EWG recommends breastfeeding if possible for at least the first 12 months of life. However, while there are valid reasons a family might need to use formula, infants who drink formula are at greatest risk of a common side effect.

Parents have plenty of reasons to worry about safe drinking water these days. From lead and disinfection byproducts to perchlorate and atrazine, heavy metals and toxic chemicals in water systems harm health and the environment.

And for decades, municipal utilities have added another chemical to drinking water: fluoride. But research shows that adding fluoride to drinking water, rather than toothpaste, is not the most effective way to combat tooth decay. And there is growing evidence that ingesting too much fluoride can be harmful.

The primary health concern is dental enamel fluorosis, and the most vulnerable are newborn babies who drink formula. And a groundbreaking new study found that exposure to fluoride during pregnancy can harm IQ and cognitive development in children.

Thanks to their low cost and convenience, powder and liquid concentrate formulas are popular with parents and other caregivers. About a quarter of infants born in the U.S. each year drink formula from birth. By the age of 3 months, 2.7 million American infants, or two-thirds of the age group, drink some amount of formula.

But when these formulas are mixed with fluoridated water, babies up to six months old drink considerably more fluoride than other age groups. At three months, formula-fed babies may drink as much as 10 times the tap water that adults drink, based on their body weight.

This overexposure can lead to dental fluorosis, which causes white lines, spots, staining or pitting to form on teeth and may weaken teeth as they emerge from the gums.

In 2011, responding to a lawsuit by EWG and other advocacy groups, the Department of Health and Human Services recommended that water utilities reduce the amount of fluoride added to water to 0.7 ppm – down from the previous federal recommendation allowing up to 1.2 ppm fluoride in drinking water. This new recommendation took effect in 2015.

The Centers for Disease Control and Prevention state that if infant formula is mixed with fluoridated water, the baby’s teeth might be have dental fluorosis, which appears as white spot markings on the teeth.

EWG recommends that parents take steps to limit the amount of fluoride their babies drink. Only reverse osmosis filters remove fluoride, so if your tap water is fluoridated, you could consider using a bottled water brand that contains no added fluoride. For more information about drinking water quality, visit EWG’s Tap Water Database.

Talk to your dentist and pediatrician if your baby shows signs of dental fluorosis or if you’re concerned about the amount of fluoride your family is consuming. Note that fluoride can come from multiple sources. As babies begin eating solid foods and reduce their formula intake, their exposure to fluoride goes down, but the chance of developing fluorosis continues until teeth have fully developed, around age 8.

As children get older, they can use fluoride toothpaste and mouthwash to strengthen teeth with less risk of overexposure. Use non-fluoridated toothpaste for children until they can reliably rinse and spit out toothpaste. Usually this happens between 2 and 3 years old. Once they begin using fluoride-containing toothpaste, choose a child-specific brand that has slightly lower concentrations of fluoride. Adults should supervise tooth-brushing to ensure that kids don’t swallow toothpaste.