High maximum annual doses of up to several hundred mSv are found in Kerala, India and Ramsar, Iran. The monazite-bearing, high-thoron-content sands of Kerala have a resident population of 200,000 living for generations at these high background levels. The frequency of micronuclei formation in newborns living in the HBRA of Kerala was not different from newborns living in normal radiation areas.
There was no increase in all cancer mortality, all cause mortality or decrease in longevity in native populations exposed to high levels of background radiation when compared with low-dose regions over an annual dose range of 0.8 - 700 mSv. More rapid DNA repair was observed in persons living in Ramsar, Iran, who were exposed to high background radiation. Studies of Brazilian HBRAs showed both nonsignificant slightly increased and descreased cancer mortality. No increase in cancer incidence has been found in inhabitants of Yangjiang , China, Kerala, India, or in Ramsar, Iran. A negative excess cancer risk was found Yangjiang, China at a natural background exposure of 9.9 mSv/year. Lung cancer in Ramsar showed a general negative correlation with natural radon levels. A negative correlation was found for all cancer with natural background dose in cities of India. No increase in overall mortality or birth defects was found in case-control studies in Ramsar, Iran or in Kerala, India.
Ramsar, a coastal city in northern Iran has the highest level of background radiation in an inhabited region of the world; the dose level is more than 100 mSv/year. This dose level is more than five times higher than the 20 mSv/year permitted for radiation workers or 55 - 200 times more than the average global dose. There have been no ill effects to populations in Ramsar exposed to these high radiation levels. No significant cytogenic effects have been observed in Ramsar compared with those living in normal background levels. Physicians in Ramsar have not reported an increase in cancer rates. An in vitro challenge dose of 1.5 Gy given to blood lymphocytes of inhabitants of Ramsar showed significantly reduced chromosomal damage compared with residents of normal background levels.