Low-Level Ionizing Radiation May Raise Hypertension Risk

A retrospective study of workers at a Russian plutonium plant adds evidence for a dose-response effect of radiation.

Low-Level Ionizing Radiation May Raise Hypertension Risk

People who work in jobs where they are continuously exposed to low levels of ionizing radiation may be at increased risk of hypertension, according to new data from a decades-long study of Russian nuclear plant employees.

These findings contribute to a better understanding of how low levels of ionizing radiation can be detrimental to human health, according to Tamara Azizova, MD (Southern Urals Biophysics Institute, Ozyorsk, Russia), and colleagues.

The rate of hypertension (≥ 140 mm Hg/90 mm Hg) in the study was 38%, which investigators say is higher than what was seen in Japanese atomic bomb survivors exposed to a single, high-dose exposure. Yet it was lower than what was observed among clean-up workers following the Chernobyl nuclear accident who were exposed to high levels of radiation for a short time. The individuals in the new study were exposed either to external radiation if they worked at the reactor or, if they worked in the radiochemical or plutonium production plant, to a combination of external (γ-rays and neutrons) and internal α-particles of incorporated plutonium.

In their analysis, published online today in Hypertension, Azizova and colleagues found a dose-response relationship: hypertension was associated with the cumulative liver absorbed dose from external γ-rays with an excess relative risk per Gy of 0.14 (95% CI 0.09-0.20).

But Brian Silver, MD (University of Massachusetts Medical School, Worcester), who commented on the study for TCTMD, cautioned that while the results suggest a relationship between the low-level radiation exposure and hypertension, a major weakness is the lack of a control group.

“They’re comparing these results with other populations that have had radiation exposure such as atomic bomb survivors and Chernobyl workers, which is fine, and finding a control group would have been very difficult to do. But [knowing] the prevalence of hypertension in matched controls of the same age living and working in the same area and measuring the radiation exposures in those people would help in interpreting this,” Silver noted.

The 22,377 people included in the study had their health and external radiation dose-exposures closely monitored over the course of their employment at Mayak, a large nuclear plant located in the Southern Urals of Russia that initially produced plutonium. All of the workers underwent medical exams at the time of initial hiring. The earliest group of workers studied—those hired in the late 1940s and early 1950s—had mandatory routine checkups every 3 months, but the checkups eventually were changed to every 6 months and then to an annual exam. By 2013, medical data and vital statuses were available for 95% of the cohort dating back to 1948. The average employment length was 18 years, with some having worked at the nuclear facility for as long as 60 years.

Mean cumulative liver absorbed doses from external γ-rays were higher in men than in women (0.45 ± 0.65 Gy vs 0.37 ± 0.56 Gy), while mean cumulative liver absorbed doses from internal α-particles were lower in men than in women (0.23 ± 0.65 Gy vs 0.44 ± 1.93 Gy).

The study is the latest in a series of investigations by Azizova and colleagues on how low-level occupational radiation exposure affected the health of the Mayak workers cohort. Their previous reports have linked this exposure to increased risk of cancer and cataracts.

Mechanism Unclear

“A fundamental problem in understanding whether there is such a radiation-related risk of CVD is the absence of an accepted radiobiological mechanism to explain how low-level exposure could increase the risk of CVD, and this is the subject of much ongoing research,” notes Richard Wakeford, PhD (University of Manchester, England), in an accompanying editorial.

“At present,” he continues, “there is indicative evidence of a link, but overall, the findings are not yet persuasive, in particular because of the uncertain influence of major nonradiation risk factors on the reported associations.”

According to Wakeford, working conditions at Mayak, “particularly in the early years of operations, were harsh and not only were exposures to radiation high but other significant occupational exposures, such as to organic solvents and acids, could influence risks.”

Another limitation of the study, which Azizova and colleagues acknowledge, is the lack of information on radiation doses to individual body parts such as the heart, brain, and blood vessels. With the existing dosimetry data and construction of theoretical models, they hope to perform circulatory organ dose reconstruction, as well as try to understand if the radiation exposure increases the risk of certain types of hypertension and leads to higher mortality.

To TCTMD, Silver said despite all the testing done on the workers, the records do not appear to have tracked the incidence of diabetes in the population, which would be another important aspect in understanding the increased hypertension risk.

Silver added that if low-level ionizing radiation exposure was responsible for the excess hypertension risk seen in the Mayak workers, it may be related to stiffening or other changes to the walls of blood vessels. “But these are just guesses, because we don’t have pathology that can show us any of that,” he noted.

Importantly, although the researchers tried to account for residual confounders, they did not have a way to measure such things as stress among people working in what would presumably have been a taxing environment.

Keeping in mind all the limitations of the study, Silver said, the dose-response relationship does warrant caution and further investigation into whether low-level ionizing radiation in the workplace contributes to hypertension, as well as efforts to identify workers most at risk.

Disclosures
  • Azizova and Silver report no relevant conflicts of interest.
  • Wakeford reports consulting for Sellafield Ltd and Tokyo Electric Power Company.

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