Geoffrey Kabat is the author of Hyping Health Risks: Environmental Hazards in Daily Life and the Science of Epidemiology.

“There are enough scientifically documented harmful effects of exposure to cigarette smoke without concocting catchy but uninformative concepts that, while likely to attract the attention of the jaded media and its audiences, confuse the important issues regarding the health effects from exposure to cigarette smoke.”—Geoffrey Kabat

The current issue of the journal Pediatrics (January, 2009) carries an article titled “Beliefs about the Health Effects of ‘Thirdhand’ Smoke and Home Smoking Bans”. The study has been reported by the New York Times, the BBC News, and many other news outlets.

The authors of the article, tobacco researchers from Harvard and several other institutions, defined “thirdhand” smoke—in contrast to the better-known secondhand smoke—as the “residual tobacco smoke contamination that remains after the cigarette is extinguished.” They characterized secondhand smoke as the smoke that is visible while the cigarette is being smoked.

Having defined this “novel” exposure, the researchers conducted a survey to determine whether respondents thought that exposure to thirdhand smoke was harmful for children compared to secondhand smoke. Their results indicated that the vast majority of respondents (both smokers and nonsmokers) believed that secondhand smoke harms the health of children, and that smaller but still sizeable proportions of smokers and nonsmokers believed that thirdhand smoke harms children.

The authors conclude that educating parents about the hazard of thirdhand smoke may contribute to making homes smoke-free. (The home is the principal remaining venue where smokers can freely exercise their habit). Few people in public health would quarrel with this objective.

However, since the article presents itself as science (with representative samples, multivariable analysis, statistical tests, and references to the literature), it is fair game to look at the validity of their concepts and inferences.

The authors’ distinction between secondhand and thirdhand smoke is artificial and betrays an ignorance of the relevant science. The preferred term used in studies of nonsmokers’ exposure to the cigarette smoke of others is “environmental tobacco smoke,” or ETS. ETS consists of a mixture of sidestream smoke emitted by the burning tip of the cigarette and the smoke exhaled by the smoker. Over time the smoke is diluted, ages, and gets deposited on surfaces.

I asked Roger Jenkins, an analytical chemist who has done extensive work on exposure to ETS and has written a major textbook on the topic, what he thought about the concept of “thirdhand smoke.” He made the following points: “ETS is made up of two phases, the vapor phase and the particulate phase. Secondhand smoke is NOT the stuff you see, and “thirdhand smoke” is NOT the stuff you don’t see. If someone wants to use the term secondhand smoke, he must acknowledge that it contains BOTH the stuff you see (particles) and the stuff you can’t see (vapors and gases).”

What is most relevant for human health is the total quantity of smoke a nonsmoker is exposed to while in the home. For this reason, studies of exposure to ETS make use of the average airborne concentration of markers of tobacco smoke, such as airborne nicotine, or other compounds in tobacco smoke, and/or markers of actual exposure, such as 24-hour urinary cotinine (a breakdown product of nicotine).

Since the distinction between secondhand and thirdhand smoke is an artificial one, questioning study subjects about their beliefs concerning the dangers of thirdhand smoke is questionable.

In the opening sentence of their abstract, the authors state that “there is no safe level of exposure to tobacco smoke.” While there are different theoretical models regarding effects of exposure at low levels on carcinogenesis, in practice there are levels below which any effects are likely to be trivial or undetectable. In fact, the dose does matter. Apart from the degree of ventilation, humidity, room size, and other characteristics of the home, the single greatest determinant of exposure is likely to be the number of cigarettes smoked in proximity to children.

One aspect of the study that appears valid is the researchers’ implied reference to toddlers, who are likely to ingest some amount of tobacco smoke constituents deposited on floors and other surfaces. If the authors had focused on this special case in order to educate the parents of newborn children about the hazards of smoking, this would have been a better-conceived project.

The special case of toddlers is yet another reason for parents not to smoke in the home, in addition to the greater susceptibility of infants and children to the adverse effects of smoke exposure, the fact that unextinguished cigarettes can cause fatal fires, and the vitally important fact that parental smoking has a strong influence on whether or not children grow up to become smokers themselves.

There are enough scientifically documented harmful effects of exposure to cigarette smoke without concocting catchy but uninformative concepts that, while likely to attract the attention of the jaded media and its audiences, confuse the important issues regarding the health effects from exposure to cigarette smoke.