Radiation: cancer risk estimates remain same
Radiation protection specialists can breathe easy due to the overall view of the U.N. Committee on current risk estimates
The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) in its latest report has reiterated that the current risk estimates for cancer and hereditary effects in humans from irradiation need not be changed, in spite of new findings about non-targeted and delayed cellular effects.
The October 2009 issue of the Health Physics Journal has summarised the latest annexures which covered “Non targeted and delayed effects of exposure to ionizing radiation,” “Effects of ionizing radiation on the immune system”, and “Source to effects assessment for radon in homes and work places”. UNSCEAR published two other earlier annexures in August 2008.
“Non-targeted and delayed effects occur in cells that were themselves not irradiated. They include genomic changes in the daughters of irradiated cells, changes in non-irradiated cells as a result of signals from irradiated cells (so-called bystander effects), and potential health effects in offspring due to irradiation of the parents,” the Committee clarified in a press release.
Scientists have observed “genomic instability” (effect appearing later in cells formed after several multiplications) and “bystander effects” (effects that manifest in cells not directly affected).
These could not be explained by conventional hypotheses. The International Commission on Radiological Protection (ICRP) did not consider them while estimating radiation risk estimates.
In 2000, I sought the views of Dr Jack Valentin, the Scientific Secretary, ICRP, on the impact of genomic instability on ICRP recommendations.
The then National Radiological Protection Board (NRPB), U.K., has argued that the estimates of radiation induced cancer risk in humans have been derived directly from epidemiological observations, and are, therefore, independent of the potential contribution from any novel cellular mechanism (Interview in AERB Newsletter 13: 1, 2000)
“At present, I find that unlikely, but it would seem wise to avoid being categorical,” he cautioned. “We must investigate it thoroughly,” he asserted.
UNSCEAR 2006 seems to have settled the issue.
Norman Gentner, Chairman of UNSCEAR conceded that these non-targeted effects may potentially amplify the biological effectiveness of a given radiation dose by increasing the number of cells that experience effects over those directly exposed to the radiation (UNSCEAR Release, 2009)
UNSCEAR noted that any clear relationship between non-targeted effects and observed health effects attributable to radiation remains contentious.
“Risk estimates are based on population health studies, which implicitly incorporate all elements including direct targeted effects of irradiation as well as non-targeted and delayed effects,” the latest UNSCEAR statement, is almost similar to NRPB’s, made nine years ago.
Radiation protection specialists and regulators can breathe easy; the overall view of the Committee is that the currently available risk estimates do not require changes.
UNSCEAR 2006 noted that some recent studies have shown that low levels of radiation can stimulate the immune system, at least for short periods.
These findings emerge from experimental studies and large scale epidemiological assessments of A-bomb survivors, residents of areas in Russia and the U.S. contaminated by weapons production, Chernobyl emergency workers and residents and patients undergoing radiotherapy. Data from these groups showed common patterns (UNSCEAR release, 2009).
The impact of ionizing radiation may be stronger during foetal development and in some diseases such as HIV AIDS; autoimmune diseases and genetic disorders also compromise the immune system making it more sensitive to irradiation.
UNSCEAR disclosed that there is direct evidence to confirm a small but detectable risk of lung cancer from living with radon in home.
The Committee evaluated recent direct studies of the public in Europe, North America and China exposed to relatively low levels of radon in their dwellings.
Thus far, researchers estimated the risk from radon from health studies of underground miners who were exposed to high levels of radon and its decay products.
United Nations set up UNSCEAR in 1955; its mandate is to report on levels and effects of radiation to the UN General Assembly. Resource crunch delayed the publication of UNSCEAR 2006. It has hopefully been corrected. The work of the Committee is crucial as it underpins international standards for radiation protection.
Raja Ramanna Fellow, Department of Atomic Energy
Send this article to Friends by