Last month, the Environmental Protection Agency issued final rules aimed at cleaning up the soot and gases emitted by heavy-duty diesel trucks and buses. That should significantly reduce respiratory harm from diesel fumes. It might also decrease their potential for perturbing normal sexual development, a previously unanticipated risk suggested by results of a new study in rats.
Japanese researchers find that when pregnant rats inhale diesel emissions—even after all soot particles have been filtered out—their male and female young develop signs of inappropriate masculinization.
As part of an assault launched by Tokyo's governor on diesels' soot and gases, Nobue Watanabe of the Tokyo Metropolitan Research Laboratory of Public Health and her colleagues have been probing reproductive risks posed by the pollution. In 1999, they found that soot-free diesel exhaust perturbed sex-hormone production in young male rats, causing the animals' sperm production to drop by one-third.
In follow-up experiments, Watanabe and a coworker exposed 48 pregnant rats to either normal or soot-free diesel exhaust for 6 hours a day, beginning 1 week after conception. She describes the inhaled dose as 30 to 60 times as much diesel exhaust as people in urban Japan would typically take in. Another 24 pregnant rats breathed only clean air.
In the February Environmental Health Perspectives, the researchers report that the testes, ovaries, and thymuses of fetuses of all diesel-exposed rat moms had delayed or abnormal development. Compared with those in the pregnant rats breathing clean air, concentrations of testosterone—the major male sex hormone—in the diesel-exposed moms' blood were significantly higher and concentrations of progesterone—a major pregnancy hormone—were significantly lower.
The most noticeable external abnormality in fetuses from the diesel-exposed mothers was a lengthening in the distance between the anus and genitals. This anogenital distance is ordinarily twice as long in male rats as in females. Among exposed male and female fetuses, this distance was some 10 percent longer than in unexposed males.
Aside from this masculinization, "exposed [fetuses] looked apparently healthy," Watanabe says.
Although the researchers removed most fetuses, some pups were born and appear normal. Watanabe is now chronicling subtle immunological changes and looking for other abnormalities.
The new findings perplex L. Earl Gray Jr. of EPA's laboratory in Research Triangle Park, N.C. "We've never seen anything in our rats that increases anogenital distance in both sexes," he notes. To get such a lengthening, he adds, "I would expect testosterone [concentrations] in the mothers to be perhaps tenfold higher [than were measured]."
The diesel gases in the Japanese study targeted the moms' ovaries, altering their structure and hormone production, notes Frederick S. vom Saal of the University of Missouri in Columbia. He rates the new findings as "a wake-up call, because if [diesel exhaust] does that to a rodent, it could potentially do it to humans." However, he notes, since ovarian function in pregnancy differs across species, human impacts might vary considerably from those seen in rats.
Adverse reproductive effects might disappear if the Japanese team ratcheted down exposures to more realistic concentrations, adds Joe L. Mauderly, a diesel toxicologist at the Lovelace Respiratory Research Institute in Albuquerque. He notes that the highest U.S. workplace exposures to diesel fumes seldom reach 20 percent of the concentration that Watanabe used, while people living near freeways might only encounter 0.2 percent of that level.
L. Earl Gray
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
Joe L. Mauderly
Lovelace Respiratory Research Institute
P.O. Box 5890
Albuquerque, NM 87185
Frederick S. vom Saal
University of Missouri
Division of Biology
114 Lefevre Hall
Columbia, MO 65211
Department of Environmental Health
Tokyo Metropolitan Research Laboratory of Public Health
2401 Hyakunincho 3 chome
Shinkuku-ku, Tokyo 69-0073
Raloff, J. 2001. Macho waters. Science News 159(Jan. 6):8.
Watanabe, N., and Y. Oonuki. 1999. Inhalation of diesel engine exhaust affects spermatogenesis in growing male rats. Environmental Health Perspectives 107(July):539.