There are many contributing factors to the air quality of a home or building.

These include contaminants in:

The HVAC System.

The Carpets.

The Attic.

The Crawlspace.

As sure as the sun rises, houses collect dust. It gathers on our knickknacks and dirties the carpets. More than just dirt, house dust is a mix of sloughed-off skin cells, hair, clothing fibers, bacteria, dust mites, bits of dead bugs, soil particles, pollen, and microscopic specks of plastic. It’s our detritus and, it turns out, has a lot to reveal about our lifestyle.

For one thing, dust is far from inert. Those shed hairs and old skin cells can soak up a constellation of contaminants originating from consumer products that we bring into our homes. Other environmental contaminants can be tracked indoors on the soles of our shoes. So in addition to fluffy hair and garden dirt, dust can hold a witch’s brew of persistent organic pollutants, metals, endocrine disruptors, and more.

Not only does dust hold a long memory of the contaminants introduced to a house, but it’s also a continual source of exposure for the residents. Dust gets resuspended when it’s disturbed and will recirculate throughout the house, picking up substances before returning once more to the floor. “Year over year, dust accumulates in the home,” says Miriam L. Diamond, an environmental chemist at the University of Toronto. Even after regular cleaning, it still accretes because homes are tightly sealed environments, and the dust gets entrenched in carpets and crevices. Dust from an old house may retain legacy pollutants such as DDT that were banned almost half a century ago, she says.

Scientists study dust to try to get a handle on both of these roles: as a proxy to better understand what chemicals are in our surroundings and how they move, and as a way to characterize what exactly we are exposed to via dust. The relationship between dust and human health remains uncertain. Researchers know that dust is an important source of exposure to certain pollutants—especially for infants and toddlers, who spend 90% of their time indoors, put almost anything in their mouths, and are more sensitive than adults to many of the compounds found in dust. But they haven’t nailed down the extent of health risks from dust exposure nor which compounds and sources are of greatest concern. And many compounds remain unknown. “The few to a hundred compounds that we know are in dust don’t encompass the universe of chemicals in commerce, which number in the tens of thousands to over a million,” says P. Lee Ferguson, an environmental chemist at Duke University. To reveal the full spectrum of chemicals in dust, researchers are turning to high-powered analytical tools. Dust is no longer something to sweep under the rug.

Scientists first realized that dust had a story to tell about environmental health in the 1940s when they measured human pathogens stuck to the dust in operating rooms to monitor cleanliness. In the 1970s, researchers began assessing house dust for lead from paint and gasoline as a way to determine the levels children might be exposed to. And in more recent studies, researchers have found carcinogenic compounds such as now-banned polychlorinated biphenyls (PCBs), once used in electrical cables and wood floor finishes, and endocrine disruptors such as phthalates, which soften vinyl flooring and other plastics.

Researchers are still building their understanding of the complex ways that volatile and semivolatile compounds interact in our surroundings, sorbing onto and desorbing from surfaces. They know that consumer products—vinyl flooring, personal care products, electronics, furniture, carpet pads, paints, cleaning products, and more—have a strong driving force to shed compounds into materials with lower concentrations of the substances. For example, a flame retardant might volatilize off the plastic parts of a TV set into the air, stick onto airborne particles, and move into dust, which settles on floors and carpets. The compounds will continue to migrate until they reach equilibrium with the surroundings, says Diamond. And heating the product, such as turning on a computer, also speeds migration into the home environment; a compound will condense in a cooler part of the room, where dust often resides.

High-molecular-weight compounds, such as the flame-retardant decabromodiphenyl ether, don’t volatilize but instead enter dust when people physically knock fibers or minute bits of plastic off couches or computer cases. “Another mechanism that we stumbled on is direct transfer or diffusion into dust,” says Stuart Harrad, an environmental chemist at the University of Birmingham. For instance, if dust settles onto a TV set or Wi-Fi router, there is a very good chance that flame retardants will migrate directly into the dust.

With people in the room, things get even more complicated. “Just like the ‘Peanuts’ comic strip character Pig-Pen, people walk around in a dust cloud all day,” says Heather M. Stapleton, an environmental chemist at Duke University. People add to the dust’s organic load as their warm bodies volatilize deodorant or fragrance compounds from personal care products. “Our skin cells and clothing fibers may also accumulate chemicals from the air before they are then shed to dust, where they can accumulate yet more chemical,” Diamond says. Those compounds can be absorbed through skin, inhaled, or ingested when people put dusty hands to their mouths, complicating the scientist’s task of determining which exposure route is most important.

Most research has focused on identifying individual classes of compounds in dust, like the polybrominated diphenyl ether (PBDE) flame retardants found in furniture foam, carpet pads, and electronics; phthalates such as those found in vinyl flooring; or pesticides tracked in on shoes or evaporated off pet collars. Now, researchers are trying to get a more comprehensive view of the mixtures people are exposed to by probing the overall contaminant load in house dust. By combining toxicity tests with emerging methods for determining a complete profile of compounds in dust, researchers may be able to determine what chemicals or combinations of chemicals are most toxic, Stapleton says.

In one new approach, scientists combed through two dozen dust studies of 45 compounds to create a snapshot of nationwide exposures, says Robin E. Dodson, an exposure scientist at the Silent Spring Institute. She and Veena Singla, a staff scientist at the Natural Resources Defense Council, ranked the substances according to the amount in dust and estimated intake and health hazard. The phthalate plasticizer di(2-ethylhexyl) phthalate, known as DEHP, topped the list. Phthalate plasticizers make plastic more pliable and are found in vinyl flooring, food containers, and cosmetics. DEHP can disrupt hormone function in human and animal studies and is linked to reduced sperm motility in men. Other compounds on the list include phenol preservatives found in deodorants and cosmetics; flame retardants; a fragrance compound known as Galaxolide, or HHCB; and perfluorinated stain repellents (Env. Sci. Technol. 2016, DOI: 10.1021/acs.est.6b02023).