Dust – what even is it? Is it dirt? Is it skin, or waste from mites and other critters? As it turns out, it’s all of those things and more, including detritus from local bacteria and fungi. That we breathe in all of this organic matter may be unsettling enough, but a study from the University of Colorado Boulder found something even more surprising: The dust in your home can tell a scientist who lives in your home, what lives in your home, and where that home is located.

In a joint effort with Rob Dunn of North Carolina State University, CU-Boulder Associate Professor in the Department of Ecology and Evolutionary Biology Noah Fierer launched Wild Life of Our Homes, a citizen science project that asked people to collect indoor and outdoor dust samples from their homes. Altogether, over 1,000 homes across the U.S. swabbed above both indoor and outdoor door frames and sent in their samples for analysis. What they found was staggering.

“We found tens of thousands of bacteria that no one knows anything about – they don’t even have names,” Said Dunn.

The analysis found some 5,000 species of bacteria, along with 2,000 species of fungi. That’s in addition to the dander and detritus given off by the people, plants and pets living in our homes. Waste from mites and other small animals also makes its way in there, resulting in a beautiful menagerie of microscopic grossness.

“Every day, we’re surrounded by a vast array of organisms in our homes, most of which we can’t see,” said Fierer. “We live in a microbial zoo, and this study was an attempt to catalog that diversity.”

Where it gets really weird is what the scientists can learn about your home from the dust they find. One major thing is location – the fungus found in dust is usually there because it blew in from outside, and fungi species vary significantly from region to region. The dust from a home outside of Chicago would have very different bacteria than one in suburban Charlotte.

Perhaps even more creepy is what the bacteria can tell scientists. Bacteria appear to have certain preferences, so they could tell, for instance, if a home had a cat as a pet. Other bacteria, however, indicated the presence of a dog. Surprisingly, they could even tell with some degree of accuracy the ratio of men to women living in a home.

“We can tell if there are more men than women in a home, for example, because those homes have more armpit bacteria,” Dunn says. “Seriously.”

The main takeaway so far, the researchers note, is that there’s precious little we can do to change the air we breathe inside our homes. Frequent dusting and a robust air filtration system help, but if you don’t like the idea of breathing certain bacteria or fungi, you’ll have to either move or find new roommates.