Wednesday, March 2, 2016

The Most Toxic Substance on Earth?

One of the noisiest guns in the anti-nuclear arsenal has been the cultivated fear of plutonium. "The most toxic substance known to man," so the mantra goes - in complete disregard to toxicity studies showing the element to be about as toxic as caffeine and 1/1,000,000,000,000 the toxicity of botulism toxin. Even in the health physics field, however, there has been a great deal of concern over inhaled plutonium, because it is an alpha emitter shooting nuclear cannon balls directly into adjacent lung tissue. If you presume radiation is a major cause of cancer, it is only logical to see chronic exposure from an alpha source within the lungs as extremely dangerous. But just as with radon exposure in mining environments, plutonium may not be nearly as dangerous as earlier believed.

During the urgent atomic bomb development period of 1944-45, some workers were exposed to plutonium fumes and extremely fine dust, which accumulated primarily in lung tissue. Twenty-six of these exposed males were followed with regular examinations every five years starting in 1952. When the initial study of these examinations began in 1973 [Hempelmann, L.H. et al. Manhattan Project plutonium workers. A twenty-seven year follow-up study of selected cases. Health Physics, 25, 461, 1973], one subject had already died of a heart attack. Anti-nuclear scientists, such as John Gofman, predicted shortened life spans from radiation-induced lung cancer. [Gofman, J.W., Radiation and Human Health, Sierra Clue Books, San Francisco, 1981.]

But apparently, someone forgot to tell the workers.

At the time of the 1986-87 examination period, with an average of age of sixty-six years, twenty-two of the twenty-five subjects had refused to die on Gofman's schedule. One had died in an automobile accident, another of a heart attack at age sixty-two, and the third - a pack-a-day-plus smoker - had succumbed to lung cancer in his seventy-second year. (It may be of some interest that this person, identified as Subject #10, was in the lower half of estimated plutonium deposits.)

Because only twenty-six individuals were involved, the study has no statistical significance - which is to say that chance could have been at work selecting certain men who were unusually tolerant to the effects of inhaled plutonium. But the data is also suggestive of a lesser response to plutonium than the LNT dose-response would predict.

Anti-nuclear activists are fond of saying that "a single gamma ray can lead to cancer." Eight of the atomic bomb workers - all living at the end of 1987 - had received a dose of more than 2,000,000,000,000,000 alpha particles, which is the equivalent of 8,000,000,000,000,000 gamma rays (assuming a Q of 4). Not only were the "victims" alive, but they were healthier than their peers who weren't lucky enough to inhale plutonium dust more than forty years earlier.

If this study were the only one indicating a biopositive dose-response from plutonium ingestion, it might be written off as an anomaly. But, quoting from a paper by Voelz and Lawrence [Voelz, G.L. and Lawrence, J.N.P. A forty-two-year medical follow-up of Manhattan Project plutonium workers. Health Physics, Vol. 61, 1991. For more information, you might also refer to Voelz, G.L. et al. Mortality study of Los Alamos workers with higher exposures to plutonium. Epidemiology applied to health physics. Proceedings of the Health Physics Society, Albuquerque, N.M. Report CONF-83010, 318, 1983]:

"Other studies of Pu-exposed workers have not demonstrated excesses of lung cancer. In 224 white male Pu-exposed workers, selected on the basis of each having a 1974 estimated Pu deposition in excess of 370Bq (10 nCi), only one death from lung cancer occurred over at 33-year follow-up period. The SMR for lung cancer based on U.S. rates was 0.2 (95% C.I.=0,1.1)."

Lest you have forgotten the definition, an SMR of 0.2 means that the lung cancer rate for the workers exposed to plutonium was one-fifth that of the general population.

Don't take this as an indication that plutonium is never dangerous when ingested. All the heavy metals are toxic to some degree, and though a relatively benign alpha emitter, it has the potential for being dangerous in large amounts. Several studies noted by Voelz in which beagles were exposed to very high doses produced extremely severe consequences. But hormesis is about the differences in effects of a toxin depending on the dose and/or dose rate. Evidence in these studies clearly suggests that plutonium may be an effective hormetin.