Indoor surfaces trap toxic chemicals and release them later

Most of us think about outdoor pollution before worrying about indoor air. But new research shows our homes and offices can trap chemicals long after their sources are gone.

Indoor surfaces such as walls, floors, and furniture act like hidden storage units for pollutants, quietly releasing them back into the air or onto our skin.

Scientists at the University of California, Irvine have now mapped this process. Their findings show that indoor surfaces absorb far more volatile organic compounds, or VOCs, than scientists previously believed.

The danger doesn’t stop when the spray can runs dry or the cigarette goes out – exposure continues.

Everyday sources of pollution

Cooking, cleaning, or even using perfume releases VOCs into the air. Tobacco smoke and wildfire pollution add extra layers of contamination.

Outdoors, wind disperses many of these chemicals. Inside, the story is different. VOCs settle into walls, painted surfaces, wood, and even cement. Once absorbed, they can stay for months before escaping back into the air.

This cycle means a single activity can create long-lasting exposure. A mop might clean the floor, but chemical traces often remain deep inside building materials. In some cases, people may breathe these compounds a year after the first release.

Chemicals test on walls

To understand this better, researchers recreated a home inside the National Institute of Standards and Technology’s Net-Zero Energy Residential Test Facility.

The team released controlled mixtures of chemicals, including a cocktail of volatile compounds and an insecticide spray with less volatile ones. Instruments tracked how the substances spread, faded, and reappeared.

The results showed that smooth glass or thin films on surfaces hold very little. Porous materials like wood and painted walls, however, absorbed large amounts. These materials acted like deep reservoirs, locking away compounds for long stretches of time.

Walls soak up chemicals

“Scientists in the air chemistry research community have known for a long time that many indoor contaminants can be absorbed by indoor surfaces, but the size of indoor surface reservoirs inside homes and buildings had not been established,” said Professor Manabu Shiraiwa.

“Our modeling found that surfaces inside homes have a much greater size to absorb and hold chemicals than previously realized. We can think of these surfaces as massive chemical sponges that soak up VOCs.”

Why walls hold chemicals

The researchers used a concept called the octanol–air partition coefficient, or KOA, to explain the behavior. Compounds with low KOA values, like isoprene or toluene, stayed in the air and left quickly.

Substances with higher KOA values, such as nicotine or some insecticide ingredients, stuck firmly to surfaces. Once there, they remained far longer than expected.

In some cases, the half-life of a pollutant on a surface was less than an hour. In others, the same process stretched to months or even a year. Very high KOA compounds proved nearly impossible to remove through ventilation alone.

Consequences for human health

Professor Shiraiwa emphasized that the findings have significant implications for human health.

“It means people can be exposed to harmful chemicals long after their initial introduction into indoor spaces, and compounds can later be released back into the air or transferred to humans through direct contact with contaminated surfaces.”

A child playing on the floor may pick up residues left weeks before. A person sitting in a room may inhale smoke compounds even if nobody has smoked there for months. Indoor chemistry makes exposure less about the present and more about the past.

Lingering chemicals, persistent threats

The research also helps explain everyday experiences. Tobacco smoke lingers because nicotine buries itself in surfaces and returns slowly, a process called thirdhand smoke.

Wildfire smoke behaves in a similar way. Some compounds from burning forests escape quickly, but others stick to walls and persist no matter how often windows are opened.

Fresh air helps with some pollutants, but it does not empty surface reservoirs. Once chemicals are inside wood, paint, or cement, they continue to leak out even when windows are wide open.

Cleaning is the only effective way to reduce them. Regular vacuuming, mopping, and dusting physically remove residues that ventilation cannot.

Dangers of indoor chemicals

Every house is different. A home filled with wood and concrete may hold onto chemicals far longer than one with glass and tile. Furnishings and even daily activities add complexity. The research suggests some pollutants can linger for the lifetime of the building.

This work pushes us to rethink indoor safety. The walls around us are not passive. They are active players in chemical cycles that influence our health every day.

Understanding this hidden chemistry may lead to smarter building materials and cleaning practices in the future.

The study is published in the journal Proceedings of the National Academy of Sciences.

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