Researchers Surprised by Hypertension Risk Among Workers Exposed to Low-Level Radiation

In a recent cohort study, researchers observed the health effects of long-term exposure to ionising radiation in 22,377 workers at a Russian nuclear facility (Mayak Production Association), between 1948 and 1982. Results have been published in the American Heart Association’s Hypertension journal.[i]

Every member of the study group had been exposed to external gamma (γ)-rays and three-quarters (76.03%) of the group had been exposed to alpha (α)-particles from internally deposited plutonium.

Figure: Electromagnetic Spectrum

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(Source: Wikimedia Commons – NASA: ‘Electromagnetic spectrum properties include wavelength, frequency and photon energy. Also includes ranges for common names of types of radiation and examples of sources of radiation’.)

50% of the workers had worked at the nuclear facility for more than 10-years. The average period of employment was 18-years.

By the end of the follow-up period (31 December 2013), 38% (8,425) of the workers had been diagnosed with hypertension (blood pressure readings of 140/90 mmHg – classified as ‘Stage 1 Hypertension’ in the UK[ii]), which was ‘significantly linearly associated with cumulative liver absorbed dose from external γ-rays’.

Graph: Blood Pressure Categories

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Of course, hypertension, more commonly referred to as high blood pressure, is a contributing factor to heart disease and stroke.

Earlier studies have associated high doses of radiation with increased cardiovascular disease risk, e.g. Murase et al (2019), which monitored incidence of congenital heart disease in the aftermath of the Fukushima nuclear accident, in 2011.[iii]

However, this latest study is the 1st of its kind to find an increased risk of heart disease where the radiation exposure was low level.

Astonishingly, incidence rates were higher among nuclear facility workers than among Japanese survivors of atomic bomb disasters in 1945, at Hiroshima and Nagasaki. That being said, incidence rates were lower than among clean-up crew members, who spent days and months at Chernobyl nuclear facility, in the wake of the 1986 explosion.

Currently, the radiobiological mechanism which connects radiation exposure and escalating blood pressure is ‘unclear’, admits Lead Author and Researcher at Russia’s Southern Urals Biophysics Institute, Dr. Tamara Azizova.

According to University of Manchester Professor, Richard Wakeford, it is this lack of clarity which has meant that systems designed and integrated to protect workers are principally targeted at preventing ‘the presumed cancer and hereditary risks’ and not the risk of hypertension and other diseases of the circulatory system.[iv]

The Professor of Epidemiology went on to warn, in light of the conclusions presented in the latest study, that:

‘Low-level exposure to radiation is experienced in everyday life [including work]. CT scans, X-rays, natural background radiation, occupational exposure, medical workers, aircrew’.

In an editorial,[v] accompanying the journal article, Prof. Wakeford went on to opine that the cardiovascular risk identified may ‘potentially’ be ‘a substantial component of the overall risk to health from low levels of exposure’.[vi]

Why ‘potentially’ and not ‘definitely’?

The English academic was judicious in his analysis of the study. He does not yet believe that the findings are ‘persuasive, in particular because of the uncertain influence of major nonradiation risk factors on the reported associations’. Such additional risk factors include exposure to organic solvents and acids. Moreover, in the first few years of facility operation, exposure to radiation was ‘high’, which confuses the supposition that worker exposure was always low level.

The Russian researchers themselves acknowledge that the study is not without its limitations. For example, analysis of medical records allowed the researchers to take into account age, smoking, alcohol consumption and body mass index as aggravating factors.[vii] However, details relating to stress, diabetes and nutrition were unavailable to them – it would not be unreasonable to assume that the facility was a ‘taxing environment’ for workers. 

Another limitation, which Dr. Brian Silver, of the University of Massachusetts Medical School, considers to be both detrimental and avoidable, is the lack of a control group. In a critical review of the research, submitted to TCTMD, he explained:

‘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’.[viii]

Notwithstanding these comments, the American clinician did not rebuff a possible dose-response relationship between low-level ionising radiation exposure and hypertension. In fact, he suggested, albeit ‘guesswork’, that ionising radiation exposure may stiffen blood vessel walls, thereby creating a pathological pathway.

Where next for follow-up research?

In regards to further investigation, the study authors remark that:

‘One of the main tasks for the coming decade is to study the mechanisms of hypertension and heart and brain atherosclerosis occurring in people who are – and who were exposed - to radiation’.

By performing circulatory organ dose reconstruction, it is hoped that researchers will develop their understanding of how distinct types of hypertension are affected differently by radiation exposure.

At this time, occupational exposure to ionising radiation presents a health risk which is yet to be convincingly substantiated. However, employers should take caution, adopt measures to protect employees and adhere to dose limits.

For more information on regulating work with ionising radiation, please see the HSE’s Approved Code of Practice and Guidance for compliance with employers' duties arising under the Ionising Radiations Regulations 2017.

 

[i] Azizova T et al., Hypertension Incidence Risk in a Cohort of Russian Workers Exposed to Radiation at the Mayak Production Association Over Prolonged Periods. Hypertension. 2019 Jun;73(6):1174-1184.  <https://www.ahajournals.org/doi/abs/10.1161/HYPERTENSIONAHA.118.11719?rfr_dat=cr_pub%3Dpubmed&url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org> accessed 12 August 2019. 

[ii] ‘Blood pressure chart’ (Blood Pressure UK) <http://www.bloodpressureuk.org/BloodPressureandyou/Thebasics/Bloodpressurechart#targetText=Follow%20a%20healthy%20lifestyle%20to,healthy%20changes%20to%20your%20lifestyle> accessed 16 August 2019.

[iii] Murasa K. et al., Nationwide Increase in Complex Congenital Heart Diseases After the Fukushima Nuclear Accident. J Am Heart Assoc. 2019 Mar 19;8(6):e009486 <https://www.ahajournals.org/doi/pdf/10.1161/JAHA.118.009486> accessed 16 August 2019.

[iv] ‘Regular low-level radiation exposure raises high blood pressure risk’ (3 May 2019 American Heart Association) <https://www.heart.org/en/news/2019/05/03/regular-low-level-radiation-exposure-raises-high-blood-pressure-risk> accessed 12 August 2019. 

[v] Wakeford R, Does Low-Level Exposure to Ionizing Radiation Increase the Risk of Cardiovascular Disease? Hypertension. 2019 Jun;73(6):1170-1171. <https://www.ahajournals.org/doi/abs/10.1161/HYPERTENSIONAHA.119.11892> accessed 12 August 2019. 

[vi] Regular low-level radiation exposure raises high blood pressure risk’ (2 August 2019 Health 24) <https://m.health24.com/Medical/Hypertension/News/regular-low-level-radiation-exposure-raises-high-blood-pressure-risk-20190802> accessed 16 August 2019.

[vii] ‘Prolonged exposure to low-dose radiation may increase the risk of hypertension, a known cause of heart disease and stroke’ (3 May 2019 American Heart Association) <https://newsroom.heart.org/news/prolonged-exposure-to-low-dose-radiation-may-increase-the-risk-of-hypertension-a-known-cause-of-heart-disease-and-stroke> accessed 16 August 2019.

[viii] L.A. McKeown ‘Low-Level Ionizing Radiation May Raise Hypertension Risk’ (3 May 2019 TCTMD) <https://www.tctmd.com/news/low-level-ionizing-radiation-may-raise-hypertension-risk> accessed 12 August 2019.