Summary

• All participants in the study were exposed to external γ-rays and some (76.03%) also to α-particles from internally deposited plutonium
• Researchers found hypertension incidence to be significantly associated with the cumulative external radiation dose but not with internal dose from plutonium

A paper published on May 3, 2019, in Hypertension, a journal of the American Heart Association stated that prolonged exposure to low doses of ionising radiation increase the risk of hypertension. This study may receive high level of attention among radiation protection specialists.

Uncontrolled high blood pressure can lead to cardiovascular diseases such as heart attack, stroke, heart failure, and other serious health problems.

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Researchers have studied the health effects of ionising radiation for the past several decades. There is clear evidence that high levels of radiation can cause cancer and to a lesser extent hereditary effects. Whether very low doses can cause cancer and or genetic effects in man is uncertain. But, radiation protection specialists consider it prudent to assume that adverse health effects exist at all dose levels.

There were some earlier studies which linked exposure to high doses of ionising radiation (for eg., radiotherapy) to increased risk of cardiovascular diseases (CVD) and death from those diseases.

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The present study is the first to find an increased risk of hypertension due to low doses of ionising radiation among a large group of workers who were chronically exposed over many years.

Researchers assessed the hypertension incidence risk in 22377 workers exposed occupationally to ionising radiation while working in the first, large scale nuclear enterprise in Russia known as the Mayak Production Association from 1948 to 1982 and followed up until December 31, 2013. On an average they worked for 18 years. Half had worked there for more than 10 years.

The workers had comprehensive health check-ups and screening tests at least annually with advanced evaluations once in every five years. This plant produced plutonium for the first nuclear weapons of the erstwhile Union of Soviet Socialist Republics (USSR).

The Southern Urals Biophysics Institute at the Federal Medical Biological Agency of Russia carried out the study as a part of continuing research on the effects of radiation exposure on human health.

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Chief findings

1. Till December 31, 2013, the researchers identified 8425 hypertension cases (38% of workers), 5745 cases in males (36%) and 2680 cases in females (49%).
2. They defined hypertension cases as those with a systolic blood pressure reading of 140 mmHg, and a diastolic reading 90 mmHg.
3. All the workers were exposed to external γ-rays and some (76.03%) also to α-particles from internally deposited plutonium.
4. The researchers found hypertension incidence to be significantly associated with the cumulative external radiation dose but not with internal dose from plutonium.
5. The hypertension incidence among the workers in the study was higher than that among Japanese survivors of the atomic bomb at the end of World War II, but lower than the risk estimated for clean-up workers following the Chernobyl nuclear accident.

The researchers explained the differences as due to the variations in exposure among the three groups. Following the atomic bombing, the A-bomb survivors experienced a single, high-dose exposure of radiation, the Chernobyl workers were exposed to radiation for a short time period (days and months), while the Mayak workers were chronically exposed to low doses of radiation over many years.

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While the development of cancer is commonly associated with radiation exposure, "we believe that an estimate of the detrimental health consequences of radiation exposure should also include non-cancer health outcomes. We now have evidence suggesting that radiation exposure may also lead to increased risks of hypertension, cardiovascular disease and cerebrovascular disease, as well," Dr. Tamara Azizova, lead author of the study at the Southern Urals Biophysics Institute in Russia stated in a press release.

She noted that in recent years, the number of people exposed to radiation in everyday life, such as during diagnostic procedures, has increased.

"It is necessary to inform the public that not only high doses of radiation, but low to moderate doses also increase the risk of hypertension and other circulatory system diseases, which today contribute significantly to death and disability. As a result, all radiological protection principles and dose limits should be strictly followed for workers and the general public," Azizova cautioned.

She conceded that how radiation exposure may increase the risk of hypertension is still a question. "So far, the mechanisms remain unclear, not only for certain cohorts but also for the general population. 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," she added.

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In an accompanying Editorial Commentary, Dr Richard Wakeford, an eminent epidemiologist noted that in spite of some reservations, “groups of workers exposed to ionising radiation are a valuable source of information on risks from low-level exposure.”

“In general, occupational exposure to radiation is monitored and doses are recorded, and good personal data for workers are also available to enable reliable follow-up of vital status and cause of death using appropriate registries. Some of these workers, particularly those employed in earlier years, have accumulated moderate-to-high doses of radiation but predominantly as a series of many low doses received at a low dose-rate,” he clarified.

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After listing the merits and demerits of several such studies Dr Wakeford asserted thus:

“There is no doubt that the relationship between the risk of CVD and low-level exposure to ionising radiation is one of the big issues in radiological protection today. The current framework of protection against adverse effects of low doses or low dose rates of radiation does not take account of any radiation-related risk of CVD, but potentially, this risk is a substantial component of the overall risk to health from low levels of exposure”.

“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,” he added.

“At present, there is indicative evidence of a link, but overall, the findings are not yet persuasive, in particular because of the uncertain influence of major non-radiation risk factors on the reported associations. As ever, observational epidemiology has great difficulties in getting a firm grip on underlying relationships in the presence of material variations in background risk factors. Until meaningful inroads can be made into dealing with this problem, reliable interpretation will remain elusive,” he concluded.

On October 7, 2010 the UK’s erstwhile Health Protection Agency’s (HPA) Advisory Group on Ionising radiation published a lucid report titled “Circulatory Disease Risk” reviewing the then existing epidemiological studies and experimental work. One of the conclusions of the report was: “Even small relative risks due to radiation could have a major impact…….. as circulatory diseases are already common in the population."

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Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.

Dr. K S Parthasarathy is a freelance science journalist and a former Secretary of the Atomic Energy Regulatory Board. He is available at ksparth@yahoo.co.uk