A recent New York Times headline warns parents to be on the lookout for "A New Cigarette Hazard: 'Third-Hand Smoke.'" The freshly coined term, introduced by a study reported in the journal Pediatrics, refers to particles and gases that linger in a room after someone has smoked there or in the clothing and possessions of people who have smoked (or been around smokers) elsewhere. The genius of the study is that it tries to stir up alarm about thirdhand smoke without bothering to show that such trace levels of toxins and carcinogens cause any measurable harm to children (or to anyone else). Instead the authors simply assume that thirdhand smoke is dangerous and then do a survey to see how many people are aware of this "fact."
You can get a sense of the researchers' method from the first sentence of their abstract: "There is no safe level of exposure to tobacco smoke." As I noted when former Surgeon General Richard Carmona said something similar in connection with his 2006 report on secondhand smoke, this is an article of faith, not a scientific statement, since it cannot be proven or disproven. But if you start from the premise that there is no safe level of exposure to tobacco smoke, it is very easy to arrive at the conclusion that thirdhand smoke (as well as fourthhand smoke, fifthhand smoke, and sixthhand smoke) is dangerous. Here is another taste of the researchers' approach:
The toxicity of low levels of tobacco smoke constituents has been proved. According to the National Toxicology Program, these 250 poisonous gases, chemicals, and metals include hydrogen cyanide (used in chemical weapons), carbon monoxide (found in car exhaust), butane (used in lighter fluid), ammonia (used in household cleaners), toluene (found in paint thinners), arsenic (used in pesticides), lead (formerly found in paint), chromium (used to make steel), cadmium (used to make batteries), and polonium-210 (highly radioactive carcinogen). Eleven of these compounds are group 1 carcinogens (most carcinogenic designation).
Noting that many of the chemicals in cigarette smoke are toxic or carcinogenic in high enough doses proves nothing about the dangers posed by tiny levels of those chemicals. One searches the article in vain for any acknowledgement of the toxicological principle that the dose makes the poison. Talking to the Times, the lead author suggests we should instead apply the smell test, which tells us to fear a fellow elevator rider who has recently smoked a cigarette:
"Your nose isn't lying," he said. "The stuff is so toxic that your brain is telling you: 'Get away.'"
On his tobacco policy blog, anti-smoking activist Michael Siegel notes that the Pediatrics article "cites just a single study to support its contention that low levels of tobacco smoke exposure are associated with health harm." That study found an association between low levels of cotinine (a nicotine metabolite) in children's blood and their scores on cognitive tests. Siegel, a public health professor who supports smoking bans but believes their advocates routinely exaggerate the hazards of secondhand smoke, details the study's weaknesses, which make it impossible to conclude from it that thirdhand smoke causes brain damage.
"There is no convincing scientific evidence that exposures of this magnitude produce any significant health harm," Siegel writes, "with the one possible exception being children who have asthma and are sensitive to tobacco smoke." He worries that propaganda campaigns focusing on thirdhand smoke may backfire, convincing parents who now make it a point to smoke outside the house or to smoke when their children are not around that such precautions are more trouble than they're worth.
[Thanks to GregA for the tip.]