6 THE PROBLEM WITH ABUNDANCE

What would an ocean be without a monster lurking in the dark?

—WERNER HERZOG

GENERALLY SPEAKING, we tend to dislike species that succeed, unless of course we can eat them. We now control so much of the planet that species that succeed almost invariably do so at our expense. They eat us or our food or the things we have made, such as our homes. Since we first built homes, species have been nibbling them back to the ground. In the story of the three little pigs, it was the wolf that knocked the houses down because he was after the pigs. In real life the species that knock our homes down are far more likely to be far smaller than wolves and yet no less dangerous. Just which species threaten our homes depends on how and where the house is built. Stone houses can last thousands of years, which is why some of the buildings from the very earliest civilizations are still standing. Mud houses, too, can last, so long as conditions are dry. But most of our houses are made of dead trees, and many species can eat dead trees. Termites, of course, can bite into wood; they then rely on specialized bacteria in their guts to digest the wood. But the grand masters of destruction are fungi.

In a dry house, fungi can be relatively inconspicuous. But when water pours onto walls or floors, it allows fungi to grow. Fungi creep up the gradient of moisture, eating. If you could hear them, the sound would be terrible, the sound of their hyphae drilling holes in the cells of ancient wood, cracking them open one by one. Fungi eat and creep using their hyphae. As fungi contract hyphae in one region and expand in another, they can actually move from place to place; they crawl in slow motion. For fungi, your walls are full of nutrients. Fungi can eat nearly everything a wooden house is built of so long as they have enough water and time. Fungi eat wood. Fungi eat thatch (fungi also compete with bacteria for the little bits of food present in dust). Given hundreds of years, fungi can even release chemicals that break down bricks and stones. In their growth, everything fungi do becomes magnified. They more quickly degrade wood and paper. They produce more spores, more toxins, more everything. When abundant, some fungi can eventually turn a house back to soil, much as they would a log. But long before that happens, they can also cause other problems. They can be dangerous if accidentally consumed. Some fungi can trigger allergies and asthma. Then there is Stachybotrys chartarum: toxic black mold. Toxic black mold can reach great abundances in houses. When it does, it seems, often, to be at our expense.

If we understand any fungus species in homes, this conspicuous mold should certainly be one of them. Stachybotrys chartarum should not be a species that offers us surprises. If you see Stachybotrys chartarum in your house, most housing professionals will advise you to call a mold abatement company. Such companies will come in and get rid of all of the visible Stachybotrys chartarum fungus in your house. Your books may be scrubbed and scrubbed (or even thrown away), your clothes treated or maybe thrown out too. This drama is reenacted again and again. The details and protagonists differ. The perceived villain stays the same, but so too does a barbarous ambiguity about what is actually going on.

Despite having spent years reading about and thinking about fungi, I didn’t really understand the story of Stachybotrys chartarum until I met Birgitte Andersen. Birgitte is an expert on the fungi of houses. She studies two things: what eats the building materials in houses and how those species, species most would regard as sinister but that she considers fascinating, get into houses in the first place. Birgitte spends a lot of time with Stachybotrys chartarum.

I emailed Birgitte asking to meet, and she invited me to travel from central Copenhagen, where I was staying, to her university, the Technical University of Denmark (DTU). I traveled by bike. The day was, relatively speaking, a sunny Danish day, which is to say that when I parked my bike at her building, I was soaked from the rain. In my damp clothing, I felt fungal. We would talk about fungi. The ambiance was perfect, albeit discomfiting.

Birgitte’s office is on the second floor of a building dedicated to technical science, the use of fancy equipment to solve applied problems. In this building, Birgitte is a hold-out. She loves fungi. She is dedicated to the study of fungi. For work, she grows fungi and then carefully, painstakingly, identifies them under a microscope, takes pictures of them, and adds them to her guide of the common and rare fungi of Denmark. After work, as a hobby, she does much the same, just not for pay. She finds the fungi beautiful and each fungus beautiful in a different way. Fewer and fewer people each year seem to have the skills or the obsession necessary to grow and identify fungi; she has both. Once, she had many colleagues who shared her passion, people to whom she could run around the corner to see and say, “You won’t believe this fungus I am seeing.” But Birgitte’s colleagues with a passion for fungi have retired. And at Birgitte’s university, like many others, few new biologists with the ability to actually grow, identify, and catalog organisms—in this case, fungi—are being hired. One article in the magazine The Scientist went so far as to ask whether scientists with expertise in naming, classifying, and growing wild species were going extinct (the conclusion was yes).¹ Such work is essential. The vast majority of fungal species are not yet named. But the effort to catalog species and their basic biology lacks glamour and so is less likely to be rewarded by hiring committees and funding agencies. Birgitte now stands alone, at the end of her hall, the last person in her building able to identify fungi well, and one of the last few in Denmark.

BY THE TIME I went to visit Birgitte, Noah Fierer and I, along with our collaborators, had worked with the public to sample dust from the door sills of more than a thousand houses. From that dust, we identified the species of bacteria in each sample by decoding their DNA. Later we also did the same for fungi and found a ferocious diversity of fungi in homes and on homes. We found forty thousand kinds of fungi.² Numerically, this was fewer kinds than for the bacteria and yet it was a bigger surprise. Fewer than twenty-five thousand species of fungi—mushrooms, puffballs, and molds—are named in North America. We found a greater diversity of kinds of fungi (or at least kinds of fungal DNA) in homes than have yet been named from North America, indoors or out. Thousands of the species of fungi we discovered in homes are likely not yet named. These nameless fungi spoke to our ignorance not only of what is in homes but also more generally. As for the named fungi, each had a unique story. Inasmuch as the life cycles of fungi are often dependent on other species, the fungi indicated not only their own presence but also the presence of the organisms on which they depend. Some of the fungi were pathogens of grapes and suggested the presence of vineyards. Others were pathogens of specific species of bees (and implied the presence of those bees). Others were parasites able to take over the brains of some (but not other) ants.³ In eastern North Carolina, we found fungi of the genus Tuber, which form symbioses with the roots of trees and then, in order to disperse from one place to another, produce truffles that mimic the pheromone produced by male pigs to attract female pigs. Wooed by the truffle, the female pigs dig it up, eat it, and then, if the truffle is lucky, poop it somewhere else in the woods near a young tree not yet colonized by truffles.

For bacteria in our homes, the emerging story is one in which we have sealed out most environmental bacteria (to our detriment) and instead have surrounded ourselves with bacteria able to deal with extremes, such as the conditions in showerheads, or with our food or our bodily waste. Superficially, because both fungi and bacteria tend to be lumped together as “microbes” along with many other small life-forms, one might assume the same for fungi. But fungi are actually much more closely related to animals than they are to bacteria, so much more closely, in fact, that one of the challenges of controlling fungi is that chemicals that kill fungal cells tend to also kill human cells. Also, unlike the case for bacteria, very few fungal species live on human bodies, whether as pathogens or mutualists. Our bodies are too warm for fungi (it has been argued that warm-bloodedness itself evolved as a way to keep fungi at bay).⁴ The story of fungi in homes, then, could be entirely different from that of bacteria. It has proven to be.

Many of the fungi in houses appear to be species that have simply drifted in from the outdoors. The fungi in houses are very similar to those we find on the outside of houses. Different fungi are found in houses in different regions primarily because the fungi outside of the houses are different.⁵ The effect of the outside fungi on the fungi inside is so great that we can identify the origin of a dust swab in the United States within fifty to one hundred kilometers based solely on which fungal species are on the swab.⁶ Swab your house, send us the swab, and we can tell you where you live (though if you do this, also send a couple of hundred bucks; this turns out to be an expensive party trick). With regard to these many thousands of species, then, the best way to change your exposure to them—really, probably the only way—is to move.

In addition to species that drift in, we also found species that seemingly specialized in life in homes, species more common indoors than outdoors. But we found so many of these species it was hard to know which ones to focus on, hard to know which were best able to move with us from place to place and thrive in our presence. To gather some more insights, I turned once again to the International Space Station (ISS), along with the Russian space station, the Mir. We know that any fungi found on the space stations are really living there, indoors. They can’t have floated in through an open window or hatch, as the case might be. Not even fungi can survive very long in the conditions outside of the space stations.⁷

We know the most about the fungal life on the Mir. Ever since its first launch in 1986, Mir was sampled again and again. Five hundred air samples have been taken to check for fungi. Another six hundred samples were taken from surfaces around the space station. These samples were then cultured, either on Mir itself or back on Earth. The samples weren’t cultured exhaustively,⁸ and yet even so, the results were unambiguous: Mir was a fungal jungle. It was full of more than one hundred different species of fungi. Fungi were found in all but a handful of the more than a thousand samples taken from Mir.⁹ These fungi were alive and metabolizing, too, so much so that one cosmonaut described Mir as smelling like rotten apples (which is perhaps better than the body odor smell of the ISS). As if that weren’t bad enough, at one point the Mir lost contact with Earth. A communication device broke down; the insulation around its wires, it was later revealed, had been eaten by fungi, so the wires shorted out.¹⁰ The fungi have been far more successful, in other words, in establishing themselves in space, having sex and living out many generations, than humans have been. Here, then, is a cautionary tale for any potential attempt to colonize Mars. Long before humans successfully colonize, live on, and have children on Mars, fungi will have done so.

Initially, the ISS was described as being, relative to Mir, if not sterile at least less fungal. Sure, the Mir was colonized by fungi, but it had a reputation for being held together by duct tape and dreams, so maybe that wasn’t surprising. But with time, the life on the ISS also grew diverse and fungal. By 2004, thirty-eight species of fungi were found to be common on the ISS. These thirty-eight species were largely a subset of those found earlier on Mir, which were in turn a subset of those we find in homes.

Many of the fungi found on spacecrafts are described as “technophiles” by the biologists who study them because of their ability to degrade the metals and plastics out of which space shuttles and space stations are made.¹¹ “Technophiles” sounds to me like the name of a boy band playing synthesizers, but it is meant to denote that these species like (“phile”) technology; they like it so much that they eat it.¹² The species already shown to be feeding on the ISS itself include Penicillium glandicola (a relative of bread mold), species of Aspergillus (relatives of the organism used to make the Japanese rice wine, sake), and a species of Cladosporium. Not all of the fungi on board are technophiles, though. On Mir, but not on the ISS, brewer’s yeast (Saccharomyces cerevisiae) was found (perhaps suggesting that the Russians had a better time out in space).¹³ The researchers also found species of the genus Rhodotorula, the pink fungus often seen growing in grout, on shower walls, and, ever so rarely, in toothbrushes and on humans on Earth.¹⁴ Here, then, living among the astronauts, were species that really, definitely thrive in the conditions indoors.¹⁵

We found all of the kinds of fungi present on the space stations in homes. In fact, the species of fungi that were present in the space stations were present in virtually every home we sampled. Just which species were most common depended on the house. Houses with more people in them tended to favor fungi associated with human bodies or foods.¹⁶ The way in which a house was heated or cooled also influenced which species were present. In particular, houses with air-conditioning tended to be more likely to have Cladosporium and Penicillium fungi. These fungi (to which some people have allergies) grow in the air-conditioning units themselves and then spread through homes and offices when the air-conditioning is turned on.¹⁷ When you turn your air conditioner on in your house or car and smell an unusual odor, it is the odor of these fungi exhaling.¹⁸

We will be disentangling the mysteries of our data on household fungi for decades, but one mystery demanded more rapid consideration, the mystery of a species that was absent inside the space stations and rare in our samples of homes, Stachybotrys chartarum. Stachybotrys chartarum is conspicuous as a problem, and yet inconspicuous in our samples. The absence of Stachybotrys chartarum on the space stations might result from the absence of its food source. The ISS, to my knowledge, is devoid of wood or even cellulose, though one might expect it to be able to degrade some of the plastics.¹⁹ But this doesn’t explain its rarity in our study of houses.²⁰

I asked Birgitte about this mystery. I explained our study. I didn’t specifically mention the ISS, but I was thinking about it, floating above us as we talked, distant and yet nonetheless fungal. Birgitte was unsurprised. “Its spores are heavy and held out on sticky slime heads. Why would you find it?” In other words: if it doesn’t float in dust, it shouldn’t be in dust. And then, for emphasis, “Why were you even thinking you might find it?” Birgitte is direct. Why would we indeed. But how, then, I asked her, was it getting into houses if it wasn’t floating in? How was it getting into houses and why had it failed to get into the space stations (when so many other species seem to have no trouble)? “We have,” she said, “done a study that might interest you.”

As we snacked on cookies and nuts foraged from a drawer (each invisibly and inadvertently powdered with a diversity of fungi from the air we were both breathing), Birgitte told me about her study. It focused on the materials out of which modern homes are made: drywall, wallpaper, wood, and cement. Birgitte is not terribly interested in the air in houses. She is interested, instead, in the materials, the pieces out of which houses are built: their bricks, their stones, their sticks, and, especially, their drywall.

EACH BUILDING MATERIAL in homes, Birgitte found, seemed to have its own kinds of fungi—much as might turn out to be true on the space stations, too, if their materials were studied in enough detail. On the cement, Birgitte found fungi of the same sorts one might find on the ground outdoors, a slurry of soil life, including some of the very first species of fungi ever to be studied by scientists.²¹ These fungi were studied by scientists because they were at hand; they were at hand because they lived in the scientists’ homes. She found Mucor, for instance, which Robert Hooke depicted in Micrographia, the book likely to have inspired Leeuwenhoek. She found Penicillium, which Alexander Fleming found by chance in his laboratory (just another building, after all) where he discovered antibiotics. Penicillium uses these antibiotics to weaken the cell walls of the bacteria with which it competes for food, causing the bacteria to explode as they try to grow. We use the same antibiotics to fend off bacteria pathogens such as Mycobacterium tuberculosis, with which we fight for our own survival.

These fungi, Mucor, Penicillium, and the like, were also kinds of fungi that had made it onto a space station.²² That they live both on cement floors and in space stations means that they are fungi we probably need to find a way to live happily alongside. They got past NASA’s control measures, and if they could hail a ride to outer space, they can presumably hail a ride nearly anywhere else too.²³ They may well be the same species that grew on the walls of our ancestral caves; if so, they have traveled from those caves with us wherever we have gone. These are some of the same fungi that, given enough time, eat away at bricks and even stones. On the floors of houses, they may be eating, too, slowly, or they may be using the cement as a habitat (their hyphal fingers holding on) while they actually eat bits of dirt too small to notice or glues and other materials on the cement’s surface.²⁴ These fungi pose problems for people who want to preserve monuments for hundreds of years, but in your basement they are likely to be little more than interesting evidence of the power of fungi to devour, if given time, nearly anything.

On the wood, too, there were fungi. We build many of our homes out of wood and long have. But wood is biodegradable. It is composed of both cellulose and lignin. Cellulose is the stuff of paper; lignin the sturdy stuff that keeps the roof up. Many microbes can break down cellulose, but only fungi and a handful of bacteria are able to break down lignin.²⁵ The fungi Birgitte found on wood in our homes included species that make the enzymes for breaking down at least cellulose and, in some cases, lignin.²⁶ The surprise is not that this also occurs among our two-by-fours and beams but instead how long we are able to prevent it from occurring. Many of the species of wood-degrading fungi that live in homes simply blow in from outdoors, such that their precise composition is determined by the kinds of trees out of which these homes are made as well as by the sorts of forests nearby. Other species, such as the dry rot fungus Serpula lacrymans, are known to have been carried on ships with humans around the world.²⁷ They followed us as we built, again and again, homes made out of their food. They follow gratefully.

It was when Birgitte considered drywall, wallpaper, and plaster covered with paper (and then painted) that things got even more interesting. These substrates were, when wet, full of fungi.²⁸ What was more, those fungi included the toxic black mold Stachybotrys chartarum a full 25 percent of the time. This is even an underestimate of the proportion of wet houses in which Stachybotrys chartarum might occur. After all, Birgitte took just small samples from each home. Stachybotrys chartarum isn’t rare in wet drywall, then. It is so common as to be the ordinary, expected fungus to occur when drywall gets wet. The mix of water and cellulose available in drywall and wallpaper appears to be a perfect substrate for Stachybotrys chartarum. This was a discovery, a big discovery, but Birgitte still needed to explain how the Stachybotrys chartarum was getting into the drywall in the first place.

Stachybotrys chartarum does not float through the air. As far as anyone knows, it does not ride on or in termites or other household insects either. In theory, it might be brought indoors on clothing. Rachel Adams, an expert on indoor fungi at the University of California, Berkeley, learned firsthand about just how many species can ride on our clothes. In one of the most careful studies of the fungi in buildings to date, Rachel found that one of the fungi she detected in a conference room of a university building was brought there inadvertently by a lab mate who had recently visited a mushroom event, where she handled puffballs.²⁹ Fungi ride lab mates. But Birgitte wasn’t really concerned about clothing. She was thinking about building supplies.

What if the mold was in the drywall all along? What if it was being introduced into the drywall when the drywall was being made and then sat there happily, quiescent, until the drywall got wet? This is just what Birgitte proceeded to test, this radical idea, this idea that, if correct, could put her at odds with the multibillion-dollar drywall industry. As she started to look, she realized she wasn’t the first one with this idea. An earlier paper had also suggested the same possibility, but the earlier paper had not tested it.³⁰ She would.

In the United States, academics have some degree of freedom in research, but increasingly it seems to be less than absolute, in no small part because of the great power of corporations. This is not to say academics don’t publish dangerous ideas, ideas hazardous to governments or businesses. It is to say that many American academics have seen enough Hollywood movies to ponder the possibility that one ought to carefully consider the consequences of research at odds with the economic incentives of powerful business leaders.³¹ It is possible that the same worry creeps into the minds of many of Birgitte’s Danish colleagues when they do challenging work. But when I asked her about this risk, Birgitte voiced little concern for the possibility that there might be negative consequences to studying what lives in drywall produced by companies that have an enormous stake in maintaining the status quo (the status quo for drywall, anyway). She just wanted to know what, if anything, was in there. Her emotions were uncomplicated. She was curious. So she looked.

First, Birgitte studied pieces of brand-new drywall, thirteen sheets in total, from four different hardware stores in Denmark. She chose two brands of drywall among those thirteen sheets and three different types of each brand (fire resistant, moisture resistant, and regular). She then cut multiple circular discs from each of the sheets and dipped the discs in ethanol (or, in alternative protocols, just to be sure, bleach or Rodalon), which sterilized their surfaces. Then, for seventy days, she soaked the surface-sterilized samples in sterile water so that any fungi inside the samples might grow. It seemed unlikely anything would really be alive in the dry, brand-new drywall. It was a painstaking longshot—one dependent on careful, tedious work, including the simple but daily task of checking each and every disc for fungi.

Finally, one day, she saw growth. Then, more growth. Birgitte found lurking inside of brand-new drywall the fungus called Neosartorya hiratsukae. This fungus has recently been implicated in the complex mix of causes of Parkinson’s disease. It is unlikely to be the sole cause of the disease, and yet its presence is nonetheless not good news. Neosartorya hiratsukae was on every single sheet of drywall, regardless of type, regardless of which store it came from, and regardless of which company it was made by. Birgitte also found the fungus Chaetomium globosum, an allergen and opportunistic pathogen.³² It was on 85 percent of the pieces of drywall. And then there it was, black and potent, Stachybotrys chartarum on half of the samples.³³ Once it started to grow, it covered the drywall discs, darkening them with life. Nor were these the only species present. Eight other kinds of fungi were also found inside the drywall, waiting.

Now came the real test of whether Birgitte harbored any more fear of the drywall companies than she admitted. Would she publish the results—results that implied the drywall industry had some role in influencing which fungi arrived in your house, some role in potentially affecting your health? Stachybotrys chartarum is often linked to health problems. Neosartorya hiratsukae can be a human pathogen. It is rarely detected on wet drywall in homes but is also hard to spot. It produces small, white fruiting bodies the color of the drywall itself. Because the fungus showed up in each sample, regardless of the store it came from, Birgitte’s results clearly implicated the drywall manufacturing companies. Of course she would publish the work. “What would they do?” she asked me. “Take my job? Then who would identify the fungi?” So it is that we now know, without a doubt, that the fungi in the drywall in homes comes preloaded in brand-new drywall. Birgitte is now working to find ways to kill these fungi in drywall before it is sent to new homes. There is unlikely to be any easy way to kill it in drywall that has already been installed. Any treatment that would kill the fungus in drywall hung in houses is likely also to destroy the drywall and be hazardous to people. Meanwhile, the fungi wait for moisture. Their patience is great.

It is unclear just how the fungi are getting into the drywall, but it is possible that when recycled cardboard is stored for use in drywall production, it becomes a hotspot for the growth of fungi. When the cardboard is then ground up and incorporated into the drywall, the fungi survive the process as spores. Perhaps, Birgitte imagines, the cardboard could be treated in some way. But it isn’t yet. And so, if Birgitte is right, the drywall that comes to your home today is still arriving with the fungi already present. It’s fine; as Birgitte notes, just don’t let drywall get wet.

For good reasons, no one has done the experiment in which one inoculates a house with Stachybotrys chartarum and then looks at the effects on the family therein. Nor has anyone taken houses and wet them to see if (or when) the Stachybotrys chartarum grows and illnesses begin. There are two ways that this fungus might make us sick, though. It could poison us with its toxins, or it could trigger and exacerbate allergies and asthma.

First, the toxins. We know that Stachybotrys chartarum, like many fungi, produces scary compounds called macrocyclic trichothecenes and atranones. Stachybotrys chartarum can also produce hemolytic proteins. If eaten by sheep, horses, or rabbits, these compounds, the proteins especially, cause leukopenia (a deficiency in white blood cells). It has been speculated that these same proteins could also cause pulmonary hemorrhage in human infants. Mice whose noses are injected with the spores of Stachybotrys suffer, though just how much they suffer depends on which strain of Stachybotrys they get. Mice given a strain that produces more toxins suffered “severe intra-alveolar, bronchiolar and interstitial inflammation with hemorrhagic exudative processes.” Put more plainly, their lungs became inflamed and started to bleed.³⁴

But just because Stachybotrys can produce toxins doesn’t mean it necessarily does so in houses. Recently, Birgitte and colleagues developed a new method of detecting the presence of toxins from Stachybotrys chartarum in dust. In doing so, they showed that the more Stachybotrys chartarum was present in rooms of a kindergarten in Denmark, the more of its toxins were present in the dust in those rooms. It is not yet known if the same is true more generally. It might be.³⁵ But to develop an illness would require eating (or, like the mice, sniffing) large quantities of the fungus. An infant in a home in which Stachybotrys chartarum was present, growing abundantly, and producing toxins might incidentally consume large quantities of the fungus and get sick in the way that lab mice and domesticated animals get sick. So far, such a case has never been documented. Neosartorya hiratsukae may be more likely to cause serious health effects from toxins than Stachybotrys chartarum, but it is even less well studied (it is no less common, but much less conspicuous). All of these complexities make Birgitte, one of the world leaders in expertise on Stachybotrys and its consequences, say that she hates when people ask what we know about the health consequences of the toxins produced by indoor fungi. As she puts it, “It is just so damned complicated and difficult to prove.”

But even if Stachybotrys toxins only rarely make people sick, the fungus could potentially still affect us in other ways. When inhaled, Stachybotrys chartarum can trigger allergies. The blood of a relatively high proportion of people shows evidence of an allergic response to Stachybotrys chartarum. In these cases, the exposure to Stachybotrys chartarum may have been outdoors, but in others it is probably from Stachybotrys chartarum grown on drywall in wet houses. In this, Stachybotrys chartarum is not alone. Many other fungi, including fungi that become more common when houses are wet, trigger allergies and asthma.³⁶ The authors of the biodiversity hypothesis, Hanski, Haahtela, and von Hertzen, would argue that what is happening is probably that our lack of exposure to diverse environmental bacteria is making our immune systems more likely to develop allergies. I think they are probably right. If they are, in houses where fungi or other organisms (such as German cockroaches or dust mites) are abundant, those abundant organisms then serve as triggers. The trigger doesn’t matter, I hypothesize, except where a lack of exposure to a diversity of bacteria, or some other precondition, sets the stage.

If the biodiversity hypothesis is right, we might expect the correlations between the presence of abundant fungi in a home and allergies to be complex and contingent. Indeed, whereas some studies find that people in houses with more fungi or more allergenic fungi are more likely to suffer from allergies or asthma, a far greater number of studies show no effect.³⁷ But it might be that reducing asthma and allergy symptoms once they appear is simpler than understanding why and when these diseases emerge in the first place. That seems to be the case. A team, led by Carolyn Kercsmar at Case Western Reserve University, found 62 children who had symptomatic asthma and were living in houses with indoor mold. Kercsmar then randomly assigned the children and their families to one of two treatments. The families of half of the children (the control group) received instructions on how best to manage the asthma, and nothing more. The families of the other children (the remediation group) received the same instruction and the study team went into their houses and removed wet wood and drywall, replaced it with new, dry material, stopped the flow of water into the home, and made alterations to air-conditioning units. After the intervention, the concentration of fungi in the air of remediation group houses decreased by half. The concentration in the control houses was unchanged. More significantly, the children living in homes in which remediation was actively carried out had fewer days in which their asthma was symptomatic than did the control group. This effect held both during the study and afterward. Just 1 in 29 of the kids in the remediation group saw exacerbations of asthma symptoms after the study. In the control group, 11 out of 33 kids did. Hooray, a simple solution!³⁸ The study was small, it was in just one city, and yet it is hopeful in terms of suggesting a way forward.

For now, what we can say is that if your house gets wet, you should figure out how to fix the water problem and dry it out. If you are building a new house, you might avoid drywall, particularly in areas that get wet, because you can’t be sure that it doesn’t already have Stachybotrys chartarum in it. And if an opportunity comes up to help support research on the biology of fungi in houses, sign up. Meanwhile, the fungi on the ISS continue to thrive, a reminder that whatever the right solution is to managing the fungi in our homes, it is, just as for bacteria, very unlikely to lead to their eradication. This is something NASA scientists, the Russians, and Birgitte can agree on.

As for the tens of thousands of other fungal species we have found in homes, each with a story as elaborate as that of Stachybotrys chartarum, they need to be studied. You are breathing them in now, these poorly understood species. Thousands of them are so unfamiliar they do not even yet have names. You could be the one to name them. You’re right to be skeptical of the idea that thousands of species around you are not yet named, but it is true. To some extent, this just reflects our broader ignorance about Earth in general. We’ve only begun to explore the planet. Most of life is not yet named. With bacteria, we haven’t even scratched the surface. For fungi, we are, perhaps, a third done with naming and far less completely finished with what comes next: the study of the details of each species’ biology. For insects, we might, if we are lucky, be half done. But I think there is also something specific at play in homes. In our homes, we tend to study species we know pose danger to humans, but there is no one assigned to study the rest of the species. Basic biologists might study them. But given the choice, most basic biologists would rather go off on trails in the woods, exploring remote locales (such as field stations in Costa Rica). We have blinders on blocking our view of the wildlife that is innocuous and close at hand, a reality that became very clear to me recently when we asked people about what lived in their basements.