Third-hand smoking (THS) has not been given enough importance in comparison to first-hand or passive smoking. While smoking in itself is a harmful practice, scientists have now found that THS also has a negative impact on people’s health, especially in the long term. THS takes place when exhaled smoke and smoke emanating from the tip of burning cigarettes gets on surfaces such as clothing, hair, homes, and cars and it may adversely affect health and the effects worsen with time. The findings, published in the journal Clinical Science, suggest t The findings, published in the journal Clinical Science, suggest that third-hand smoke toxins could be damaging to liver and brain tissues. Using a system in which the exposure of mice to third-hand smoke mimics that of human exposure in the homes of smokers, the researchers investigated the effects of third-hand smoke exposure on biological molecular markers – or “biomarkers” – found in serum, and in liver and brain tissues. The liver plays a major role in metabolism and detoxification; the brain plays significant roles in behaviour. “Our goal was to determine the minimum amount of time required to cause physiological changes in mice when they are exposed to THS, using an exposure system that mimics human exposure,” said lead researcher Manuela Martins-Green, Professor at University of California. “We found that THS exposure as early as one month resulted in liver damage. THS exposure for two months resulted in further molecular damage, and at four to six months caused even more such damage. We also found that the mice showed insulin resistance after long-term THS exposure.” Damage to the liver can hinder its capability to detoxify the body, leading to more damage by third-hand smoke toxins. Martins-Green and her team examined the brains of THS-exposed mice and found that stress hormones, such as epinephrine, increased in one month of exposure. Additional stress hormones were seen at two months, four months, and six months, eventually causing immune fatigue in the mice. “THS is a stealth toxin, a silent killer,” Martins-Green said. “Contaminants can be absorbed through the skin and through breathing. Although our research was not done on humans, people should be aware that hotel rooms, cars, and homes that were occupied by smokers are very likely to be contaminated with THS.” The third-hand smoke toxins, which are invisible but can be smelled, remain on surfaces for many years, and are resistant to even strong cleaning agents.