Sunday, February 28, 2016

On to Ontario

In a 1983 study by J.D. Abbatt et al., the standard mortality ratios (SMRs) of 4,000 nuclear workers were compared with those of 21,000 unexposed "thermal" workers and to those of the general population of nearby Ontario, Canada. [Abbatt, J.D., et al. Epidemiological studies in three corporations covering the Canadian nuclear fuel cycle. From: Biological Effects of Low Level Radiation, International Atomic Energy Agency, STI/PUB 646, Vienna, 1983.]

Exposures for the nuclear cohort in this investigation, which covered twenty years of plant operation, averaged 7 cGy (7,000 mrad) or about twenty-three years of additional annual background radiation per worker.

Hold the phone. Just what is a "standard mortality ratio" (SMR)? And what does HWE mean? (It hasn't come up yet, but it's getting ready to.) Glad you asked.

First the SMR: If, in the United States, it is observed that 20,000 of the 1 million makes aged fifty-nine die per year, we can say that the rate of deaths for this group is 2% and that becomes the basis for comparison of other smaller groups of fifty-nine-year-old, left-handed Presbyterians, there are only ten deaths, giving us a death rate of 1%. To obtain the standard mortality ratio of the Presbyterian lefties we divide its 1% rate by the 2% rate for the total population, yielding the ratio 0.50.

If we wanted to know the reason for this lesser mortality ratio, we'd call in an epidemiologist. This special type of statistician might note that most U.S. Presbyterians are Caucasians, who have a lower death rate at age fifty-nine than that of the population in general. Hence race would be considered a confounding factor that explains, in part or in total, the difference in the death statistics.

One of the most often used confounding factors in attempts to rebut the hormesis phenomenon is that of the healthy worker effect (HWE).

Any employer requiring reliable workers will want to know, at the time of employment negotiations, the health history of the prospective employee. [Politicians who pander to certain groups attempt to thwart such reasonable actions - as seen by several Federal laws making health questions illegal.]

Since the healthy applicants, who tend to have a history of less health-related absenteeism, are the first hired, it is logical to assume that the workforce will be healthier than a group of people including those who had applied but were not hired for health reasons. In studies relating to, for instance, tooth decay, we would generally tend to find that the employed contingent had sounder teeth than the total population, some of whom might have lifestyles that didn't include the use of a toothbrush. It would be appropriate in this case to attribute the better dental health to the fact that the individual in question was a "healthy worker."

There is a case, however, where LNT proponents use this confounder to confuse: It is when the employees are drawn from the same pool and work in the same or very similar work areas. Since there is no screening test for cancer, there is no way to predict whether the prospective employee will contract it. How, then, can an employee be hired on the basis that he will not contract cancer later? We shouldn't discount the HWE, but we shouldn't let others use it to discount the hormesis phenomenon when it is not applicable. Please pardon the interruption, and now back to our story.

The close agreement shown between the thermal worker SMR and that of the general population in Figure 17 would indicate a high degree of reliability for the overall study with a slight degree of "healthy worker effect" (HWE) noted for the non-nuclear cohort. Since there is no HWE in the comparison of nuclear and thermal workers - both being drawn from the same pool - the only difference in the cohorts is the additional radiation exposure of the nuclear workers. This, it would appear, strongly indicates a beneficial effect of low-level radiation exposure - which, of course, is our definition of radiation hormesis.

Source for Figure 17 Cancer Mortality of Nuclear Plant Workers: Abbatt, J.D., Hamilton, T.R., and Weeks, J.L. Epidemiological studies in three corporations covering the Canadian nuclear fuel cycle, in Biological Effects of Low Level Radiation, International Atomic Energy Agency, Vienna, 351, 1983.