Revision 1
March 19, 1998
by Radiation, Science, and Health, Inc.,
Edited by J. Muckerheide

1.2.6
Natural Radiation and Radioactivity

1.2.6.2
Natural Background: Populations

"The newly classified dose groups by the external radiation levels, number of the cohort members and the person-years of observation (1979-1990) are listed in Table 1.

Classification of Dose Groups (Based on the measurements of external gamma-radiation, indoor and outdoor, in 526 hamlets of the investigated areas)*

* Based on Ref. (Yuan 1997)
** Calculated only for fixed cohort

"It should be noted that if we consider the HBRA as a whole (including three groups of ‘high’, ‘medial’ and ‘low’) thus the ratio of external radiation dose rates (absorbed dose rate in air) between HBRA and CA was 3.5 for indoor, and 3.7 for outdoor."

"Table 3 demonstrates the adjusted RRs (90% CI) of site-specific cancers for the four dose groups (1979-1990 data), still it shows no statistical difference for all the site-specific cancers. It also shows that there is no increase of any cancer mortality with the increase of annual dose of external radiation (Tao et al 1997).

"The prevalence rate of 31 kinds of hereditary diseases and congenital defects in children (<12 years old ) in HBRA was compared with those in CA, which showed the prevalence rates between HBRA and CA were almost identical except the rate of Down Syndrome (higher in HBRA statistically), however, the prevalence rate of Down Syndrome was in the normal range of spontaneous rate if compare it with other places in the same Province (Guangdong Province) and some other places in China. (Wei 1996)"

"Some organizations and authors, based on the "no-threshold, linear" hypothesis estimated the risk for low dose exposure and stated ‘no matter how small the dose, there will be increment of cancer induction.’ But the results obtained in the HBRA of China have not demonstrated any increment of cancer mortality."

"The High Background Radiation Research Group stated in 1985 (Wei 1985):

‘However, the fact is that the increase of frequency of chromosome aberrations were observed in the high background radiation areas (HBRA), and the possibility of effects induced by ionizing radiation can not be excluded, although the doses were low. Nevertheless, on the contrary, cell-mediated immunity examination revealed that there is a tendency of strengthening of immune functions among the HBRA inhabitants. If we say the former is disadvantageous, thus the latter is beneficial. So, there is a competition between these two kinds of effects. As above mentioned, up to now we have not found the increase of mutation-based diseases. Possibly, it implies that the beneficial effects are superior to the detrimental effects in case of low level radiation of HBRA.’ "

"Recent literature described the fate of damaged DNA and the process of multi-step colorectal carcinogenesis (UNSCEAR 1993). In case of radiation, the DNA damage may be caused by direct action or indirect action. Besides the apoptosis, damaged DNA may be balanced by elaborate defense and repair process, although it is not complete. Thus, there is probability of converting DNA lesions to mutation, if the increase of rate of cell division occurs. The process of carcinogenesis needs multisteps, and it needs several mutations (UNSCEAR 1993). In case of low dose exposure, the stimulatory effects strengthen (defense and immune functions strengthen) for inhibiting and preventing the formation of mutations. Many reports of epidemiology and radiobiology and the results of investigation in HBRA of Yangjiang denoted that in case of low dose exposure the defense and repair system might be superior to the disadvantageous effects. The results of immunological function obtained from the examinations of inhabitants in HBRA and CA demonstrated the strengthening of these functions (Liu 1997, Zou 1997). Table 12 shows the results obtained from laboratory experiment which shows low dose ionizing radiation may reduce the incidence of mammary tumor. (Makinodan 1992)"

"Summary:...
"The long term epidemiological study (1972-1990) in the high background radiation areas (HBRA) in Yangjiang, China and the nearby control areas (CA) demonstrated that no increase of cancer mortality, or incidence of hereditary diseases and congenital deformities was found in the HBRA after careful study on comparability between HBRA and CA and between the dose groups within the cohort (1970-1986 data, and the 1979-1990 data which were used for fixed cohort analysis on cancer mortality). However, a minor increase of frequency of chromosomal aberrations was found in HBRA, the results were reproducible."

"Literature on mechanisms of carcinogenesis revealed that DNA lesions have a certain probability of giving rise to mutations when the cell divides, and mutations in several critical genes can lead to tumors. However, in the meanwhile there exist defense and repair system for inhibiting and preventing the formation of mutations. Perhaps, this system even effective during the Promotion stage of carcinogenesis. Thus these two kinds of system constitute a series of competition In case of radiation exposure, the results will be dependent on the dose and dose rate. Many reports and the results of investigation in HBRA of Yangjiang denote that in case of low dose exposure the defense and repair system may be superior to the disadvantageous effects. More evidences (both epidemiological and biological) are needed to prove this hypothesis. Thus, international cooperation is very important to corroborate whether the findings above mentioned are reproducible, and to know whether the findings can be interpreted by radiobiology."

The incidence of spontaneously occurring mammary tumor of mice subjected to LDR* and CRD**

Based on T. Makinodan 1992, in "Low dose irradiation and biological defense mechanisms", Sugahara, T., Sagan, L.A. and Aoyama, T. (eds) . Elsevier Science Publishers, Amsterdam, pp. 233-237

CRD--chronically restricted diet (calorically 70% of ad-libitum diet)