Sunday, December 03, 2006

Shrill smokescreens and radioactive bananas

In a dashing blaze of opportunistic fear-mongering, the New York Times op-ed page is reporting that cigarettes, in addition to their usually toxins, also contain radioactive polonium-210. This is the same isotope used to assassinate Victor Litvinenko. The article, long on analogy and short on math, even goes so far to suggest that the total polonium dosage of second hand smoke in London could equal that which killed the former Russian spy. So, how much radiation is 0.04 picocuries?

Why, 1.48x10-3 decays per second, of course. That’s about one decay every ten minutes. You’d have to be in a very deep, shielded room to detect that sort of signal above the cosmic ray background, and if your shielded room was made of cement, sandstone, or granite, the decays from naturally occurring radioactive minerals would also dwarf your polonium signal.

For analogy lovers, here’s a more correct one that what Professor Proctor has dished out: Potassium, which is a vital nutrient, has a slightly radioactive minor isotope, 40K. With an isotopic abundance of .01% and a half-life of 1.25 billion years, a banana with 450 mg of K will kick out 14 decays every second. So a banana is over nine thousand times more radioactive than the polonium in a cigarette.

Now, how many cigarettes would it take to get a lethal dose? Well, the LD 50 for ingestion is around 8 million becquerels (decays/sec). So with 1.48x10-3 Bq per fag, you would need about 5.4 billion of them to accumulate a lethal dose of polonium. I reckon the nicotine would get you first.

Professor Proctor writes, “London’s smokers (and those Londoners exposed to secondhand smoke), taken as a group, probably inhale more polonium 210 on any given day than the former spy ingested with his sushi.” Can this be true? Well, with a lethal dose 5.4 billion times greater than that of a fag, and assuming that 5.4 million Londoners smoke, they’d have to suck down a thousand cigs a day (50 packs) in order for the figures to be correct. Muscovites may think a 50 pack day is cold turkey, but Londoners? I doubt it.

Professor Proctor obviously thinks that the risk of smoking justifies incorrect arithmetic and easily refutable generalizations. Hopefully, my calculations will allow all my smoking readers to rest easy tonight, secure in the knowledge that it will be the tar and the nicotine that kills them, not the 210Po.

p.s. As a geologist, I usually work in years, not seconds, so the first time I did the banana calculation, I instinctively calculated decays per year, and assumed I had seconds. However, I quickly decided that if 17 billion Bq was the dose from a typical banana, then I had bigger things to worry about than this blog.


C W Magee said...

Fixed an arithmatic error in the banana.

Anonymous said...

From my response to your comment at motd:


Radiation exposure from cigarettes is real, ask your health physics guy!

C W Magee said...

Dude, what a great paper. I was amazed that the entire first page of a science paper was given over to describing the method- only then did I notice the date. Back in 1964, Science still published science, as opposed to conclusions and pretty pictures like today.

Anyway, a few thoughts...

Firstly, the paper notes that smokers have high urine contents. This requires the polonium to detach from the smoke particulates and be absorbed into the body. Unless the kinetics of this process are understood, it is difficult to tell how localized the damage will be to the lungs before getting "diluted" into the general blood supply.

Secondly, the paper uses rem to estimate cancer risk. I'm not convinced that is appropriate. The rem and Sievert scales were designed for acute radiation poisoning, which involves cell death. The weighting factor (Q) for alpha decays is greater than that of beta and gamma because alphas are more likely to kill cells. However, this does not mean that they are more likely to cause cancer. If you think about the genetic damage necessary to make a cell cancerous, a small beta or gamma is much more likely to make a minor, survivable mutation than a great big alpha, which will probably tear up most of a chromosome.

So even though alpha particles kill cells more effectively, the beta decay or electron capture from potassium might be more likely to cause the minor genetic defect that turns a healthy cell cancerous.

In any case, my main complaint about the original article was the deliberate playing on the public's fear of radiation for the author's purposes. There are plenty of sound reasons to discourage smoking; we don't need to discard them for sensationalized scare tactics.