A 1976 explosion in an Italian factory rained TCDD over thousands of nearby homes. This unintended byproduct of herbicide manufacture is the most potent dioxin known. A new study finds that men who received large TCDD exposures from the accident have fathered fewer sons than expected. Indeed, the more dioxin a man picked up, the more likely he was to have daughters.
“In New York, Washington, Milan, and throughout the world, 106 males are typically born for every 100 females,” observes Paolo Mocarelli of the University of Milano-Bicocca in Italy. He and his colleagues report that after the accident in Seveso, this sex ratio tilted sharply in the other direction. In the men with highest TCDD exposure, only 38 percent of their subsequent children were boys. That’s only 62 males per 100 females.
An earlier study had found hints of such a trend in Seveso’s most heavily exposed victims (SN: 4/4/98, p. 212: https://www.sciencenews.org/sn_arc98/4_4_98/fob1.htm). The new, larger analysis of 535 people exposed to a broad range of doses “shows that only the fathers’ exposures—not the mothers’—are responsible for the altered sex ratio,” Mocarelli says.
In the May 27 Lancet, his team also highlights the importance of the initial exposure. Though TCDD and other dioxins can persist within body fat for decades, their concentrations steadily fall if there’s no further exposure. In many Seveso victims, dioxin concentrations had dropped to background levels before their children were conceived. Still, even if a man suffered high initial exposure from the accident before age 20, his likelihood of fathering sons was diminished.
Early exposures “must therefore be having a permanent effect on some structure” in the reproductive tract, Mocarelli told Science News. He suspects that this change perturbs chemical signals relayed to sperm in the epidydimal tract.
Indeed, observes David Ozonoff of the Boston University School of Public Health, endocrine disruptors such as dioxins represent “a new source of noise” in hormonal communications. He suggests that the resulting cross-talk might have reduced the number of male fetuses.
Dioxins are ubiquitous trace contaminants. In Seveso, altered sex ratios were linked to TCDD concentrations below 80 parts per trillion (ppt) in body fat immediately after the accident. “That’s not very high,” notes Linda S. Birnbaum of the Environmental Protection Agency in Research Triangle Park, N.C. Though U.S. background totals of dioxins have dropped since 1976, she says, they were “probably in the neighborhood of 40 to 60 [ppt in body fat]” at the time of the Seveso accident.
In some places, such concentrations still occur. In the same issue of Lancet, for instance, Jouko Tuomisto of the National Public Health Institute in Kuopio, Finland, and his coworkers report up to 170 ppt of dioxins in fat from local fishermen over age 50. However, Tuomisto notes, since the men acquired these pollutants gradually, “very few” would have had high concentrations by the time their children were conceived.