While the dangers of secondhand smoke (SHS) are widely recognized, a new study led by Prof. SUN Yele at the Institute of Atmospheric Physics of the Chinese Academy of Sciences, has uncovered a more persistent and stealthy hazard lurking in indoor environments: thirdhand smoke (THS). This residue—left on surfaces long after a cigarette is extinguished—is not merely a stale odor, but a dynamic source of continuous air pollution.

The study was recently published in Building and Environment.

THS refers to the toxic mixture of chemicals that adhere to walls, furniture, carpets, curtains and other surfaces once tobacco smoke dissipates. Unlike SHS, which is inhaled directly, this residue can continuously re-emit pollutants into the air and undergo chemical transformations over time, posing a long-term exposure risk.

For the first time, the research team mapped the real-time evolution of THS in indoor settings. Using advanced air monitoring technologies, the team tracked both fine particles and invisible gases released from contaminated surfaces.

The findings reveal a notable distinction between secondhand and thirdhand smoke. While SHS particles dissipate rapidly, THS maintains a stable, low-level presence in the air for prolonged periods. More importantly, the chemical "fingerprint" of the particles changes—indicating the residue becomes more nitrogen-rich and potentially more harmful over time.

Additionally, the study identifies how homes trap this pollution. Porous materials such as wool carpets or fabric upholstery act as deep "reservoirs," absorbing tobacco chemicals. These reservoirs then slowly release pollutants, making them resistant to simple ventilation and extending contamination for hours or even days.

The research outlines a three-stage release pattern: an initial burst of gases, followed by a multi-hour phase of steady release of nicotine-related compounds, and finally a long, slow "tail" phase of other chemicals.

"The key takeaway is that THS is not a static stain; it is an active, ongoing source of pollution in a room," said Prof. SUN, the study's corresponding author. "A smoking session may end, but the release of hazardous compounds persists, exposing people to low levels of toxins long afterward. This turns our homes into environments of chronic, low-dose exposure."

The research underscores the need to incorporate THS into tobacco control policies and indoor air quality standards. It also provides new chemical markers for detecting this lingering contamination.

A wool sweater exposed to thirdhand smoke is suspended within a custom-built Teflon film chamber for controlled ozone oxidation experiments. (Image by YANG Liu)