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Ecology for Insiders

The indoor biome covers as much as six percent of the world’s landmass—and we know almost nothing about it

By Peter Andrey Smith

The view from the HazeCam, which is situated just west of the Jackson Street Bridge in Newark, New Jersey, usually extends for about eight miles. To the east, the skyline of New York City rises and falls along the horizon like a bar graph, buffered by a blue haze of humidity and particulate emissions and ozone. The towers are planted into the rock of Manhattan, and the island of steel-and-concrete canyons covers 59 square kilometers and accommodates 1.5 million people. Technically, the city is much larger: its indoor environment comprises some 172 square kilometers—nearly three times the size of Manhattan’s landmass.

One day recently, I called Rob Dunn, an ecologist who spends much of his days in an office at North Carolina State University in Raleigh. Manhattan, he explained, is hardly an outlier; humans spend roughly 90 percent of each day indoors, inside climate-controlled spaces. By his estimates, this indoor biome—the space that has been constructed for shelter and environmental control—covers as much as six percent of the world’s landmass, more than all the world’s flooded grasslands. This includes the great unbroken megalopolis that covers the US eastern seaboard, along with Mumbai and Maharashtra, Mexico City, Manila, and about 24 other urban areas spread out around the world—each with over 10 million residents.

A relatively inactive person, sitting at a desk at room temperature, emits a convective plume of microbes at a rate of about 37 million per hour.

Dunn reveres old-school naturalists who saw familiar surroundings with new eyes. He’s personally combed through patches of grass in the median strips of Manhattan, looking for ants. “It’s really abstract nature,” he says. “It’s not quite Mark Rothko science, but it’s getting there.” His undeniable takeaway: we are surrounded by nature no matter where we are. (His next book will be titled Never Home Alone.) Dunn, who trained as an entomologist, says he cannot shake the unmistakable differences between ants and humans. “If you look at a lot of insect societies, they have really sophisticated ways of managing the microbes in their nests to keep themselves healthy. As humans, we don’t have that; we’ve been pretty good at killing the most-deadly things, whether through vaccines or reducing our exposure to feces. But when it comes to doing anything much more sophisticated, we have not done very much.” The great indoors remains a largely underexplored and overlooked ecosystem—one that scientists still don’t understand or know how to manage.

In early 2013, when I was living in a third-floor tenement apartment in Brooklyn, I received an envelope from Dunn’s lab. It held four plastic tubes, each containing a cotton-tipped swab. I ran them one by one across my cutting board, my pillow, the interior door frame of my apartment, and finally, outside on the sidewalk, above the frame of the front door. Then I mailed the swabs back. The project, known as “The Wild Life of Our Homes,” involved about 1,000 other citizen-scientists spread out across the US. It wasn’t the only project of the sort, and in the ensuing years, scientists began sequencing the DNA pulled from all the swabs and identifying what microbes live in our homes.

Several years later, when I learned the full range of organisms that had been sharing my apartment (not just the human partner, the cockroaches, and the mosquitoes), the firehose of data was at once overwhelming and reassuring. Dunn and others have identified upward of 200,000 species of bacteria, viruses, fungi, and protozoa that pass through the air and water and the heating, ventilation, and air conditioning systems of indoor spaces. And the vast majority of them do not ordinarily cause disease.

In fact, the biodiversity found indoors underscored the important interactions between humans and microbes. In private homes and apartments in the US, the source of most bacterial DNA tends to be associated with humans and pets, especially dogs. One researcher observed that a relatively inactive person, sitting at a desk at room temperature, emits a convective plume of microbes at a rate of about 37 million per hour. Subsequent studies show that this personal microbial cloud can be used to tell, with relative certainty, whether an inhabitant is male or female. Not surprisingly, increasing air flow (by ventilating a home) appears to displace, or clear out, these microbial clouds. The next challenge is to trace how microbes are transmitted from person to person indoors with little or no direct contact, and which organisms exert a protective effect against disease.

In a study published in Proceedings of the National Academy of Sciences that looked at patterns of fungal DNA collected from people’s door frames, Dunn found he could identify the source of that dust sample within about 50 kilometers; samples from urban areas were generally more homogeneous than those of rural areas. In ecological terms, the indoors may function as a kind of nutrient-starved cave. Perhaps little grew on the flat walls and surfaces in the home, which functioned more as a dumping ground—an assemblage of invisible organisms that waft in from outside or emanate off our bodies. Particularly where water accumulates, true ecological communities may begin to form and adapt to living in specific niches. Dunn, for instance, has studied microorganisms living in salt shakers and in dishwashers. This is not without consequence. Washing dishes by hand correlates with lowered risk of childhood allergies, although the cause remains something of a mystery. More recently, he and his colleagues sent out 500 kits for people to swab their showerheads. “Almost certainly, some shower heads are more likely to favor pathogens than others,” Dunn says. Simply knowing which showerheads are least likely to favor pathogens could make bathing, well, cleaner.

The world has always been awash in microbes, and as far back as humans began recording stories, cities were cast in opposition to nature as epicenters of plagues and pestilence. Interventions in building infrastructure are often predicated on comfort, and occasionally on protecting against pollution and disease. The view of indoor spaces as unique biological environments worthy of study has taken a back seat to the medical view of human biology. For good reason: urban sanitation and chlorination campaigns are among the most important feats in public health, eliminating waterborne diseases such as typhoid, cholera, and dysentery.

Yet even as more and more people in urban areas gain access to improved drinking water and modern medicine, the end result has not been a world free of disease. Our insistence on wiping out certain germs inadvertently eradicated and perturbed other species, all of which comes with unintended consequences.

Ironically, sealing off buildings to save energy, exacerbated by the oil embargo in the 1970s, coincided with the emergence of so-called sick building syndrome. White-collar workers experienced beguiling symptoms associated with poorly ventilated indoor spaces, although many experts initially questioned whether sufficient evidence linked molds or the “off gassing” of industrial pollutants to reports of illness. Then, in 1976, epidemiologists decisively traced an outbreak of infectious disease among American veterans at a convention center in Philadelphia to the building’s design. They showed that the respiratory illness now called Legionnaires’ disease was caused by bacteria-laden airborne particles that drifted from air-conditioner cooling towers into the ventilation intakes.

Since then, one thing has become clear: Uninvited guests, commensals and parasites, live in our midst, and not all of them exert an equal effect on our lives. Some domesticated animals, especially dogs, appear to lower allergies in children by altering the composition of bacteria of house dust—whereas chickens kept in proximity to homes can serve as hotspots for the emergence of antibiotic-resistant bacteria, even when the animals are not routinely fed these drugs.

Indoor ecology is a lot like ecology as a whole: direct evidence is hard to find, and simple notions of cause and effect don’t apply to complex systems. Because there are so many variables, so many changes over time, and so much that remains unknown about microbial exposure, many theories attempt to explain a single epiphenomenon. Take the prevalence of digestive disorders. Some blame a westernized diet, along with pasteurization and the eradication of certain microbes traditionally found in fermented foods. But another idea, known as the “cold chain” hypothesis, suggests that changes to our health may have come in the wake of widespread refrigeration and the unintended introduction of cold-hearty pathogens.

Intriguing questions in the field abound. Does dilution of species diversity increase disease risk, or does biodiversity amplify disease? If outdoor parks and green spaces act as deterrents (and sometimes accelerants) for certain diseases, can the indoor equivalent—our microbial garden—ameliorate diseases? To improve human health, should we restore certain native species—or artificially seed the air with soil and dust kicked up by domesticated farm animals?

As Dunn sees it, humans never had a choice about whether to invite thousands of species into our homes. But he suspects we may soon have a choice about which species are intentionally cultivated in the indoor environment where we spend 90 percent of our lives. Ultimately, indoor ecology could reshape architectural design.


Peter Andrey Smith is a Brooklyn-based reporter who has contributed to the New York Times Magazine, Smithsonian, and Scientific American.
Header Image: ©David McClenaghan
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