Original link: http://www.thehindu.com/todays-paper/tp-features/tp-sci-tech-and-agri/article671607.ece
Cancer risk: The study found that there is no excess cancer risk to
people living in the area of high natural background radiation in
Kerala.
Now it is official. In the January
2009 issue of the Health Physics Journal, researchers from the Regional
Cancer Centre (RCC), Thiruvananthapuram, and their collaborators have
shown that there is no excess cancer risk to people living in the area
of high natural background radiation in Kerala from exposure to
terrestrial gamma radiation.
The Journal highlighted the importance of the paper by carrying a photo of the beaches in its cover page.
Gamma radiation
The coastal belt of Karunagappally, Kerala, is known for high background radiation (HBR) from thorium-containing monazite sand.
In
the coastal panchayats, the median outdoor gamma radiation levels are
more than 4 mGy y{+-}{+1} and in certain locations, the levels are as
high as 70mGy y {+-}{+1}.(Gy is a unit of radiation dose; mGy is one
thousandth of a Gy; the annual gamma radiation level in normal locations
is on an average one mGy).
During 1990-97, survey
teams collected data on 359,619 subjects in 71,674 households using a
standardised questionnaire which covered socio-demographic factors,
lifestyle, dietary habits and tobacco and alcohol use.
Follow up study
Based
on radiation level measurements, by a method perfected by scientists of
the Bhabha Atomic Research Centre, they chose a radiation sub cohort
consisting of 173,067 residents and analysed the cancer incidence in the
sub cohort, aged 30 to 84y (N=69958 followed up for 10.5 years).
They
estimated the cumulative radiation dose to each individual in the age
group based on the radiation doses received indoors and outdoors and
taking into account how long and where they stayed during the period.
By the end of 2005, they identified 1379 cases of cancer including 30 cases of leukaemia.
The results
Statistical analysis of the data showed no excess cancer risk from exposure to terrestrial gamma radiation.
In
site-specific analysis, they did not find any cancer site or leukaemia
to be significantly related to cumulative radiation dose.
“Although the statistical power of the study might not be adequate due
to the low dose, our cancer incidence study, together with previously
reported cancer mortality studies in the HBR area of Yangjiang, China
suggests it is unlikely that estimates of risk at low doses are
substantially greater than currently believed,” the researchers
concluded.
It appears that the researchers were in a
hurry to publish the paper. They did not use the complete data but
selected four coastal panchayats (Chavara, Neendakara, Panmana and
Alappad) which had HBR and two control areas (Oachira and Thevalakkara)
which have relatively low natural radiation levels.
They
estimated the excess risk as -0.13 Gy{+-}{+1} (95 per cent confidence
limit:-0.58, 0.46). The authors pointed out that the upper limit of 95
per cent confidence limit was lower than 0.97, which other researchers
got for pooled analysis for nuclear workers from 15 countries (BMJ,
2005) and slightly lower than 0.47 Gy{+-}{+1} reported in the study of
atomic bomb survivors in Hiroshima and Nagasaki (Radiation Research,
2007)
Authors highlighted some unique features of
their data. Unlike the nuclear workers study, RCC study included smoking
habits, an important contributing factor. The estimate of atomic bomb
survivors is a sex-averaged estimate for solid cancer unlike the RCC
study. The currently accepted radiation risk estimate is mostly based on
atomic bomb survivor study.
Regrettably, the
researchers did not estimate the substantial contribution of airborne
radon and thoron daughters to the individual radiation dose. This may
not affect the main conclusion that there is no excess cancer in areas
of high natural background radiation.
The limitations
Though
the analysis limited to six panchayats cannot be faulted
scientifically, they should use complete data including internal dose
from all panchayats for a reanalysis to do justice to the project and to
examine whether precise radiation risk estimate can be arrived at from
this study
Highlighting the negative radiation risk
coefficient of -0.13 Gy{+-}{+1}, proponents of those who believe in the
beneficial effects of radiation (hormesis theory) may argue that low
level radiation is helping to lower cancer risks!
They may not agree that lack of statistical power may be the reason for the negative result.
K.S. PARTHASARATHY
FORMER SECRETARY, AERB
