Dear EarthTalk: I heard that children are reaching puberty at earlier ages now and that it may have to do with environmental toxins and even their TV viewing habits. Can you enlighten?

-- Mark Abbot, via e-mail

To say that kids are growing up faster than ever these days may be more than just cliché. Recent studies have shown that children are reaching puberty at younger and younger ages, and researchers are starting to see links between this trend and other societal ills such as ubiquitous pollution and sedentary lifestyles.

In a 2007 report for the Breast Cancer Fund entitled “The Falling Age of Puberty in U.S. Girls: What We Know, What We Need to Know,” ecologist Sandra Steingraber argues that unfettered access to computers and TVs over the last 30 years has led to an increasingly sedentary lifestyle among kids in the U.S. and beyond. Active kids produce more melatonin, a natural hormone that serves as the body’s internal clock and calendar. This could explain why sedentary kids are likely to go through puberty sooner: Their bodies think their decreased melatonin production is a trigger to move into puberty. “[Melatonin is] an inhibitory signal for puberty,” says Steingraber. “The more melatonin you have, the later you go into puberty.”

Of course, sedentary lifestyles are also linked to childhood obesity, a condition that often continues—along with the many health problems that can accompany it—into adulthood. A recent National Health and Nutrition Examination Survey (NHANES) found that, between 2001 and 2004, 17.5 percent of children ages six to 11 were overweight—an effective doubling of obesity rates three decades ago. A study by the non-profit Obesity Society came up with a slightly higher figure—20 percent—with the percentages higher for Hispanic, African-American and Native American children.

Obesity is certainly one factor in the surge in so-called “precocious” adolescence, but chemicals are also thought to play a role. According to Erin Barnes, writing in E – The Environmental Magazine,a study comparing the body mass index of Danish and American girls found that the former group hit puberty a full year later than the latter even though their weights were in the same range. Another study found that wealthy girls in South Africa reach puberty a full year after their African-American counterparts. “Many researchers,” writes Barnes, “are studying the relationship between chemical pollutants like PCBs (polychlorinated bphenyls) and phthalates (commonly used plasticizers) and premature development.”

Some researchers believe that the preponderance of synthetic chemicals in more developed societies are interfering with human endocrine development and essentially “tricking” kids’ bodies into going through puberty prematurely. Also, precocious puberty in girls has been linked to breast cancer, as well as higher rates of drug abuse, violence, unintended pregnancies, problems in school and mental health issues.

“Shortening childhood means a shortening of the time before the brain’s complete re-sculpting occurs,” says Steingraber. “Once that happens, the brain doesn’t allow for complex learning.” She adds that the brain can only build the connections used to learn a language, play a musical instrument or ride a bike before it gets flooded with the sex hormones that come with the onset of puberty.

CONTACTS: Breast Cancer Fund, www.breastcancerfund.org; National Health and Nutrition Examination Survey, www.cdc.gov/nchs/nhanes.htm; Obesity Society, www.obesity.org.

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