Out of 110,645 Chernobyl clean up workers, 19 might have contracted radiation related leukemia

On November 8, 2012, Environmental Health Perspectives, a monthly journal supported by National Institute of Environmental Health Sciences, National Institutes of Health, U.S. Department of Health and Human Services, published a report titled Radiation and the Risk of Chronic Lymphocytic and Other Leukemias among Chornobyl Cleanup Workers.

The report details the final results of a study conducted during the period from 2006-2012 and includes the names of twenty authors. It was funded in part by the National Cancer Institute, the US Department of Energy, the Nuclear Regulatory Commission and the French Institute for Radiological Protection and Nuclear Safety. The study objective was to estimate relative risks of chronic lymphocytic leukemia (CLL) and non-chronic lymphocytic leukemia (non-CLL) from protracted exposures to low-dose ionizing radiation.

The authors studied a population of 110,645 people who had been involved in efforts to clean up contaminated areas after the 1986 Chernobyl accident. The exposures were all received sometime during the period from 1986-1990.

Out of that large population, 162 (0.146%) were diagnosed with leukemia between 1986-2006. Of the 162, radiation dose estimates were not available for 25 people, leaving a group of 137. Out of the 137, 20 people were interviewed within 2 years after they started chemotherapy for their disease. That group demonstrated a significantly different dose-response from other cohorts so the people conducting the study decided to exclude them from the study results, leaving 117 cases of leukemia (both CLL and non-CLL) in the group to be studied.

Through a variety of statistical methods, the study authors determined that approximately 19 cases of leukemia (16% of the remain 117 cases, but just 0.017% of the initial group of 110,645) could be attributed to radiation doses received during the clean up effort.

Aside: Here is a quote from page 12 of the report that explains the “anomalous” dose-response of that group of 20. “However, preliminary analysis identified a significant (p=0.021) difference in the dose-response for 20 cases (6 non-CLL and 14 CLL) with direct in-person interviews <2 years from start of chemotherapy compared with other cases (ERR/Gy= -0.47, 95% CI: <-0.47, 1.02, p=0.244 for 20 cases vs.ERR/Gy=2.38, 95%CI: 0.49, 5.87, p=0.004 for the remaining 117 cases, Table 2 and Supplemental Material, Table S2). Due to this marked disparity, we limited our primary analyses to cases who were interviewed 2-15 years after start of chemotherapy, did not have chemotherapy, or for whom proxy interviews were used and their matched controls (85% of all cases and 83% of all controls)."
(Emphasis added. A negative number in ERR/Gy indicates a positive health response to increased radiation. Hmmm.) End Aside.

Using a technique called Realistic Analytical Dose Reconstruction with Uncertainty Estimation (RADRUE), which is a time-and-motion dose reconstruction method that uses questionnaires and field radioactivity measurements to model radiation exposure that was created specifically for this study and a related one, the study authors estimated doses to bone marrow of the people involved. According to page 11 of the report, the mean estimated bone marrow radiation dose for the studied population was 132.3 mGy (13.2 Rem). “Seventy-eight percent of study participants had doses below 100 mGy (10 Rem) and 87% below 200 mGy (20 Rem).”

There is an interesting contrast between my report summary above and the report abstract that Environmental Health Perspectives published.


Background: Risks of most types of leukemia from exposure to acute high doses of ionizing radiation are well known, but risks associated with protracted exposures, and associations between radiation and chronic lymphocytic leukemia (CLL) are not clear.

Objectives: To estimate relative risks of CLL and non-CLL from protracted exposures to low-dose ionizing radiation.

Methods: A nested case-control study was conducted in a cohort of 110,645 Ukrainian cleanup workers of the 1986 Chornobyl nuclear power plant accident. Cases of incident leukemia diagnosed in 1986-2006 were confirmed by a panel of expert hematologists/hematopathologists. Controls were matched to cases on place of residence and year of birth. Individual bone marrow radiation doses were estimated by the Realistic Analytical Dose Reconstruction with Uncertainty Estimation (RADRUE) method. A conditional logistic regression model was used to estimate excess relative risk of leukemia per gray (ERR/Gy) of radiation dose.

Results: A significant linear dose-response was found for all leukemia (137 cases, ERR/Gy (excess relative risk per gray)=1.26 (95% confidence interval 0.03, 3.58)). There were non-significant positive dose-responses for both CLL and non-CLL (ERR/Gy=0.76 and 1.87, respectively). In our primary analysis excluding 20 cases with direct in-person interviews

Conclusions: Exposure to low doses and low dose-rates of radiation from post-Chornobyl cleanup work was associated with a significant increase in risk of leukemia, which was statistically consistent with estimates for the Japanese atomic bomb survivors. Based on the primary analysis, we conclude that CLL and non-CLL are both radiosensitive.

NOTE: There is a section from the abstract of the actual paper that is missing from the abstract that is published on the abstract. On the web site, the “Results:” section ends with the word “interview.” In the paper itself, that section continues with the following:

…<2 years from start of chemotherapy with an anomalous finding of ERR/Gy= -0.47 (<-0.47, 1.02), the ERR/Gy for the remaining 117 cases was 2.38 (0.49, 5.87). For CLL the ERR/Gy was 2.58 (0.02, 8.43) and for non-CLL ERR/Gy was 2.21 (0.05, 7.61). Altogether, 16% of leukemia cases (15% of non-CLL, 18% of CLL) were attributed to radiation exposure.

I cannot explain why that section was left out, but it is possible that a the “less than” symbol interfered with a cut and paste. It is also possible that someone decided to obscure the fact that the final study cohort excluded a significant portion of the sample (15%) because their results did not match the stated objectives of the study.

A statistician considers a result to be “significant” as long as it is unlikely to have arisen simply by chance. Most people who are not statisticians or who have not been trained in statistical techniques use a rather different definition of the word, probably one or more of the following choices: “Having or expressing a meaning; meaningful” or “Having or likely to have a major effect; important” or “Fairly large in amount or quantity”.

Most radiation health specialists would not use the terms “low doses” or “low dose rate” to describe the exposures to a population with mean exposures of 13.23 Rem (132.3 mGy) received over a median period of just 35 days. (page 12)

The words that the study authors chose for their conclusions will be misinterpreted by many people. They will be repurposed by individuals and organizations with antinuclear or radiation protection agendas as “proof” that exposure at any level will inevitably result in an an increased risk of cancer.

People with antinuclear agendas or an already developed fear of radiation will not mention the fact that the increase is so slight as to be barely detectable. They will not mention that the small quantity of excess leukemia was found in a population that received far higher doses than anyone is expected to receive from even the worst postulated accident from a light water reactor designed to internationally accepted standards. They will fail to mention the fact that there was a hint found in the study that indicated that the leukemia, even if initiated by a radiation dose, was treatable. (Recall that the people conducting the study excluded a group of 20 people whose dose response indicated a ERR/Gy of -0.47 with a confidence interval that ranged from <-0.47 to 1.02 because that group had been treated with chemotherapy.) I hope you are now armed with enough information to understand and to challenge the people who are going to try to misuse the results of this credible statistical modeling effort, which was conducted by nearly two dozen credentialed researchers during six years of intensive effort. There is a back story explaining why I wrote this post. At about 10:00 am on November 8, a friend posted the following note on an email list populated by people who are interested in using social media tools and techniques to share accurate information about nuclear energy.

I can’t find the report itself in downloadable form (a little bit of help?), but if this article is anywhere near accurate in its data reported, this could be some serious fuel for the no-safe-level crowd.


There was a little flurry of discussion about the article from others who participate on that email list. One of the more tuned-in members noted that the reliably antinuclear NIRS (Nuclear Information Resource Service) had tweeted about the release of the study within minutes after its announcement.

At 6:00 pm on November 8, 2012, I received the following email from an Atomic Insights reader:

Dear Rod,
First, thank-you for all of the work you do in maintaining this website, and providing an oasis of reason amidst an almost overwhelming sea of alarm regarding nuclear power and radiation.

I live in Japan, am a mechanical / electrical engineer by training, and have learned more than I ever expected to about nuclear power over the past year and a half.

I write in to bring your attention to a paper published on Nov. 8 in “Environmental Health Perspectives”, entitled “Radiation and the Risk of Chronic Lymphocytic and Other Leukemias among Chornobyl Cleanup Workers”.

A link to the main journal site is here:

The paper itself is here:

I am curious whether one or more of the people who comment on your articles might examine this paper and provide insight into the conclusions and/or methods and analysis used in the paper.

The analysis of bio/medical data is just enough beyond my skill set that I would like to hear from others before the common FUD folk start proclaiming the validity of the linear dose rate theory all the way down to zero.

I decided to do what many journalists who work on tighter deadlines than I do will not do – I read the press release, the abstract and the full report. Once again, my rather eclectic educational mix that includes deep study of English literature, statistics, radiation biology, politics, business and nuclear technology has provided a unique lens for reading between the lines.

Value for Value Request

If you found this summary to be useful, maybe you could say thanks by making a contribution on my Movember page. During the month of November I’m growing a new mustache (what my Aussie friends might call a ‘Mo’) to replace the one I shaved off on October 31 when I began my participation in Movember. The participants are working to raise funds that will help treat forms of cancer that are not leukemia – specifically prostate and testicular cancer.

About Rod Adams

14 Responses to “Out of 110,645 Chernobyl clean up workers, 19 might have contracted radiation related leukemia”

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  1. Robert Hargraves says:

    Rod, thanks for this. I had to look up abbreviations to refresh my statistics. Tell me if I’m wrong and I’ll fix the comment.

    ERR is excess relative risk. So if normal risk of something is 1%, then with ERR = 2.38 the observed risk is 2.38%.

    p is probability the observation is just noise. So p = 0.24 means that if the experiment were repeated 100 times and there really is no correlation, a similar correlation result would be observed 24 times.

    Gy is gray (not gigawatt-year) which is 1 watt-second of ionizing energy absorbed per kilogram of flesh — a large amount. [Below I assume 1 Sv = 1 Gy for simplicity.]

    The mean dose of 132 mGy is high compared to low level ionizing radiation concerns. The article is concerned with doses as high as 1.4 Gy. Looking at the graph on page 30 the confidence error bars are consistent with ERR = 1.0 up to doses of 500 mGy.

    It’s not clear whether the doses received are chronic or acute. I would think cleanup workers would be exposed to most of the radiation at the beginning of the cleanup. The concerns people have with low-level chronic ionizing radiation are not addressed in this study.

    At Fukushima, 9900 members of the public were exposed to radiation < 10 mGy and none more than 23 mGy. These exposures are way smaller than those in the study.

    The US and ICRP limits public exposure to 1 mGy/year. Jaworowski proposed raising the limit to 10 mGy/year — the level of background radiation as life evolved on earth. Wade Allison proposes 100 mGy/month.

    But the takeaway is that only 19 people, 0.017%, developed leukemia attributable to radiation from the Chernobyl cleanup.

    • Rod Adams says:


      That’s just about perfect.

      The only correction I will offer is that there is an indication in the study that each person received their total doses during a relatively brief period of performing clean up work – the median time in the zone for all workers was 35 days with just 38% of the people in the zone for more than 60 days (two months). The maximum duration of the work inside the zone was 711 days.

      All of that can be found on page 12 of 31.

    • Carl Lumma says:

      Note that these results exclude more cases than they attribute to radiation. The authors exclude 20 cases for which the ERR/Gy was negative!

  2. Joffan says:

    Let me add another couple of translations:
    The 20 cases were excluded because they were “incompatible with our current understanding of radiation-related leukemia risk.”
    = We took out the cases that conflicted with our expectations.

    “However, it must be recognized that our final results derived from a post-hoc subgroup analysis”
    = We cherry-picked.

    I am still mystified, though, as to how these 20 cases with an “unexpectedly” LOW dose were assessed as being a negative ERR. And even more mystified as how taking them out would increase overall ERR, too, since last time I looked zero is less than twenty.

    • Rod Adams says:


      Can you point to the section that led you to say that the 20 excluded cases had an unexpectedly “LOW dose”?

      I do not think their doses were low; they showed a negative ERR/Gy which indicates to me that they must have had higher than average doses and/or lower than expected excess risk. (Or am I getting confused here?)

      • Joffan says:

        page 18: “However, a preliminary examination of differences in various characteristics of participating cases, ascertained using the two methods described above, indicated that cases with direct in-person interviews <2 years from start of chemotherapy treatment had mean bone marrow radiation dose estimates significantly lower than other cases interviewed in-person (16.8 vs.121.4 mGy, 7-fold difference in means, p Wilcoxon=0.036), and these doses were uniformly lower across all types of work performed in the 30-km zone, while the mean doses for controls from both groups were almost identical."

        I think it is intrinsically confusing. I would be surprised (and delighted) to find someone who was not confused.

        • Carl Lumma says:

          They’re bad investigators for not looking more closely at this. These directly-interviewed subjects starting chemo within 2 years of being interviewed got lower rad doses than other subjects. Why?

          For one thing, their proxy interviews do seem to exaggerate doses compared to their direct interviews (see my reply to Rod here).

          For another, I’m not sure I believe them when they say there’s no relationship between ERR/Gy estimates and “calendar period of diagnosis”. The calendar periods they consider are ’86-94, ’95-00, and ’01-06… relatively long compared to 2 years. Presumably these recent-chemo folks are the last to be diagnosed. That’s consistent with a low dose giving protection.

          But you’re right, I don’t think any conclusions can be drawn from the work as presented. Aside from issues already mentioned, their confidence intervals look a bit crazy. They used EPICURE, and BEIR warns about how easy it is to misuse.


      • Carl Lumma says:

        “…cases with direct in-person interviews < 2 years from start of chemotherapy
        treatment had mean bone marrow radiation dose estimates significantly lower than other cases interviewed in-person (16.8 vs.121.4 mGy …)… while the mean doses for controls from both groups were almost identical."

        A negative ERR/Gy means less risk of leukemia than the general population. To get the relative risk per Gy, add 1 to the ERR/Gy. So -0.47/Gy + 1 = 0.53/Gy, which means: every Gy of radiation cuts your risk of leukemia approximately in half compared to the population at large. Of course that's the relationship only in the low-dose cohort. For the high-dose cohort, it's 1 + 2.38 = 3.38 times more risk per Gy. Which is to say, they observed a nonlinear dose-response.

        However, their data looks pretty crap. All doses were inferred by interview (basically, "how long did you spend in each area?" times the known contamination in each area). And despite them saying "There was no significant difference in ERR/Gy estimates by proxy or direct interviews", the differences do seem significant. From the "S2" supplemental table

        Proxy 69 5.10 (<-0.81, 29.29)
        Direct 68 -0.10 (<-0.38, 1.74)

        The third column is ERR/Gy. Proxy interviews give a much bigger bang than direct interviews. When they say the difference wasn't significant, they mean the p-value wasn't < 0.05. But it's not clear this "p-interaction" calculation is meaningful. And the p-interaction is 0.42 for their cherry-picked group of 117 cases, but is already down to 0.1 for the full 137 cases (see the supplemental table PDF).

        Now, a lot of proxy interviews may have been necessary because the subject had died, and perhaps they were more likely to die if they got a higher dose. So some difference in dose may be expected, who knows. The potential for coworkers and family members to exaggerate after the death of a loved one is also something to consider.

  3. Carl Lumma says:

    What a find! You can’t make this stuff up: “The ERR/Gy estimates for cases with direct interviews < 2 years from start of chemotherapy (ERR/Gy=-0.47) and the remaining cases (ERR/Gy=2.38) differed substantially (p=0.021), with the former estimate incompatible with our current understanding of radiation-related leukemia risk. ERR/Gy estimates in the former group were negative overall and by time since first exposure, for cases diagnosed in 1986-2000 and 2001-2006, and for CLL and non-CLL cases (data not shown). The discrepancy could have arisen by chance or from an unknown ascertainment anomaly. Other possible reasons were that the 20 cases were undergoing therapy at the time of interview or were in poorer health compared to other cases, which could have influenced the accuracy of recall. In our primary analyses, we omitted these 20 cases, so results were not unduly influenced."

  4. Pete51 says:

    A question. If the 20 excluded cases were for people interviewed less than 2 years from starting chemo, then by definition these people must have had leukemia. So how could the ERR be negative? A negative number would indicate some hormetic effect, would it not? But they DID get cancer. I am obviously confused here.

    • Carl Lumma says:

      A negative ERR does mean exposed subjects had a lower incidence of disease than the general population. The authors basically divided their subjects into groups by estimated dose. The ERR for a given dose D can be solved from

      (leukemia rate at dose D / normal leukemia rate) = 1 + (ERR/Gy * D)

      The ERR/Gy figures in the paper are basically averages of the these ERR/Gy figures across all groups.

  5. Paul Smith says:

    Thank you for writing such a detailed synopsis of this study. If it weren’t for this article, I wouldn’t have even noticed this study. I’m always tuned in to these sorts of studies because I’m interested in hearing the evidence against nuclear. Being a nuclear engineer, my livelihood depends on a strong nuclear industry and it’s upsetting to hear so much negative propaganda about nuclear energy.

    After reading the paper, it seems clear the researchers were strongly influenced by a predisposed belief in the LNT model. They even say that the 20 cases thrown out are inconsistent with their current understanding of “radiation-related leukemia risk”. I read this as, “it is inconsistent with some monotonic relationship between leukemia risk and radiation dose.” They are admitting that they have to throw out cases because it doesn’t fit their thesis.

    They go on to present results for the entire data set. The ERR seems significantly lower (which is underplayed by the authors) and the p-value is higher. The higher p-value indicates that the 0.74 ERR/Gy is not even statistically significant and that the high ERR calculations could be entirely due to chance. (Note: I base that statement on my rudimentary understanding of hypothesis testing and I don’t really know how they are actually calculating the p-value).

    This paper is why I generally dislike statistics. Authors go in with a strongly held belief, blatantly skew statistics to fit their belief, then hide their malpractice by using complicated arguments. Also note that the entire data set and equations used are not readily available. God forbid somebody else come in and do a full peer-check of their results.

  6. James Greenidge says:

    What model does NASA employ to gauge rad exposure for Shuttle/ISS crews? What’s the solar storm cosmic ray flux tipping point to abandon ship? Can’t Google it up. I assume NASA would use a model with the most fidelity and accuracy — and can such be applied to terrestrial situations?

    James Greenidge
    Queens NY

  7. Ruth Gravitt says:

    My father worked at Oak Ridge for two years (1944-1946) and got a diagnosis of CLL at age seventy. He died in 1993 at 78. We applied for EEOICPA compensation and were successful. I think it was because the cause of death stated on his death certificate was “lymphoma”.