HEPA filters are actually more efficient at capturing very very small particles (<0.1 micron) than they are at capturing simply small particles (≈0.3 micron). The tiniest particles are light enough to be captured using electrostatic forces rather than the more typical mechanical filtration that is effective at capturing large particles.

More precisely, many odors are not particles at all but rather gases/vapors. Activated charcoal can capture larger organic molecules because it has an incredibly jagged/porous surface - just 100 grams of activated charcoal provides over 1,000,000 square feet of "effective" filtration surface area.

Yes, that's true that HEPA is very efficient at particles in the 0.01 to 0.1 micron range. It may also be below that, but I personally haven't seen that data. But because HEPA fails to filter gases/vapors, its performance must fall off before 1 angstrom = 0.0001 micron.

> odors are not particles at all but rather gases/vapors

I think the confusion comes from people referring to "particulate" as particles of >= 0.01 micron, whereas in physics we say that gases/vapors are also composed of particles. They may be single atoms floating (can't actually come up with an example though) or small molecules like O^2, CO, etc, but I'd still call those particles.

> 'but I'd still call those particles'

But then in your world there are no such things as gasses? If so, the world uses all usefullness

I don't know any way to filter NO2 effectively from air, and I have tried practically all commercial air filters with the largest carbon filters on the market. The worst parts of air pollution cannot be filtered out without industrial devices (that are part of the air filteration system) as far as I know.

NO2 and other acidic gases are typically filtered using activated alumina balls impregnated with an oxidizing agent such as potassium permanganate or sodium permanganate.

There are residential air purifiers available that use this in combination with activated carbon for their gas stage.

I bought IQAir and tried multiple filters (I think they use potassium permanganate), and I wasn't able to significantly reduce NO2 in my room. 3M masks similarly may work if the toxic gas concentration is far above the high levels, but I couldn't get it from normal/high to low levels.

Maybe you are talking about an integrated air filtration system, as I haven't tried that yet (though I'm planning to try it).