A new study from researchers at Drexel University has found that third-hand smoke – chemical residue from cigarette smoke – can spread more pervasively than previously thought, even moving throughout a seemingly "smoke-free" building. But despite this compelling research, the dangers to human health still remain unclear.

The research came about when a doctoral student stumbled across some unexpected data. The air inside a non-smoking classroom was being studied for an unrelated project examining the movement of particles in the air from outdoors to indoors. Upon examining the results, the student, Anita Avery, came across some startling data.

"In an empty classroom, where smoking has not been allowed in some time, we found that 29 percent of the entire indoor aerosol mass contained third-hand smoke chemical species," says Avery. "This was obviously quite startling and raised many questions about how that much third-hand smoke could be lingering in a non-smoking, ventilated room."

Third-hand smoke is generally considered to be particles from cigarette smoke that settle on surfaces long after a cigarette has been extinguished. This particulate matter can be absorbed through the skin, ingested or even inhaled if reemitted into the air.

This new Drexel research reveals a new exposure route for these third-hand particulates. Subsequent research from the team found that these chemicals can attach to passing aerosols and get back into the air, ultimately being transported into environments considered to be "smoke-free".

While this study importantly highlights the way chemical pollutants can travel into seemingly protected indoor environments, questions still remain over how harmful third-hand smoke exposure actually is to humans. Co-author on the research Michael Waring suggests, "This study shows that third-hand smoke, which we are realizing can be harmful to health as with second-hand smoke, is much more difficult to avoid."

But not everyone is convinced that this study does anything more than simply describe a novel way for chemical particulates to move between environments. Peter Hajek, Director of the Tobacco Dependence Research Unit at Queen Mary University of London points out that no connections to health concerns can be made from this research.

"As is increasingly common however in publicity hungry press releases, the claims about health hazards and toxicity suddenly appear out of thin air," says Hajek. "The study documents no health risks, but without such claims, the findings are not newsworthy."

The health hazards to humans surrounding third-hand smoke are notably unclear. Several studies have recently begun to make a case for toxic health effects but they are primarily animal experiments or studies on cell cultures. Mice studies do suggest third-hand smoke exposure can increase the risk of lung cancer and liver disease.

It has been suggested that infants are the biggest group at risk of health problems related to third-hand smoke. Young children crawling on the ground and mouthing objects are the most prone to coming in contact with, and ingesting, these third-hand particulates.

But is third-hand smoke more of a danger than simply inhabiting a smoggy city? How dangerous to human health is a room that was previously inhabited by a smoker? Whatever the answers, it is undeniably clear that third-hand smoke is a real thing. A study published earlier this year examining the legacy of years of smoking in a casino revealed chemical residues present months after smoking was banned from the premises.

However, further research still needs to be done to clarify the dangers of third-hand smoke to human health. As Peter Hajek makes clear, "It is the dose that makes the poison and to make any statements on this, the exposures detected here would need to be compared with e.g. air quality or occupational health standards."

The study was published in the journal Science Advances.