I recently stumbled upon an interesting article that was published in Science News a while back about phthalates and their possible effect on the sexual development of girls in Puerto Rico (In case you don't know, phthalates are a group of chemicals that are most commonly used to soften plastic).
According to the article, "for more than two decades, the island has hosted an inexplicable epidemic of premature breast development, or thelarche. The incidence there—at least 7 or 8 per 1,000 girls—is the highest known. Most of the affected girls begin developing breasts between the ages of 6 and 24 months..."
To figure out what was going on, researchers in San Juan undertook a small study of 76 girls, 41 who had premature thelarche and 35 who were developing normally.
The researchers expected to find that pesticides were the cause of this premature development. However, they found no evidence of this.
Instead they found that the girls with premature breast development had significantly higher levels of phthalates in their blood: 68 percent of the girls developing prematurely had blood containing phthalates, compared with just 17 percent for the girls developing normally.
Another interesting fact: "In the [normally developing girls] whose blood contained DEHP (the most commonly used phthalate), its concentration averaged 70 parts per billion; among the girls with premature breast development and detectable DEHP, the concentration averaged 450 ppb."
Unfortunately, the data from this one study is not enough to prove that phthalates are the cause of what's going on in Puerto Rico, but it's still quite interesting...