Hygiene leaves kids with loads of triclosan.

Levels of a controversial chemical meant to kill bacteria spike in the bodies of young children after they brush their teeth or wash their hands, according to a new study.

U.S. manufacturers are phasing triclosan out of hand soaps after a Food and Drug Administration banned it effective last year amid concerns that the compound disrupted the body's hormone systems. It remains in Colgate Total toothpaste, some cleaning products and cosmetics. Health experts say exposure is best avoided for babies in the womb and developing children.

The latest study, published in the journal Environmental Science & Technology, is one of the first to show that children's levels rise through their first few years of life. Hand washing and teeth brushing have speedy, significant impact on levels, the researchers found.

“There's very little data on the exposure in those first years of life," said the senior author of the study, Joe Braun, an assistant professor and researcher at Brown University. “There are a lot of behavioral changes in the those years, and environmental chemicals can play a role."

Braun and colleagues tested the urine of 389 mothers and their children from Cincinnati, collecting samples from the women three times during pregnancy and from the children periodically between 1 and 8 years old.

They found triclosan in more than 70 percent of the samples. Among 8 year olds, levels were 66 percent higher in those that used hand soap. And more washing left the children with higher loads—those who reported washing their hands more than five times per day had more than four times the triclosan concentrations than those washing once or less per day.

Children who had brushed their teeth within the last day had levels 2.5 times higher than those who had a toothpaste-free 24-hour span.

“It's a thorough, well-done analysis," said Isaac Pessah, a researcher and professor at University of California, Davis, who was not involved in the study. “Given the high concentrations [of triclosan] in personal care products, you're seeing that the concentrations in the end user are also quite high."

Braun said the levels of triclosan rose as the children aged, eventually leveling off. “Their levels were almost to moms' levels by the time they reached 5 to 8 years of age."

"There is evidence that triclosan is positive for dental health but if it's not recommended from your dentist that you use this product, then don't."-Joe Braun, Brown UniversityThis, he said, is likely due to more frequent use of personal care products as the kids aged. Despite the hand soap ban, triclosan remains on the market because it is effective at fighting plague and gingivitis.

Colgate uses 0.3 percent of the antibacterial to “fight harmful plague germs."

Colgate's parent company, Colgate-Palmolive, did not respond to requests to comment on Braun's study. The company has, however, maintained triclosan's safety in its toothpaste and its role in fighting plaque and gum disease.

“As for claims of endocrine disruption, the World Health Organization defines an endocrine disruptor as something harmful to one's health, and that is simply not the case here," wrote Colgate's head of R&D;, Patricia Verduin, in response to media reports on triclosan's endocrine impacts.

Braun, however, said there is “quite compelling" evidence from animal studies that triclosan decreases thyroid hormone levels. Properly functioning thyroid hormones are critical for brain development.

Just last month, using the same mothers and children, Braun and others reported that mothers' triclosan exposure during pregnancy was linked to lower birth weights, smaller heads and earlier births.

In addition, Pessah and colleagues reported triclosan hinders proper muscle development. The researchers used mice and fish, finding that triclosan affects the process responsible for muscle contraction.

Braun said the study was limited in that they didn't know what brands of toothpaste or soap kids were using. Also, product use was self-reported and mothers may not want to say their kids weren't brushing their teeth, he said.

Researchers do not argue triclosan's effectiveness at reducing bacteria. And Pessah said there is a role in protecting oral health. “Perhaps if we would have made triclosan available by prescription for some people, like those with secondary infection on their gums from diabetes, we wouldn't reach the contamination levels we now see."

Braun said people might take pause next time they're browsing for bathroom and cleaning products.

“There is evidence that triclosan is positive for dental health, but if it's not recommended from your dentist that you use this product, then don't," Braun said. “Especially if you're a young child or pregnant woman."