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An increase in the male to female (M/F) ratio at birth supposedly under the impact of radiation exposures from nuclear testing (worldwide) and Chernobyl fallout (in Europe) has been investigated by Victor Grech, professor at the Department of Pediatrics, Mater Dei Hospital, Malta [1,2]. The conclusions were that “elevated levels of man-made ambient radiation may have reduced total births, affecting pregnancies carrying female pregnancies more than those carrying male pregnancies, thereby skewing M/T (male live births divided by total live births) toward a higher male proportion” and “birth rates are greatly reduced and the M/T ratio is skewed upward significantly with population exposure to ionizing radiation, even at great distances from major nuclear events” [1,2]. However, social factors that could have influenced M/F ratios at birth were not analyzed. The natural radiation background (NRB) was not mentioned, although additional doses due to contamination were often negligible compared the NRB. Worldwide annual doses from NRB are generally expected to be in the range of 1-10 mSv, with 2.4 mSv being the estimated global average [3]. Some national averages are ≥10 mSv [4]. In Europe, mean annual doses from NRB are ≥5-7 mSv in several countries [5]. There are many places in the world where the dose rate from NRB is 10-100 times higher than the average e.g. 260 mGy/a in Ramsar, Iran [6], or 70 mGy/a at certain locations in Kerala, India [7]; yet there are no reliable data on shifts of sex ratios at birth in such areas. For example, a study based on ≥150,000 consecutive live singleton newborns in Kerala did not indicate any impact of elevated NRB on the sex ratio [8]. The maximum annual dose from the global fallout due to nuclear tests was estimated to be 0.14 mSv in 1963, having decreased by almost an order of magnitude by 1979 [3]. According to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), “as far as whole body doses are concerned, the six million residents of the areas of the former Soviet Union (SU) deemed contaminated received average effective doses for the period 19862005 of about 9 mSv, whereas for the 98 million people considered in the three republics, the average effective dose was 1.3 mSv, a third of which was received in 1986. This represents an insignificant increase over the dose due to background radiation over the same period (~50 mSv)” [9]. In other countries, individual doses from the Chernobyl fallout were lower: the first year doses after the accident reached 1 mSv only at singular locations in Central Europe; all country overages are ≤1 mSv/a [5,10]. For comparison, a single computed tomographic (CT) examination produces a dose within the range 2-20 mSv, while doses from interventional diagnostic procedures usually range from 5 to 70 mSv [11]. Health risks have never been proven for the abovementioned doses [12]. Annual individual doses in the vicinity of reactors have been in the range 0.001-0.5 mSv [3], so that the above dose comparisons pertain also to the reported shift of sex ratios at birth in people residing near nuclear facilities [13].

Male to Female Ratio at Birth: the Role of Background Radiation vs. Other Factors