The Valyn are a Roman analogue who have been exiled to live in a hostile mushroom forest with daily mist in the vein of a Cloud forest. The air is absolutely brimming with mold, spores, and aeroplankton. The amounts of air particulates are a concern over a long time period. A month of breathing it is enough for lung inflammation with a sickly cough while a year is usually enough to kill a person ala "Mushroom Worker's disease" in real life. A note: This society has access to 10% hydrogen peroxide via a dog sized animal with a Bombardier Beetle's defense mechanism, so you can add that to your toolbox. (Link to medical journal on Mushroom Worker's Disease)

How do they purify their air enough to live while using a technology base from 100AD?

I want to list some avenues of research I've explored, but am not fully satisfied with.

  1. Some settlements are built mostly inside of old salt mines, with some buildings above ground attached via ducts. Air that is pumped through salt mines (Via beasts of burden) naturally becomes purified of particulates. (Link to an article on Wieliczka Salt Mine) The issue with this solution is that there are only a few salt mines spread around. It also means that settlements cannot continue to mine salt. Mining salt pollutes the air with too much salt particles and makes the air quality dangerous.
  2. Houses are air tight with an airlock style mudroom. Every time someone wants to enter they go through a "Purity" ritual which involves hot coals, hydrogen peroxide, and supplication to the gods. After 5 or 10 minutes of the proper prayers the room is sterilized and they can enter the house. Periodic purification is also done inside of houses. This is like "shocking" a pool. It will purify the air, but not keep it pure.(Link to article mentioning several studies on the efficacy of Hydrogen Peroxide Sterilization via steam.)
  3. Houses are air tight and use aerial filter feeders to purify the air. The sheer amount of detritus suspended in the air allows the existence of terrestrial sponges. Sponges are nearly exactly the same as their sea based cousins. They pump air through themselves and catch anything suspended. A 1kg sponge can filter 40 cubic meters of air per day. These sponges are placed in each room like house plants. This improves air quality, but doesn't really prevent spores from entering the house. It is also not compatible with salt mine air which would dry out the sponges. I initially thought to use these sponges as a forced air system, however the speed of airflow is a measly 0.5 ft/s or 0.15m/s. (Link to relevant wikipedia article on Sea Sponge feeding/filtering)
  4. The Valyn use a primitive gas mask with one-way valves like a bellows. The gas mask is attached via a leather tube to a canister they wear on their back. The canister is a tube shape with perforations along its length. Inside the canister is a filter feeding sponge. When the wearer breaths air is drawn through the sponge and the air is purified. The rate of airflow is much greater than a sponge is normally used to. Due to this the sponge's filtering mechanism gets "Filled up" over several hours and becomes ineffective after that time. It then needs a rest to digest the particulates. Long trips outside require two canisters. Periodically the sponge must be culled from overgrowing the canister, as well as it needs to be "cored/hollowed out" so it does not fill up the inside of the canister too much and make airflow difficult. (Given the technology level I believe a filter feeder would be more effective at purifying air than anything the 100AD romans might think up. They generally used bandanas tied around the face in their salt mines which really wasn't too effective.)

Edit to address questions.

  1. Cutting down the forest is not a solution. Light levels across the world are 18% of Earth levels due to a cataclysm long ago. The area they were banished from has an artificial light source which cannot be replicated. The artificial light prevents fungal growth because of mushroom sensitivity to UV. The mushroom forest is fed by photosynthetic aeroplankton high in the atmosphere (part of the light level issue) which periodically die off from solar phenomenon every 180 days. This makes the mushroom forests insanely productive and filled with edible flora and fauna if you can survive. The banishment was meant to be a death sentence because no one else has survived the forests.
  2. Town population is dictated by religious law. New colonies start at 10,000 people. Maximum size of a city is 100,000. After that religious law dictates excess population from every town meets at the capital to found new colonies. There are no smaller towns. The people here are more like sedentary hunter gatherers because of the nature of the forest which provides a glut of edible flora and fauna. Large cities are neccesary for defense against hostile fauna which cannot be made extinct, additionally there are fantastical dangers which necessitate magical wards which are expensive and best implemented over a city sized area. "Safe" areas are not even possible due to astral phenomenon which basically teleports additional creatures from another planet to this one every 180 days. Every 180days when the plankton dies off and sinks to the surface there is new growth. The whole city works together to harvest and preserve the bounty which only takes them 6 weeks with an extra 30% food than needed (9months food harvested every 6 months) In essence, each person is a hunter/gatherer 23% of the time, and spend the rest in their chosen profession.

Lets try to focus on the air quality issues though.

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    $\begingroup$ burn the forest down, plant something not actively killing you, if there is that much organic particulates in the air the whole thing might explode anyway the first time some creates a cooking fire. $\endgroup$
    – John
    Jul 2, 2021 at 1:01
  • $\begingroup$ How big is the population in one of these towns? Is there just the one town? Just the one mushroom forest? $\endgroup$
    – Tom
    Jul 2, 2021 at 3:53
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    $\begingroup$ I don't think you can. You're basically asking them to live like astronauts. And the problem with air tight houses no oxygen can get in and new CO2 you can get out. So you die from CO2 poisoning first, and then you die again a few hours later from lack of O2. $\endgroup$
    – DKNguyen
    Jul 2, 2021 at 4:43
  • $\begingroup$ Which is why Im not satisfied with my research. They dont really need to be wholely protected from spores. As it stands its like hay fever that never ends. If trees produced pollen constantly like they do during pollen season then we would have this same problem on earth. An irritation becomes an allergy which eventually kills you. What we need to do is decrease the spores as much as humanly possible, and maybe handwave away that "Only those with less severe immune reactions would survive the initial banishment leaving people who are less sensitive." Maybe that's it! You might have solved this $\endgroup$ Jul 2, 2021 at 4:45
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    $\begingroup$ I think your city size needs a frame challenge. There's no way you can feed 100 000 people with only gathering food and hunting, no matter how bountiful the forest is. One hundred people in a tribe is more plausible. To have a city in 100AD, you need a lot of farmland surrounding it, because farming was still not very effective (but oh so much more effective than gathering). $\endgroup$ Jul 2, 2021 at 16:45

5 Answers 5


Particle Traps and Face Masks

As others have pointed out, face masks are likely an effective solution outdoors - high quality cloth can filter on the order of 100s of nano-meters, while mushroom spores are on the order of 10s of micro-meters. So face masks work.

But what about when you sleep or eat or relax indoors? Presumably you don't want to wear a mask 24/7.

Tunnels and Ladders

Mushroom spores are particles, not gases. (obviously)

In the absence of strong wind, particles will settle to the ground.

All building entrances will have a deep, "V" shaped tunnel protecting them.

To enter a building, you open the first door, climb down a 20 foot ladder, walk a few feet, and then climb up a second 20 foot ladder. Only now do you actually enter the house through a second door.

Without a strong wind to move them, spores will fall into the pit, and be trapped there - the rest of the building will be spore free. You'll have to clean the pit regularly, obviously while while wearing a mask. The home should be otherwise as air tight as achievable.

Particulate traps are especially nice because they are passive - no machines or continuous effort required.

Edit, based on comments:

I'd also add a metal grate above the floor of the pit, and some kind of water - either a puddle or stream - to wet the spores, and hold them in place. You could add some kind of mud-room or cleaning area to the top of the second ladder to remove spores from your cloths and body.

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    $\begingroup$ Bonus points if the bottom of the particulate trap has water or mud to stop particles being disturbed by passers by. $\endgroup$
    – Joe Bloggs
    Jul 2, 2021 at 13:51
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    $\begingroup$ @JoeBloggs - I almost included a metal grate over a mud puddle for this reason, but I was trying to keep the answer brief. :-) +1 $\endgroup$
    – codeMonkey
    Jul 2, 2021 at 15:39
  • $\begingroup$ You could also incorporate a ritual wash at the top of the second ladder. $\endgroup$
    – codeMonkey
    Jul 2, 2021 at 15:45
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    $\begingroup$ @codeMonkey Def add the metal grate to your post. This is not only a good answer, but leads directly to a flavorful architecture that would become characteristic of the society. Seems ripe for storytelling. $\endgroup$
    – Tom
    Jul 2, 2021 at 17:29
  • $\begingroup$ It should be noted that in other comments someone linked a source which made facemasks of that period unviable because greco-romans could only create at maximum 250 threads per inch. (Thats the highest wr have recovered. Most fabric was in the mid double digits. They didnt have the tech for facemasks that block spores. $\endgroup$ Jul 2, 2021 at 22:00

It doesn't look good

By strange coincidence, I've been idly musing about a very similar problem for a few years (since I read Wool by Hugh Howey). So, I already had some ideas to investigate.

Sadly, none of them were available to Greco-Roman civilization, which is basically what 100 AD means.

Wet scrubber

My first thought was to use water sprays. I think this could actually work, although I don't know whether it would be efficient enough.

For any kind of automatic system, you'll need mechanical energy.

The ancient Greeks had a grasp of hydraulics, so the Valyn would have both Archimedes' screw, and the piston pump. The article on hydraulics says the Greeks got pretty sophisticated. At a smaller scale, they had the aeolipile, which is a little steam engine, but it's not clear they considered it mechanically useful; it may have been treated as a novelty, but this could be useful in single-family homes.

The other thing you'll definitely need is spray nozzles, and this is the deal-breaker: they weren't invented until the 1800s. However, a spray nozzle doesn't seem to depend on other advanced tech like electricity, chemistry, atomics, or post-Newton math or physics, so, per Joe Bloggs, it may be reasonable to just assert that the Valyn did have them.

I really don't know if you could filter particulate matter out of the air, or keep it out of a clean room, using sheets of falling water. And it seems like that would require a lot more water.

Wet scrubbing may also go by a couple other names, if you approach it from a chemical engineering perspective: "separations process," which is a subtopic within "mass transfer" (according to a reddit thread you may find interesting). You may find something useful down that road, but I suspect 100 AD will kill it.


Modern air filters come in a bunch of varieties, but none of them were available to the ancients. HEPA-style filters weren't invented until the 1950s. Lots of filters are made of paper, but while paper was invented around 100 AD in China, it was kept secret for centuries.

Cotton was known to the Greeks, and it doesn't take a genius to realize that you can breath through it. In fact, the NIH says that cotton is actually pretty good:

a 600 TPI cotton sheet can provide average filtration efficiencies of 79 ± 23% (in the 10 nm to 300 nm range) and 98.4 ± 0.2% (in the 300 nm to 6 μm range).

NIH also says that mushroom spores are between 34×28µm and 3.5×0.5µm. 1µm = 1000nm, so I think cotton filters will make a big difference, although how much will depend on how big your mushroom spores are, and how dense their cotton is.

I have no idea if 600 TPI is realistic for 100 AD textile manufacturing. Per Nosajimiki, 600 seems out of reach:

Ancient textiles only ranged form ~25-75 TPI ... The highest TPI on record for the ancient world was about 250 TPI, but this is a significant outlier

The Greeks also had silk, although it was very much a luxury item. While I don't doubt the Valyn would think it worth the price, the question is more about scarcity. Silkworms weren't smuggled out of China until a few centuries after your tech date. And, you'll again run into the question of threadcount.

Alternative solutions

It seems like your best bet is the hydrogen peroxide and the salt mines.

If the air quality is really so bad, I think the Valyn won't construct their buildings as freestanding, single-family homes. We don't do that in space, or underwater. When the surrounding environment is unbreatheable, we link our habitats (like a hamster village).

So, what I imagine is not the kind of residential area that's common in big, open places like the United States, but a veritable warren of interconnected buildings that share a ventilation system.

In that case, it might be practical for the entire system to have its intake in a salt mine, and also to use peroxide somewhere upstream of homes and businesses. If they have the technology to construct airtight homes, it stands to reason they can also build airtight ductwork.

It wouldn't all have to connect to a mine, either. They might have a freestanding building that's just a giant lung: filled with sponges, sucking air from the atmosphere into the ventilation system. They'd space these out to ensure adequate airflow throughout their sealed complex.

And a complex is definitely what it would be. Like a moonbase in the forest.

ETA: now that I see your population numbers, I'm skeptical this could work.

The googles says that the average human breathes 2,000 gallons of air per day. Scale that up to a population of 100,000, and you've got 200,000,000 gallons.

It's hard to imagine ancient technology handling that kind of airflow. Admittedly, I don't normally think about that kind of thing, so I could be way off-base, but the analogy that seems clear-cut is: Greeks building just the ventilation system for a base on the Moon that supports 100,000 people. It doesn't seem anywhere near plausible.

You might want to try a spin-off question that focuses just on airflow requirements and ventilation design for a population of that size, tagged with hard-science (just to get a baseline).

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    $\begingroup$ It’s not outside the realm of possibility that a Roman-era engineer could make a spray nozzle by accident. I certainly wouldn’t question it! $\endgroup$
    – Joe Bloggs
    Jul 2, 2021 at 6:11
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    $\begingroup$ Also: water soaked cotton is a better filter than cotton alone, though a damp face mask would need very regular and thorough cleaning. $\endgroup$
    – Joe Bloggs
    Jul 2, 2021 at 6:14
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    $\begingroup$ Sticking a pin through balsa wood can be used to create a (very rudimentary) spray nozzle. Author fiat in an alternate world can easily cover that regardless of the actual length of time it took us to invent it. $\endgroup$
    – Joe Bloggs
    Jul 2, 2021 at 6:20
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    $\begingroup$ @theonlygusti 600 TPI is unbelievable labor intensive to do without modern machines, and hand spinning threat thin enough for that is nearly impossible because the thinner you make your thread the more precise you need to be with your tension and feed. Ancient textiles only ranged form ~25-75 TPI. cambridge.org/core/journals/antiquity/article/… $\endgroup$
    – Nosajimiki
    Jul 2, 2021 at 18:32
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    $\begingroup$ I should specify 25-75 is for Roman cloth. The highest TPI on record I can find for the ancient world was a Greek cloth that was about 250 TPI, but this is a significant outlier. $\endgroup$
    – Nosajimiki
    Jul 2, 2021 at 18:38

They Would Die

Sounds brutal, but it's true. Rome faced many similar problems in settlements where lead, asbestos, or salt mining rendered the area toxic and the archaeological evidence shows that they never came up with a way of protecting themselves from the hostile environment. Written evidence shows that the Romans knew about the dangers of these mines, yet over the course of hundreds of years, they never invented a way to make these mines safe.

Instead they simply made these mines the work of penal slaves: criminals and POWs who were intentionally sent to work these mines knowing that they would live very short lives. So, if your Roman Era people had somewhere safe to live, and this forest is just one place on thier world with some resource worth exploiting, then thier solution will be to send thier undesirables there to gather the resource of interest and send it back to the safe places. If this environment is your whole world, they will all be dead LONG before they solve the problem.

In 100AD, people simply did not understand enough about air-borne hazards to effectively combat them.


Filtration hypocausts and big building complexes..

There are three issues, airflow, filtration, and leaky construction techniques.

Airflow is pretty obvious: you need to bring air in from outside or you will suffocate. Leaky construction techniques is also fairly obvious. Large constructs like a series if connected homes or an apartment complex will not be airtight.

So we need to make sure all air flowing into the building goes through a filtration system, a you need to set up positive pressure within the large building so all leaky parts leak air out, not in.

Mine some salt. You can abandon one mine for this purpose. Use the salt to line a hypocaust. Usually these are underfloor heating systems that use hot, dry air in a subfloor cavity to keep the floor warm. You will use it for that purpose, but also allow the hot air to escape into a central atrium in the house. You can do this with clever placement of heat sources or using your animal-driven air pumps. Air flowing through the hypocaust will be naturally purified by the heat and dryness. Setting up large sponge colonies at the air intakes will also further improve air quality. Small peroxide reservoirs would further improve the air, though you’d want to balance that very carefully to avoid a whole new set of problems. Airflow will then be from inside the house (clean) to outside.

Why do this instead of just having passive sponges everywhere?

Well: it lets you vastly increase the size of building that you can provide conditioned air to. That in turn means that big, complicated warrens of interconnected ‘pure’ buildings are perfectly possibly as long as you have suitable ventilation to pull cool air into the hypocaust. Good airflow will also increase the efficacy of any sponge filters (either by the air intakes or passively inside the housing complexes).

These homes will be hot, dry, and cramped, but you won’t die of pneumonia.

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    $\begingroup$ Im imagining the traditional Roman Domus, but enclosed with the Impluvium pool removed and a hole to the hypercaust instead. I wonder:Maybe inside the hypercaust there are fan sponges that completely block the "ducting" Forced air underneath the Impluvium could suck air through the fan sponges to be filtered. It seems the fan sponges would be self regulating. They will expand in size until their food intake is about equal to their maintenance. $\endgroup$ Jul 2, 2021 at 7:33
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    $\begingroup$ @Hippeus_Lancer: depending on the exact requirements of your sponges you could even make it a whole self-regulating ecosystem down there, with specialist filter slaves (Filtratorae?) dedicated to maintaining the health of the sponge-biosphere and stopping passage-blocking overgrowth instead of having hypercaust slaves feeding the furnaces. You might have to play with your sponge efficiency or have the hypercaust be quite big though. $\endgroup$
    – Joe Bloggs
    Jul 2, 2021 at 7:48
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    $\begingroup$ Plus it wouldn’t be a hypocaust (just noticed I spelt it wrong throughout!) anymore. Hypoliquet, possibly? $\endgroup$
    – Joe Bloggs
    Jul 2, 2021 at 7:57

Here is my final solution using several answers in conjunction with each other, as well as some additional research inspired by the answers. I don't normally answer my own question, but in this case it seems to be justified because each answer contributed just a piece towards the final whole. I've upvoted all of the answers that I used to create this as recompense for not choosing an answer from among them.

Housing: Homes are ventilated via a combination of stack effect and Venturi effect. Houses are large communal complexes 25m in diamater, housing 200 people on 3 stories (Inspired by Chinese Tulou). Every house has a central courtyard which is fully enclosed except for a windcatcher/windtower. The house itself is mostly sealed without windows facing the outside world. Wind is near constant here, so the windtower's opening is opposite the flow to create suction that ventilates the house. Additionally, a hanging magical lightbulb creates a stack effect for near constant suction even when the wind fails. (It additionally grows some rare plants/vegetables in the courtyard.) A stone grate in the floor of the courtyard connects to the basement where the air is sucked up from.

The basement is unremarkable, except at the periphery are several vertical shafts. The shafts lead to the surface and suck in unpurified air. At the bottom of the shaft is a particulate trap with a grate covering a pit. Many of the particulates fall into this grate where they feed a local scavenger (A giant isopod/roly-poly.) Air flows into the basement proper but it is not entirely clean. It eventually is sucked towards the floor grates leading to the courtyard. It is here that it encounters a huge sponge. A variety that is shaped like a barrel. It draws air through the outside, filters it, and expels it into the center. Suction from the courtyard speeds this process. The sponge itself is self regulating. It will continue to grow until its maintenance calories matches its intake. A wooden lattice supports the sponge beyond its normal size. These supported sponges grow up to 3m tall (floor to ceiling) and 30cm in wall depth with a diameter of 3m.

There is a large mudroom that is the only entrance to the clan structure. It is where the people store their outside gear, heavy boots, tools, and breathing apparatus. As someone strips their gear, the still air allows particulates to settle somewhat where another scavenger eats it. Incidentally it acts as a primitive airlock because of two armored doors. Some of the air entering will be contaminated, but it is of no worry because of the great number of air changes per hour that keeps the air fresh.

Personal Ventilation: When leaving home, every person carries with them a cylinder made of thin hammered bronze. This cylinder is perforated at the sides and is filled with a living sponge colony. The cylinder is attached to a stylized half-face mask made of bronze and leather with a leather tube. The person breathes, sucking air through the sponge, and into their mask. The cylinder is 50cm long and is worn on the back. The Valyn have developed a leather harness which supports this cylinder. It has two shoulder straps/suspenders, and a large wide belt around the stomach. Additional leather pouches are attached to the belt for everyday use. The masks themselves are stylized based on clan, personal style, and wealth. Thicker armored versions are made for use in war which attach firmly to helmets.


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