smaller particles are also more difficult to filter (and therefore more likely to make their way indoors without dedicated filtration), and also expe

Presumably smaller particles are also more difficult to filter (and therefore more likely to make their way indoors without dedicated filtration), and also expected to remain airborne for a greater distance and duration (because of square cube things).

hedgehog 2y ago
PM2.5 is around the hardest size to filter, I don't understand the physics but both larger and smaller are actually easier to remove from the air. Given that 2.5 is a useful estimate of worst case performance for a given setup.
- TylerE 2y ago
  Basically a HEPA/electrostatic filter works in two ways. As a “physical” filter (think fishing net), works well on big particles. As an electrostatic filter, works best with really small, works by electrically attracting particles and having them stick. 2.5 micron is in the middle crossover range where total net effectiveness is worst. Respirators are rated at the same size for the same reason.
LordDragonfang 2y ago
>Presumably smaller particles are also more difficult to filter So, that may or may not be true for just passive filtering (like door and window frames), but it's actually not true for actual hepa-style filters, because the physics of particulate filtering is totally unintuitive from the macro level -- past a certain point, finer particulates actually become easier to filter, because the physics is dominated by two different effects at different sizes. https://dynomight.net/ikea-purifier/#on-physics https://dynomight.net/ikea-purifier/#on-physics
- tsimionescu 2y ago
  I don't think it's all that un intuitive, as long as you know that there is an electrostatic effect used for filtering. We all know that it's easier to attract a lighter object with an electrostatically charged ruler, so it's quite easy to imagine that very fine (so, light) particles are easier to to attract than larger heavier ones as well.

hedgehog 2y ago

PM2.5 is around the hardest size to filter, I don't understand the physics but both larger and smaller are actually easier to remove from the air. Given that 2.5 is a useful estimate of worst case performance for a given setup.

TylerE 2y ago
Basically a HEPA/electrostatic filter works in two ways. As a “physical” filter (think fishing net), works well on big particles. As an electrostatic filter, works best with really small, works by electrically attracting particles and having them stick. 2.5 micron is in the middle crossover range where total net effectiveness is worst. Respirators are rated at the same size for the same reason.

LordDragonfang 2y ago

>Presumably smaller particles are also more difficult to filter So, that may or may not be true for just passive filtering (like door and window frames), but it's actually not true for actual hepa-style filters, because the physics of particulate filtering is totally unintuitive from the macro level -- past a certain point, finer particulates actually become easier to filter, because the physics is dominated by two different effects at different sizes. https://dynomight.net/ikea-purifier/#on-physics https://dynomight.net/ikea-purifier/#on-physics

tsimionescu 2y ago
I don't think it's all that un intuitive, as long as you know that there is an electrostatic effect used for filtering. We all know that it's easier to attract a lighter object with an electrostatically charged ruler, so it's quite easy to imagine that very fine (so, light) particles are easier to to attract than larger heavier ones as well.