Apparently the issue is doublefold:
http://www.villagevoice.com/2005-03-...-diesel-fumes/
Though the issue of decling sperm count and feminization has received more attention, and carries more risk.But these figures just scratch the surface, apparently. Research has found, according to some experts, that diesel fumes degrade the immune system and interfere with hormones, reducing sperm production, masculinizing female rats, and changing the development of baby rats.
EDIT - Here's a more detailed summary: http://www.ehponline.org/docs/2001/1.../abstract.html
Our study provides evidence for the first time that inhalation of diesel exhaust during pregnancy masculinizes fetuses through accumulation of testosterone in mother rats. These experiments also showed that the elevated testosterone levels did not result from accelerated maternal adrenal functions. The feto-placental-ovarian unit and the inhibition of aromatase activities and synthesis caused by inhalation might have played essential roles in the accumulation of testosterone.
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