Ryan, JJ, Z Amirova and G Carrier. 2002. Sex Ratios of Children of Russian Pesticide Producers Exposed to Dioxin. Environmental Health Perspectives 110:A699-A701.

Ryan et al. link dioxin exposure of fathers to alterations in human sex ratio: fewer boys are born to fathers with high dioxin levels than expected. This is the most recent in a series of papers showing similar effects by dioxin and dioxin-like compounds, for example, following dioxin exposure in Seveso or PCB/dioxin/furan exposure in Taiwan.

What did they do? The city of Ufa sits just west of the Ural Mountains at the eastern edge of Europe, in the Bashkortostan Republic of Russia. An agrochemical production plant has operated in Ufa since the early 1940s. Products manufactured at the plant have included 2,4,5-trichlorophenol (TrCP), hexachlorophene and 2,4,5-trichlorophenoxy acetic acid (2,4,5-T). Production of the latter ended in 1967.

In previous work, Ryan and Schecter had demonstrated that a sample of men, women and children associated with the chemical plant in Ufa carried high levels of dioxin (TCDD) and another, equally toxic dioxin like compound (PnCDD). Dioxin toxicity (TEQ) levels in this cohort were more 30 times higher than in the general population in Bashkortostan.

In this paper, Ryan and coworkers combine data from that previous study with samples from additional chemical factory workers, who were also found to have high contamination levels. They then examine birth records for men and women within the sample to calculate the sex ratio of the exposed sample, and compare that sex ratio to that of the general population in Ufa and to the Republic of Bashkortostan.

What did they find? For the entire cohort of workers, the sex ratio of offspring was 0.40. This differs significantly from Ufa's and Bashkortostan's sex ratio (0.512) and from expectations based on world averages (also 0.512).

An additional pattern emerged when they considered the sex ratio of male vs. female workers. For offspring born to exposed female workers (but whose father was not a worker), the sex ratio (0.51) was not different from world averages. But for the offspring of exposed fathers, the observed sex ratio was significantly depressed, , to 0.38. And when they examined the a subset of workers who had produced TrCP, the sex ratio was even lower, 0.23. The median dioxin toxic equivalent value (TEQ) for this subset of male workers was 715 ppt, compared to the overall median in the workers of 243.

What does it mean?

Three factors limit Ryan et al.'s ability to link altered sex ratios specifically to dioxin and dioxin-like compounds.

• Workers were undoubtedly exposed to a mixture of chemicals, not dioxin and dioxin-like compounds alone.
• The sex ratio and dioxin TEQ levels used in the control group were from regional averages instead of from a specific sample of people for whom both parameters were measured.
• Contamination levels were characterized long after exposures, and to a varying degree, long after the offspring included in the sex-ratio calculations were born. Hence the TEQ measurements are at best an estimate of exposures involved in altering the sex ratio.

Nonetheless, when Ryan et al.'s findings are viewed in light of research with other dioxin-exposed cohorts a clear picture is emerging: High levels of a father's exposure to dioxin and dioxin-like compounds is associated with a lower sex ratio, that is, a lower chance of having a male child.