Once upon a time, decades ago, college students across the nation could smoke cigarettes in class, in their dorm rooms and common areas; and workers could smoke in their offices and factories. The chemicals from those long-ago cigarettes are still around, however, and can re-enter the air and, presumably, the lungs of a new generation of students and workers.

The sticky residue from those once-smoked cigarettes coats walls, furniture and other indoor surfaces. And it can make its way back into the air and into your lungs many years later, researchers have found. Even homes where no one smokes may not really be smoke-free.

“...[T]his research reminds us of just how many chemicals we are exposed to that we probably aren't aware of.”

Welcome to the world of thirdhand smoke.

Cigarette smoke collects on any surface it comes in contact with — whether it's building walls or floors and furnishings such as carpets, couches and curtains. But it doesn't always stay there. Just take a wet sponge to a wall in the home of any smoker and look at the brown residue that comes off.

Researchers at Drexel University studying the particles floating in the air of a classroom where no one had smoked for years recorded substantial amounts of chemicals found in cigarette smoke.

To investigate using more tightly-controlled conditions, the Drexel researchers took a glass container and pumped cigarette smoke into it. They then pumped out any residual smoke from the container and sent outdoor air through to make sure all the secondhand smoke was gone.

A day later, they sent filtered outdoor air through the container and analyzed the composition of the aerosol particles in that air, comparing its composition to outdoor air that hadn't passed through the container. They found a 13 percent increase in chemicals related to thirdhand smoke in the air that went through the container, showing that residue from cigarette smoke can indeed, re-enter the air.

Moisture accelerates the smoke residue's attachment to the aerosols/particles in the air. And ammonia, often found in bathrooms, accelerates the re-entry of thirdhand smoke from walls back into the air.

“Thirdhand smoke is not something we're currently thinking about as a society when we talk about air quality,” said Avery. “It's easy to recognize the presence of chemical pollutants if you can see or smell them, but this research reminds us of just how many chemicals we are exposed to that we probably aren't aware of. That's why we need to keep studying these indoor spaces where people spend so much of their time, so we can build a full profile of what exactly is in the air.”

So how can you eliminate or minimize the thirdhand smoke that's lurking indoors? The MD Anderson Center has some recommendations for those seeking to remove thirdhand smoke:

  • Have carpets, furniture or car upholstery professionally cleaned to remove residual smoke or toxins.
  • Clean all air ducts and replace air conditioning filters in your home or car.
  • Wash or replace curtains, blankets and other fabrics that collect tar and other chemicals. Replace the floor mats in cars.
  • Wash or repaint walls and ceilings to remove or control old toxins.

    • The study appears in Science Advances.