‘Thirdhand’ smoke can hitchhike to non-smoking sites

Even buildings that strictly enforce bans on indoor smoking may never be smoke-free. A new study shows that “thirdhand smoke” — the type that sticks to surfaces — can end up indoors, even in supposedly smoke-free spaces.

So people could end up breathing in harmful chemicals from cigarette smoke, even when they were in a room where no one ever smoked.

Inhaling smoke directly through a cigarette can lead to cancer and many other health problems. The cigarette smoke that people nearby inhale is known as secondhand smoke. It, too, can harm the lungs. Thirdhand smoke is what’s left behind after someone has finished smoking. It’s a mix of nicotine and other chemicals that can create that stale, smoky smell after any visible smoke clears. It sticks to clothing, furniture and walls, where it can linger for hours or days.

Thirdhand smoke is not benign. It, too, can react with other molecules in the air to form cancer-causing substances. 

Peter DeCarlo was surprised to find thirdhand smoke in a classroom at Drexel University in Philadelphia, Pa. DeCarlo is an atmospheric scientist at Drexel. He was setting up an experiment to study the effects of heating and air conditioning on particles in the air. As far as anyone knew, no one had ever smoked in this room being tested. It certain didn’t smell smoky.

DeCarlo wasn’t looking for thirdhand smoke. But once he found its chemical “fingerprint,” he did some detective work to investigate how it got there.

Harmful hitchhikers

Air is a mixture of gases, including nitrogen and oxygen. Microscopic particles of things like pollen, water vapor, soot and pesticides can float in the air. These are called aerosols. DeCarlo’s research team sucked up a small amount of air from the classroom.

Then they used a machine to identify the aerosols in that air. This mass spectrometer (Spek-TRAH-meh-tur) found that nearly three in every 10 of these particles appeared to come from thirdhand smoke. To explore why, they took a closer look at the chemistry of those particles in the air.

Anita Avery is a graduate student working with DeCarlo. She pumped cigarette smoke into a glass container. To do this, Avery used an artificial-smoking machine. This device draws air through a burning cigarette (much as a smoker would). Some of the emitted smoke particles remained stuck to the glass walls, even after the air had cleared.

The next day, Avery pumped fresh air through the container. Then she measured what was in the air as it left the container. Thirdhand smoke particles showed up in what should have been clean air.

Those particles must have come off of the glass walls and hitched a ride on passing aerosols, the researchers now conclude. Nicotine and other chemicals in thirdhand smoke are alkaline — chemicals with a pH higher than 7 (on a scale of 14). Nicotine and other weak bases have a pH close to 7. Those with a far higher pH are considered strong bases.

The researchers now think that as some strong base in air passed by the glass, the nicotine reacted with it and became unstuck. It then attached onto neighboring aerosols, DeCarlo suspects. That would allow the nicotine molecules to float into different rooms as air circulates throughout a building’s heating and cooling ducts.

Ammonia is one example of a strong base that’s common inside buildings. In fact, DeCarlo points out, ammonia is an ingredient in many cleaning products. An ingredient of sweat and urine, it is also released by the human body. Similar reactions with thirdhand smoke particles on someone’s clothing might also release them back into the air.

The researchers described their findings May 9 in Science Advances.

Story continues below video.

---

Drexel University chemist Peter DeCarlo explains how thirdhand smoke stuck to indoor air particles and traveled through a non-smoking classroom.  
DeCarlo Laboratory /Vimeo.

---

Stealthy smoke

“This study opens our eyes to the steps that can occur after tobacco smoke enters the air,” says Charles Weschler. He is a chemist at Rutgers University in Piscataway, N.J., who did not work on the new study. But Weschler’s team does focus on indoor-air pollution and the role that clothing can play in transporting it.  

DeCarlo’s group did not study what the new findings might mean for health. The researchers do note, however, that thirdhand smoke could be a hidden danger for nonsmokers in rooms or other spaces shared with smokers. For example, thirdhand smoke could linger in rental cars. Or nonsmokers might encounter the particles on a crowded bus — even in a “non-smoking” hotel room.

DeCarlo usually studies outdoor air pollution. But his new data show why indoor pollution is important. “We spend most of our days in buildings,” DeCarlo points out. What that means, he says, is that “indoors is where most of our exposure to air pollution happens.”