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Comment by wnevets | Hacker News Reader

Focusing on PM 2.5 makes sense because anything that size and smaller tend to cause the most harm. [1]

[1] https://www.epa.gov/pm-pollution/particulate-matter-pm-basic...

> Particulate matter contains microscopic solids or liquid droplets that are so small that they can be inhaled and cause serious health problems. Some particles less than 10 micrometers in diameter can get deep into your lungs and some may even get into your bloodstream. Of these, particles less than 2.5 micrometers in diameter, also known as fine particles or PM2.5, pose the greatest risk to health.

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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).

>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

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.

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.

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.

Yes, but isn’t lead getting into your bloodstream worse than pollen? I mean oxygen gets into your bloodstream via inhalation…

Which plant produces pollen at PM2.5 tho? Pollen usually isn't entering your bloodstream through your lungs, its too large.

Pollen particles are quite fragile and tend to break up into PM2.5 and smaller, especially during rain events. See for instance the reference below.

https://doi.org/10.1016/j.aeaoa.2022.100177

In addition to pollen, there are spores from both mushrooms and ferns. Both of which are common cloud-seeding particles and can often be PM2.5

Most pollen is captured at pm10 or higher level. PM2.5 tends to be created by humans

Crossing the blood brain barrier exacerbates not only asthma but other vascular/heart and related diseases, including diabetes, as well as lung diseases.

Worse in what way? Lead is a long term neurotoxin, but pollen triggers the immune system and makes you miserable. Both are bad but in different ways.

It’s really unclear to you why a long term neurotoxin that leads to lifelong disability is worse than allergy producing pollen?

It isn't clear until you are suffering from allergies, and notice that old people who used to chew on lead fishing weights are not all that bad off. Most people with high lead in their bodies don't realize the issue nor to the people they know.

> used to chew on lead fishing weights

Chunks of lead just aren't that bad. https://doi.org/10.2146/ajhp060175 lists common sources of lead poisoning, and none of them are lead weights.

Common sources involve things like airborne lead powder (from grinding or smelting), lead compounds dissolved in food & water, paint, and lead in soil.

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