What would Sherlock Holmes have made of the dust in your house? Besides whether or not you’re a scrupulous house cleaner, it says more about you than you than you probably would have ever guessed.

With electron microscopes, DNA sequencing and other fancy tools, researchers are looking at dust like no one has before and discovering some interesting surprises. For instance, the bacteria and fungi it contains can predict the ratio of men to women in your house. And a dab of your dust can reveal — with 92 percent accuracy — whether you own a dog.

In a joint study from three universities, including North Carolina State University (NC State), researchers asked more than 1,100 households to to swab the dust from the tops of their door frames and computer screens, among other places, and send in dust samples. (Why the tops of doors? Because people rarely clean there.) The project was called The Wild Life of Our Homes.

DNA testing revealed the dust from each household contained an average of 2,000 species of fungi and 5,000 species of bacteria. (If this doesn’t make you want to haul out the vacuum cleaner, what will?)

“We found tens of thousands of bacteria that no one knows anything about — they don’t even have names,” said NC State biologist Robert Dunn, PhD, co-author of the study.

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What’s in dust?

Dust is made up largely of human and animal dander, specks of insect carcasses and droppings, microscopic fibers of bedding and clothing, and of course, microorganisms. The fungi in dust tends to come from outside, either by being tracked into the house or through the window or ventilation system.

Researchers say mapping the community of organisms that inhabit household dust — its microbiome — is a first step in finding out the composition of the microscopic wildlife teeming in our homes. Dunn likens these “maps” of dust’s denizens to maps of the New World in Columbus’ time. He predicts they will eventually be used in everything from home design to allergy research.

In the meantime, here are a few things you didn’t know about your dust.

It can tell the ratio of men to women in your home. For example, men shed more bacteria than women do, including armpit bacteria (Corynebacterium) that contribute to body odor. So the dust in male-heavy households has more of this bacteria. Similarly, a fecal bacteria, Roseburia, is more plentiful in homes with fewer women. Scientists speculate this may have something to do with skin biology, body size or even hygiene practices.

It contains bacteria you track in from outside. In fact it shows the bacteria inside your home is more diverse than that outside it. Why? People bring different varieties home from work and public places. Outdoor bacteria is more likely to be blown or rained away.

It’s teeming with fecal bacteria. A thin patina of fecal microbes was found in houses across the United States (and in an earlier pilot study, even in the most squeaky-clean homes). As Dunn wrote in his blog, “It is the patina of success, the success of animals, if that is your point of view, or perhaps more accurately, of microbes, those beasts who evolved to take rides all around the world, in guts.”

The dust on your toilet seat and on your pillowcase are remarkably similar, microbially speaking. But it’s not as bad as it sounds. “The two largest contributors to dust in the house are human skin and soil from outside the home,” explains study co-author Albert Barbarán, a graduate researcher in the University of Colorado Boulder’s department of ecology and evolutionary biology. “Toilet seats and pillowcases look similar because both have so much bacteria from human skin.”

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Fungi in your dust can reveal where you live. Dust fungi can tell scientists, within a few hundred kilometers, where your house is located. Houses in the East had different fungi than those in the West (although dust on both coasts was more similar than dust from the Midwest), and houses in drier areas had different fungi than more temperate climes. Dunn suggests that this might be useful in forensic applications, something he points out Holmes was doing in his fictional world with dust back in the 1800s.

Dust sticks around. Researchers found dust containing DDT, a pesticide that has been banned in the United States for decades.

Not all bad

House dust may have some useful effects. Barbarán told SafeBee that children in rural homes with more dust have been found to have stronger immune systems compared to kids who grew up with less dust. In addition, children who grow up from an early age among cats and dogs (and the dander and microbes they add to household dust) have fewer allergies.

Related: How to Survive a Dust Storm

Kathryn Olney is a freelance writer and editor who has served as a reporter and editor for California, San Francisco and Mother Jones magazines.