Background radiation in the environment comprises of variety of sources including both natural and artificial radiation. Natural background radiation (NBR) is defined as radiation of the non-artificial sources of apparent origin and sometimes simply referred to as background radiation (BR) for the environmental interest. Its origin is inhalation of air (mainly ²²²Ra), ingestion of food and water (⁴⁰K, ¹⁴C, etc.), terrestrial radiation from ground (depending on soil materials), and cosmic radiation (solar and galactic origin). The levels of natural background radiation vary with location, time and altitude. The global average effective dose to humans has been estimated to be 2.40 mSv/year (UNSCEAR Report 2000). This level in Japan is 1.50 mSv/year (http://search.kankyo-housyasen.go.jp).
     However, several regions in the world are known as high background radiation area (here we call HBRA), where the natural background radiation is far excess of the world average one. They include Guarapari (Brazil), Kerala (India), Ramsar (Iran) and Yangjiang (China). Since the exposure levels in these areas are several times (some tenths of times in particular area) higher than the recommended limit for exposure to the public from artificial sources (ICRP 2007 Recommendations; 1 mSv that may have a 5.5% chance of developing cancer), the health consequences of the residents has been continued concern in radioprotection and radiobiological sciences.

Scenario:       Kerala is a southwest Indian state facing on the Arabian Sea. It coastal belt, about 0.5 km width and about 250 km long areas spanning from Neendakara in Kollam district to Purakkad in Alappuzha district is known as world highest level of natural radioactivity among densely populated areas.       The high level of natural radiation area (HLNRA) is due to local abundance of monazite, a mineral containing high concentration (weight average of 8-10.5 %) of thorium phosphate (232ThPO2). The residents are exposed to radiation released by its decay and decay products externally (gamma- and beta-rays emitted in the 232Th series) and/or internally (dietary and respiratory intake of radionuclides).      Several studies on the frequencies of congenital malformation (Jaikrishan et al., Radiat. Res., 152:S149-S153, 1999), Down syndrome (Kochupillai et al., Nature, 262:60-61, 1976; Sundram, Nature, 267:728, 1977; Edwards and Harnden, Nature, 267:728-729, 1997; Verma et al., Nature 267:729, 1977) have been done but with no conclusively significant difference as compared to those in the normal level natural radiation areas (NLNRA). More recently, the acceleration of germline mutation of mitochondria DNA has been reported (Foster et al., Proc. Natl. Acad. Sci., 99:13950-13954, 2002).

Study populations
      A large scale cytogenetic investigations have been carried out from 1986-2007 for inborn abnormalities and acquired damage to chromosomes in children. This investigation is unique in that the study design has primarily focused on a potential effect of high level natural background radiation on inborn abnormalities of children born to parents living in areas of high natural background radiation.
     The data compiled here are those presented in the mose recent publication by Ramachandran et al., Int. J. Radiat. Biol., 89:259-267, 2013). In the course of the same research project, several interim reports have been also published (e.g., Cheriyan et al., Radiat. Res., 152:S154-S158, 1999; Das and Karuppasamy, Int. J. Radiat. Biol., 85:272-280, 2009; Das et al., Int. J. Radiat. Biol., 88:642-647, 2012; Jaikrishan, et al., Indian J. Community Genet., 4:21-31, 2013)

     The umbilical cord blood was collected from 27,295 consecutive newborns at the four Government hospitals (17,298 from HLNBRA and 9,997 from NLNBRA). The chromosome preparations were screened (1) for karyotype abnormalities with the G-banding confirmation, and (2) for types and frequencies of chromosome structural aberrations.

Dosimetry
     The dosimetric profiling of the study area was carried out by dividing the area into small meshes of 100 m². The ambient dose of each mesh at 1 m above ground was estimated by means of several Geiger-Muller-based environmental radiation dosimeters. Since the normal background areas of Kollam district is 1.2 mGy/y (range <1.0 to 1.5 mGy/y), the areas with radiation exposure below 1.5 mGy/y were considered as NLNRA and those above 1.5 mGy/y were considered as HLNRA.

References 
     Ramachandran, E. N., Karuppasamy, C. V., Cheriyan, V. D., Soren, D. C., Das, B., Anilkumar, V., Koya, P. K. M. and Seshadri, M. (2013): Cytogenetic studies on newborns from high and normal level natural radiation areas of Kerala in southwest coast of India. Int. J. Radiat. Biol., 89:259-267.

     This study also included the frequencies of inborn abnormalities and answered on the earlier question on the elevated levels of Down syndrome. After comprehensive studies including background radiation levels and parental age effects, the authors concluded that there was no significant difference in the inborn abnormalities, including Down syndrome, between two levels of natural background radiation areas, and from newborn abnormalities in other nations as well. Since the authors did not find any difference in the frequencies of inborn abnormalities between two areas, the data on inborn abnormalities were combined and presented in conjunction with other population studies on [inborn errors].