• sex ratio
• Iran
• environmental toxin

Little is known about the factors that affect the sex ratio in humans. Many animal and human studies have indicated an association between environmental toxins and altered sex ratios.^1,2 Masjid-i-Sulaiman (MIS) is located in the Khozestan province, southwest of Iran. The first oil well in the Middle East was located in MIS (excavated in 1908 by William Darcy). Petroleum—whether gas, oil, or liquid asphalt—that exudes in the form of springs and seepages may reach the surface. Active seepages of oil and gas overlie the MIS oilfield.³ Unfortunately, some parts of MIS (named Darr-e-khersoon, Posht-e-Borj, and Camp Brench) are contaminated by subsurface leakage of natural gas, which contains hydrogen sulphide. It has been reported that the gas dissolved in the oil of the MIS oilfield contains 40% hydrogen sulphide.⁴ We determined the sex ratio at birth of families resident in the contaminated area of MIS, and compared this with the sex ratio of the general population of MIS.

We identified 359 offspring within 51 families resident in the above mentioned area, born between 1987 and 2001. Data concerning live births in MIS are routinely compiled, using reports from the Statistical Center of Khozestan province from 1987 to 2001.

To test the null hypothesis that the probability of a male live birth in the contaminated area is equal to that in the general population of MIS, χ² analysis was conducted. A probability of p < 0.05 was considered statistically significant. The sex ratio was expressed as the proportion of total live births that were male.

The overall sex ratio in general population of MIS from 1987 to 2001 was 0.504 (total number of live births 44 040). In the contaminated area, the sex ratio was increased (0.563). This difference is statistically significant (χ² = 4.91, df = 1, 0.025 < p < 0.030).

We are aware of no other similar reports. Yang et al reported that the association between exposure to petroleum air pollution, near a petroleum refinery plant in Taiwan, and sex ratio at birth was not significant.⁵ However, to clarify the effect(s) of subsurface leakage of gas on sex ratio and hormone concentrations in parents, further investigations need to be carried out.