Monazite sand does not cause excess cancer incidence

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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.
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.




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