On thorium and Lynas — Chan Chee Khoon
MARCH 27 — The Sun declined to publish my response (below) to Dr Looi Hoong Wah’s earlier letter. In the interest of reasoned exchanges, I hope this response to Dr Looi’s latest letter sees the light of day.
In this letter, Dr Looi’s cites the Argonne National Lab’s fact sheet on thorium to argue that only a miniscule portion of thorium-232 which is ingested via food or water is absorbed into the bloodstream, of which only 4 per cent gets deposited in the liver where it is retained with a biological half-life of 700 days.
He neglects to mention that thorium-232 is much more readily absorbed into the human body via an inhalation route, and furthermore that 70 per cent of the amount entering the bloodstream gets deposited in bone where it is retained with a biological half-life of about 22 years, all that while irradiating the much more radio-sensitive blood-forming tissues there with highly mutagenic alpha-particles (20 times more damaging to cellular genetic material than beta or gamma radiation).
Could this be the reason for the cluster of childhood leukaemias observed among the children of Bukit Merah? (Recall also the inverse square law — the intensity of radiation from a radioactive particle a metre away from a human body increases a trillion-fold when that same particle sits at micron-level distances on the body’s cells and tissues.)
Dr Looi considers that inhalation exposures to thorium-containing dust is solely an occupational problem which is not relevant to the greater Kuantan-Kemaman community. Let’s recall that the ARE rare earths refinery at Bukit Merah, like LAMP, had no long-term waste management plan. Ad hoc arrangements, including the aborted Papan dump site, eventually led to a situation of indiscriminate, clandestine dumping of radioactive thorium-cake wastes at Lahat, Menglembu, Pengkalan, Jelapang, Buntong, Simpang Pulai, among other locations.
The Kuantan-Kemaman community similarly faces the prospect of unknown numbers of dump sites at unknown locations scattered in and around the cities if Lynas does not come up with an acceptable plan for long-term waste disposal.
Allow me also to bring to Dr Looi’s attention a 1993-1994 study of male miners at the Bayun Obo rare earths and iron mine in Inner Mongolia which was reported in the Journal of Radiological Protection in 2005.
In that study, highly dust-exposed miners had 5.15 times the age-adjusted lung cancer rate compared to the rate among Chinese males in the general population. The less-exposed mining staff had 2.30 times the general population rate. Both groups had similar smoking rates (78 per cent vs 67 per cent for the general adult male population).
On this basis, the authors concluded that the excess lung cancer risk among the less-exposed was largely due to above-average smoking, and the further difference between the two miner groups was due to high exposure to airborne crystalline silica particulates (mainly) and to thorium-containing dust and its radioactive daughter nuclides such as thoron gas.
These conclusions are highly debatable, and it is precisely in this situation of uncertainty and lack of consensus that the Kuantan-Kemaman community shouldn’t end up as tikus makmal (lab rats) in a natural experiment for Lynas.
Finally, it should also be noted that the ores that the Chinese miners were exposed to contained 400 ppm of thorium. The rare earth oxide concentrates that will be arriving shortly at Kuantan port will have 1,600 ppm of thorium.
The US Public Health Service (1990) reports that the natural background level in North American soil is typically ~ 6 ppm of thorium.
The unpublished letter to The Sun.
Dr Looi Hoong Wah, in dismissing the Kuantan community’s apprehensions over Lynas rare earths refinery in Gebeng (The Sun, March 7, 2012), has glossed over a number of points:
1. Potassium-40, when absorbed, is distributed more uniformly throughout the human body, mostly in muscle tissue which is among the least radio-sensitive of the body’s tissues (the most radio-sensitive tissues are the lymphoid organs, bone marrow, blood, testes, ovaries, intestines)
2. Thorium-232 is very poorly absorbed from the gastrointestinal tract, but much more readily via an inhalation route. Seventy per cent of thorium entering the bloodstream localises to the bones where it is retained with a biological half-life of 22 years, all that while irradiating the blood-forming tissues with alpha-particles which are 20 times more damaging than gamma or beta radiation.
3. In contrast, potassium-40 is excreted from the body much more rapidly, with a biological half-life of 30 days.
4. The net result of all this: the lifetime cancer mortality risk per Curie of inhaled thorium-232 is 200 times that of inhaled potassium-40, and 2,000 times that of ingested potassium-40 (1 Curie = 37 GBq).
5. The “safe thresholds” of 1 mSv/yr (public) and 20 mSv/yr (occupational) that Dr Looi, Dr Che Rosli Che Mat (MP, Hulu Langat), Lynas, AELB, and IAEA repeatedly invoke are derived from ICRP risk models which are currently under critical scrutiny and challenge, in the wake of excess childhood leukaemia near nuclear power plants that can’t be explained by radiation exposures which are much below the “safe thresholds”. A UK expert panel for instance (2004, www.cerrie.org) could not arrive at a consensus regarding the health risks of low-level exposure to internal emitters (inhaled or ingested radioactive particles). Opinions among the UK panel members ranged from negligible adverse effects to an underestimation of risk by at least a 100-fold.
6. In short, nobody really knows at this point how hazardous the Lynas refinery may turn out to be, given that much of the radioactive solid wastes will be in powdery form, i.e. respirable as suspended particulates, or ingestable from contaminated surfaces.
7. In asserting that the LAMP refinery is unquestionably safe, Dr Looi, Dr Che Rosli, Lynas and AELB have recklessly abandoned the precautionary principle. The Kuantan-Kemaman community may end up as lab rats in a natural experiment.
* Chan Chee Khoon, ScD (Epidemiology) is from the Centre for Population Health, Department Social & Preventive Medicine Faculty of Medicine, University of Malaya.
* This is the personal opinion of the writer or publication. The Malaysian Insider does not endorse the view unless specified.