[jagraff]

Interesting, it seems that the actual surface material of walls and/or furniture makes a large difference in how long VOCs stick around, due to differences in surface area at the microscopic scale.

I have a couple HEPA filters in my house that hopefully keep particulate exposure down. Does this mean that I have to run them longer? That I need more of them continuously running to keep exposure to VOCs low?

[whatevertrevor]

As pointed out in another comment HEPA filters don't work well for VOCs (Volatile Organic Compounds), which are gaseous in nature. They're intended to filter particulate matter.

For VOCs you need activated charcoal/carbon filters usually and replace them from time to time.

[jopsen]

Or a ventilation system I'm guess?

Where I live all new houses are pressure tested and have a ventilation system, replacing all air once every 1-2 hours or something (I think).

[sneak]

TFA is specifically about how they attach to porous surfaces and how simple ventilation is way less useful than originally assumed.

[whatevertrevor]

The GP comment is talking about active ventilation though, through an ERV/HRV system. Also the article states this:

> The lifetime of these compounds indoors can be extended via partitioning to the surface reservoir as modulated by ACR. Higher ACR, which may be achieved by opening windows or through mechanical ventilation, leads to shorter t_half_surf because once indoor compounds partition from the surface reservoir to the gas phase as controlled by gas diffusion across the boundary layer, they would be removed from indoor air more quickly before repartitioning to the surface reservoir.

So they do state active ventilation can help, as you reduce the vapor pressure of VOCs allowing them to partition back into the gaseous env, where they can be promptly ejected. How much exactly is hard to ascertain from their graph since I don't have the exact data they used in the plots. But from squinting at it, it seems 1 OOM change in ACR gives you close to 1 OOM change in the VOC half life, which seems substantial to me.

So adding an active ventilation system might be a good idea for this particular concern. Of course it will add to your energy bill.

[jopsen]

Modern ventilation systems, like the one I have will recover a lot of heat, when exchanging air.

[whatevertrevor]

I know, I have one too. It's still energy negative because they're not 100% efficient and you're paying energy to run them in the first place.

Still worth it for better air quality.

[fuzzfactor]

True, but simply using a low volume exhaust like a bathroom fan can give you a phenomenally greater effect than zero.

And that's for the entire house, zero is such a small number.

Then when you run it 24/7 it's 24 times as effective compared to a single hour. That's an impressive multiple itself, on top of bumping the baseline above zero to begin with.

This can really add up to a lot more ventilation than commonly assumed from some of the crummiest fans.

If you can't tell the difference when you walk in, between zero and running one of these all day before you get there, you're gonna need a bigger fan.

But you may be surprised and you never know until you try.

[jopsen]

But when stuck inside the porous surfaces isn't the problem mostly when they become airborne again?

Most of us don't eat wooden furniture -- granted my toddler didn't get the memo :)

Thus, continuous ventilation (while not perfect) is hopefully still a decent alternative. Probably better than active charcoal filter.

Granted I should probably out a charcoal filter on the ventilation intake to reduce pollutants coming in from nearby traffic. (All depending on your level of paranoia)

[whatevertrevor]

If the porous surfaces are saturated then you'll basically be maximizing the vapor pressure of these gases in the air you breathe. Check out my sibling comment, extrapolating just from the data in the article an active ventilation system should help.

EDIT: And yes, charcoal filters aren't as effective if they're not part of your critical airflow/ventilation path. :D

[Gigachad]

Every air purifier I’ve seen has both a hepa filter and carbon filter.

[throwway120385]

This kinda makes sense. Water vapor diffuses out through the building materials so why wouldn't VOCs diffuse into those materials?

What you're looking for are not HEPA filters but organic vapor filtering. If you were shopping for a respirator it would be easy but organic vapor extractors I think are a lot more expensive than HEPA filters. I looked in to it when I was doing a couple of oil based coatings for a home renovation project.

[wongarsu]

A lot of air purifiers are advertised as HEPA but really contain a filter stack consisting of a pre-filter, a HEPA filter and an activated carbon filter. Those would presumably help against VOCs, assuming you change the filter frequently enough

[bonesss]

Compare those air ‘purifiers’ with the activated charcoal setups they use on cannabis grow operations, and you’ll get a sense of what volume of charcoal and air circulation is necessary to combat those small particulates. Purifiers help in theory but are nowhere near effective or active enough to combat off gassing or VOC dispersals in practice.

[s0rce]

Frequent replacement is critical, my understanding is the activated carbon filters typically provided have very limited capacity. More so when compared to the lifetime of the hepa.

[plorkyeran]

"Frequently enough" with the size of the carbon filter a typical air purifier has would be close to daily.

[lm28469]

> HEPA filters

They won't do anything against VOCs, you need activated charcoal filters

[hxorr]

If you are in a temperate climate, just make a habit of keeping a couple if windows open through the day

[bflesch]

Thats why ecological buildings use lime and clay for plastering indoor walls. They can absorb a lot of things (water, fumes) and thereby regulate air quality and humidity.

[bahmboo]

The paper posits this is a problem. Large amounts of VOCs are absorbed by these complex structures. Then the structures with the embedded VOCs flake off and are absorbed by breathing, dermal contact and ingestion. Particularly by small children. This is literally their point.

[lxgr]

Do they absorb VOCs forever, though, or do they actually make it harder to vent them out once absorbed by a surface with a large capacity?

[backprop1989]

I’d think you’d want the VOCs to be captured by something, rather than floating around in the air where you could breathe them in. Combined with a HEPA filter in the air circulation system, this should be a good solution.

[lxgr]

Absorption is usually not a one-way street, though: Surfaces absorb gasses when the concentration in the air is higher than that on the surface boundary, but often also release them back into the air otherwise (which is why you can e.g. smell cigarette smoke in clothes – if they only captured it, there would be nothing for you to smell).

The only difference are some materials like charcoal, which does permanently bind many substances (but as a result can also saturate).

No idea which kind lime and clay are (i.e. "absorb and permanently bind with limited capacity" or "act as a buffer both ways").

> Combined with a HEPA filter in the air circulation system

HEPA filters are not effective against VOCs.

[fuzzfactor]

Actually not that many types of things will bind to the carbon permanently, mostly it's the affinity for such a wide variety of contaminants to the carbon, combined with the porosity of the carbon structure which can have a very impressive amount of surface area to come in contact with the fluid being filtered. Whether filtering air or water. It hangs onto contaminants tightly.

Because carbon is such an effective adsorbent for contaminants, the partitioning coefficient for contaminants to remain in the solvent being filtered is lowered quite dramatically compared to so many other kinds of affordable alternative filtration media.

Most times people do need to afford to discard the carbon eventually, but it doesn't even really absorb contaminants like it's supposed to unless it is activated carbon to a good degree.

Activation only means that is it porous enough to begin with so it has enough surface area to be effective, then it is heated with adequate air exchange to about 250 Celsius for as many hours as it takes for virtually all of the VOC's or moisture it may have accumulated to be baked out. Then sealed up tightly, otherwise it can sit around for ages and gradually become saturated passively with any contaminants or humidity admitted through leaks to the ambient environment.

Sometimes, you can reactivate almost indefinitely to keep reusing the same carbon, and it works with VOCs because by their volatile nature they are basically baked back out easily and virtually completely each time. Different amounts of time if using different temperatures though, if equipped.

The stronger the activation, the more tightly with higher capacity the carbon wants to absorb things it encounters that are dissimilar to the fluid being filtered.

[bflesch]

I assume they absorb VOC until you tear down the chalk or clay plaster.

With clay the indoor problem is more about radioactivity, but it's best in terms of humidity control. Chalk creates an alkaline environment on the surface which makes it inhabitable for mold (however the wooden furniture you put in front of it can still get mold if the indoor air humidity is too high).

[scottlamb]

Does that work if it's painted over? Or can you mix colorants in as with (exterior) stucco? (Maybe this is considered a kind of stucco? I just had to look it up: wikipedia says "The basic composition of stucco is lime, water, and sand".)

[bflesch]

Nope, I dont think it works when painted over. Some vendors recommend colors which are very open for diffusion such as chalk colors, but every other "common" color based on acryl/latex/etc basically seals it from the air and destroys it over long term.

For clay I know you can add color pigments to the clay itself, most likely you can do the same with stucco for some limited amount of colors. But painting over it with modern products mostly destroys the diffusion properties.

Many people put plastics or other sealing products on top of a clay or lime-based wall and it's a shame.

[ender341341]

I would assume if you paint it over with a latex based paint at least it would massively affect absorption. For oil based paints I have no idea though.