The anti-tobacco extremists’ motto: if you can measure it, it must be deadly. Last week, a group of chemists and toxicologists published a largely theoretical study claiming that “infants and children are particularly at risk” from thirdhand smoke. The first author was Mohamad Sleiman; another, James Pankow, claimed several weeks ago that smokeless tobacco users were overdosing on wintergreen flavor.
The report appeared in the Proceedings of the National Academy of Sciences, a scientific journal that usually publishes solid research studies. Not this time.
Sleiman claims that nicotine residues in thirdhand smoke, which can become attached to skin, dust and items in houses or cars, can react with nitrous acid, which is formed by unvented combustion appliances (such as a gas cooktop) and smoking. The reaction produces NNK, which is a tobacco-specific nitrosamine (TSNA) that is considered a cancer-causing agent.
The report is largely theoretical, since it is very difficult to isolate and measure tiny levels of these chemical agents. For example, Sleiman estimated that smoking might expose human skin to as much as 15 nanograms of NNK per SQUARE METER. Keep in mind that a square meter is about 9 square feet. This is about 5 times the surface area of a normal adult. He also estimated that about 15 nanograms of NNK could be present over 9 square feet of furniture surfaces.
A nanogram is one billionth of a gram, and a gram is less than 4 hundredths of an ounce. So we are talking about vanishingly small amounts.
Sleiman didn’t discuss how infants and children could actually consume a 15 nanogram dose of NNK from that 9 square foot surface. He collected the residue by using special solvents, including methanol, so uptake by humans is likely to be very inefficient, perhaps negligible.
But let’s assume for a moment that a person is exposed to, and completely consumes, Sleiman’s dose of 15 nanograms of NNK every day for decades. What potential cancer-causing effect might this have?
The answer might come from what we know about NNK in a product used by millions of American men -- smokeless tobacco. In 2003, I published a study reporting the level of NNK in American moist snuff. I found that a typical 1-can per day user may be exposed to one thousand times more NNK, about 15-30 micrograms (actual consumption is probably far lower). Readers of this blog know that there is very little evidence that lifelong use of smokeless tobacco is associated with any cancer.
I am not the only tobacco researcher who is critical of the Sleiman study. Even Steven Hecht, a Minnesota professor who holds extreme positions with respect to smokeless tobacco, has doubts: “I personally feel that exposure by this route would be minimal…”
How harmful is thirdhand smoke? It probably carries about as much risk as third hand rain.