Ionizing radiation; Demography; Gender imbalance

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 1986-2005 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 above-mentioned 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].

Experimental and other relevant research has not been discussed in [1,2]. The following studies should be cited in this connection. Experiments using 18 generations of exposed mice with the daily dose ∼0.29 mGy suggested that low-dose low-rate exposures do not affect the sex ratio in mouse litters [14]. No radiation-induced sex ratio changes in the offspring of mice were found by other researchers [15-19]. On the contrary, a study and review from the year 1968 concluded that there is a sex ratio shift following spermatogonial exposure in rats [20]. It should be commented that doses used in animal experiments are much higher than average doses to the residents of contaminated territories after the Chernobyl accident. These latter doses are generally within “the window for maximum adaptive response protection” [21]. According to experimental data, this window occurs at doses between 0 and 100 mGy (or higher) from a low dose rate, low LET (linear energy transfer) radiation exposure, where the risk is expected to be reduced below the spontaneous level of cancer risk [21]. In a study from 1958, radiation was found to influence the sex ratio of infants born to survivors of the atomic bombing [22]; but the association was not seen in later studies, while the data on the total number of births in Hiroshima and Nagasaki in the period 1956-1962 indicated no significant difference in the sex ratio of infants [23]. Male radiologists tended to father even a lower proportion of boys compared to the control group [24]. Significance of supposedly radiation-related shifts of sex ratios calculated by Grech and co-workers has been questioned [25-28]. A comprehensive review concluded that “there is little consistent evidence that ionizing radiation affects the sex ratio” [29].

The data by Victor Grech and co-workers should be viewed taking into account possible mechanisms unrelated to radiation. So, except for Baltic States, all regions of the former SU showed a significant increase in M/T ratio from 1986 on [2]. The highest M/F ratios at birth were reported from the South Caucasus (Azerbaijan, Armenia and Georgia) [2,30], being explained by the son preference and sex-selective abortions [30]. The same is probably true for the North Caucasus, where the birth rate has been the highest in Russian Federation. Masculinity has traditionally high value in the Caucasus. The elevation of the M/T and M/F ratios at birth in the former SU coincided with the increasing availability of the prenatal ultrasonic gender testing in the late 1980s [2,30]. A relatively high M/T ratio at the time of generally unavailable prenatal gender testing (1981-1985) in Caucasus [2] might be seen as indication to female neonaticide, which is the ancient family planning tool [31-33]. Gender imbalance due to the son preference and sex-selective abortions occurs in China, India and some neighboring countries [31] as well as among Asian immigrants to Europe and the USA [34,35]. On one hand, there are many immigrants from the Caucasus in the former SU (except for the Baltic States mentioned above); on the other hand, similar tendencies of son preference might exist also in other groups of the ex-Soviet population favored by manliness propaganda remarkable since the early 2000s [36]. Insufficient security coupled with the tolerant attitude towards violations of the law might have motivated some families to have sons - for protection and more success. All these social phenomena in the former SU coincided with the elevation of M/F or M/T ratios. Analogously, dynamics of M/T ratio in Central Europe [2] could have been influenced by the ongoing immigration from countries with son preference and gender imbalance [34].

The conclusions by Victor Grech 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 toward a higher male proportion” [1] and that “the M/T ratio is skewed upward significantly with population exposure to ionizing radiation, even at great distances from major nuclear events” [2] have not been sufficiently corroborated. A significant role of radiation from nuclear testing and Chernobyl fallout as a factor modifying the sex ratio at birth is improbable. Dose-response relationships at low radiation doses should be studied in large-scale animal experiments involving different mammal species, comparable doses and dose rates, reliably shielded from biases and conflicts of interest.