By the end of the 1920s, scientists already knew that tobacco smoke contained a small encyclopedia's worth of risky chemical compounds: carbon monoxide and hydrogen cyanide, hydrogen sulfide and formaldehyde, ammonia and pyridine (a component in industrial solvents).

I discovered that list when I was researching my book about early 20th century toxicology, The Poisoner's Handbook. And I remember being surprised because I had believed that it wasn't until the mid-20th century, maybe a little before the famed 1964 U.S. Surgeon General report on the dangers of smoking - that we really knew anything about the health risks of smoking.

Of course, that 1920s list turns out to only be the bare start of the one we've assembled today. By some counts, there are a good4,000 chemical compounds in cigarettes and, of those, the U.S. Food and Drug Administration classifies more than 100 as dangerous (from carcinogenic to addictive). Given the body of evidence, linking cigarette smoking to disease, it's not necessarily a surprise to find that the smoke contains well-known bad actorsranging from arsenic to toluene.

Still, I'll confess to being startled last week when I was researching the suspected radiation poisoning of Palestinian leader Yasser Arafat and discovered that one of the most common sources of radiation exposure is through smoking cigarettes. I wrote about that in the context of the recent exhumation of Arafat's body and the toxicity texts underway in a post called "Yasser Arafat and the Radioactive Cigarette."

And when I read the FDA list of hazardous compounds in cigarette smoke and found not only polonium-210 (the radioactive element suspected in Arafat's death) but two well-known isotopes of uraniumbest associated with nuclear reactors (uranium-235 and uranium-238), I thought - wow, how did I miss that?

As it turns out, there's a real case to be made that I - and really all of us - missed this because the tobacco companies hid the information, that cigarette makers flagged the problem internally by 1960s and studied it in secret. The best evidence for that comes from the companies' confidential documents, which were released in the 1998 Tobacco Master Settlement Agreement between four major companies - Philip Morris, R.J. Reynolds, Brown & Williamson and Lorillard - and attorney generals from 46 states.

An analysis of those documents by public health researchers at the University of California-Los Angeles was published last year in the journal, Nicotine and Tobacco Research. As that study (paywall) notes:

The documents show that the industry was well aware of the presence of a radioactive substance in tobacco as early as 1959. Furthermore, the industry was not only cognizant of the potential “cancerous growth” in the lungs of regular smokers but also did quantitative radiobiological calculations to estimate the long-term (25 years) lung radiation absorption dose (rad) of ionizing alpha particles emitted from the cigarette smoke.

This wasn't the first study to note the corporate coverup;an earlier report in American Journal of Public Health reached the same conclusion. Still, let's call the information an imperfectly kept secret (as so many are). In 1964, for instance, we find scientists from the Harvard School of Public Health reporting that they had discovered hot spots, fizzing with polonium-210, in the lungs of regular smokers. They published that finding in the highly visible New England Journal of Medicine in 1965, warning that "we believe 210Po may be an important factor in the initiation of bronchial carcinoma in humans". It wasn't, actually, that tobacco companies were entirely successful at hiding the radioactive nature of cigarettes; it was that the rest of us weren't entirely successful at paying attention.

But, as the UCLA analysis points out, internal documents revealed something else. Not only did cigarette makers know about polonium-210 contamination of their product for decades - they knew how to fix it and chose not to. And to understand that, you need to know why tobacco plants become such little radiation factories.

The radioactive elements occur naturally in the Earth's crust. So it's not surprising to find them in soils where crops are grown. In the case of tobacco, this effect tends to be amplified because the most commonly used fertilizers for that plant are phosphate-rich mixtures based on the mineral apatite. And apatite is known to mix up with radioactive elements. Or as the U.S. Environmental Protection Agency puts it: "When phosphate fertilizer is spread on the tobacco fields year after year, the concentration of lead-210 and polonoium-210 in the soil rises." When the soil is stirred up - by planting, plowing, wind, whatever - radioactive particles drift into the air, attach to dust and other particulates there. As these settle back down to the ground, they are often trapped by the naturally sticky leaves of the tobacco plant.

These radioactive residues can be removed by acid-washing the plants. But the documents obtained by the California researchers showed that manufacturers refused to do that for fears that the acid would alter the nicotine and decrease the chemical kick that helps make the products popular. The UCLA analysts went on to calculate the resulting radiation health risk from regular smoking, based in part on the industry's own analysis. They set the cost of such alpha radiation in the lungs at 120-138 cancer deaths per 1,000 regular smokers.

As a story by British science writer Ed Yong points out, these are tricky numbers to set because the radiation dose comes in a smoke fog of treacherous chemistry. But as he also points there's no disagreement that having polonium-210 delivered directly to the lungs is a very bad idea. This is a highly energetic element, with a half-life of only 138 days; it's considered 5,000 times as radioactive as radium. Like radium, it primarily emits alpha particles which, although, not particularly dangerous outside body (they lose energy on impact and don't penetrate skin) wreak havoc once inside.

But inside the body, alpha particles are capable of doing wide-spread harm. Polonium-210 lodges in cells of the lungs like little hissing balls of radiation. It travels easily elsewhere in the body, irradiating tissue as it goes. It settles into and destroys bone marrow, causing a host of blood-related disorders. At smoker-related exposure levels, health expects thus warn of diseases, such as cancer, that follow a kind of chronic, radiation-induced injury. At high levels, though, polonium-210 kills with relative speed.

The classic example is the 2006 death of former Russian spy Alexander Litvinenko, who was purportedly killed by KGB agents who slipped polonium-210 into his drink during a meeting in London. Litveninko died just three weeks after that November meeting. British police say there is enough evidence to charge two Russian agents with his death but Russia has refused to extradite them and - even today - angrily denies the accusations.

Which brings us back to the other possible assassination, the suspected poisoning of the Palestinian leader Yasser Arafat, who died in 2004. A months long investigation by Al Jazeera, which included testing of his clothes and even his famous checked kaffiyeh, found some unexpectedly high traces of polonium-210. The publication of those results in July led to calls for further testing and last month his body was exhumed and tissue and bone samples sent to three laboratories (one, ironically, in Russia). Results are not expected until early next year.

In my post last week, I pointed out that a possible explanation for evidence of polonium-210 exposure could, in fact, be cigarette smoke. Arafat and his colleagues in the Ramallah compound were known to be heavy smokers. Of course, I also somewhat undermined that idea by also pointing out that Israel had been known to restrict Arafat's access to tobacco as a form of petty punishment. In other words, it's worth exploring all possibilities but keeping them ones that make most sense.

It might be, as I've suggested, that a smoky environment accounted for some of the polonium-210 traces in Arafat's clothing. But there's still no clear evidence that smoking killed him; no clear evidence that he was a victim of one of those polonium-201 induced lung cancers or similar illness. So, beyond the first stage of this investigation, if forensic work is able to show a lethal exposure then - as in the case of Litvinenko - we will indeed be talking about assassination and all its ugly and messy implications.

But while we wait, let me just emphasize my other point. Let me just quote you the closing line of that UCLA look at radiation in cigarettes: "The evidence of lung cancer risk caused by cigarette smoke radioactivity is compelling enough to warrant its removal." After all these years, it would be gratifying to see that message get a little traction too. And that conclusion, we can safely call an understatement.

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