Most people are aware of the dangers of exposure to second hand smoke. But researchers have found that a surprising quantity of “third-hand” smoke — the lingering chemical residue of tobacco that clings to surfaces long after a cigarette is extinguished — can also be found indoors where there has been no active recent smoking.

“Third-hand smoke, which we are realizing can be as harmful to health as second-hand smoke, is much more difficult to avoid,” says Associate Professor Michael Waring.

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Air quality measurements taken from an unoccupied, nonsmoking classroom indicated that almost 30 percent of the particulate matter small enough to be absorbed by the human lung was associated with smoke residue.

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Researchers pumped cigarette smoke into a Pyrex container and then pumped it back out and cleared the container with fresh air. A day later, they circulated filtered outdoor air through the container and measured the aerosol particles inside. Compared with outdoor air, the container had 13 percent more third-hand smoke chemicals.

To investigate this surprising finding, Waring teamed up with Associate Professor Peter DeCarlo and graduate student Anita Avery to simulate third-hand smoke exposure in a lab. In their controlled test using a closed container, they found a 13 percent increase in third-hand smoke chemical species, which meant that, though it seemed as though the smoke had cleared, a residue remained.

“Aerosol particles are ubiquitous particles suspended in the air — they come from a variety of sources and are known to be detrimental to health,” DeCarlo says. “The fact that third-hand smoke can attach to them, like it would to the clothing or furniture of a smoker, means that the potentially toxic chemicals associated with third-hand smoke are found in places we wouldn’t have expected.”