Omega-3 fatty acids from fish oil can often be found in adult vitamins.
And they are increasingly being added to some kids' vitamins, as well.
But Jeff Ashley, Ph.D., a chemistry professor at Philadelphia University, says that might not be a good thing.
He's found PCBs - a toxic industrial chemical - in some brands of kids vitamins and gummy supplements.
PCB's have been banned since the 1970s, but can still be detected in water-ways across the country.
"Fish are exposed to it through eating smaller fish, or ingesting sediment, or the water," says Dr. Ashley.
"Larger fish will have larger burdens of PCBs," he continues.
Although the PCB levels are low, they can accumulate in the body if kids take those vitamins every day.
Dr. Ashley urges parents check the labels of vitamins containing fish oil, to find out where the fish were harvested.
And parents should encourage kids to eat fish that's high in Omega-3, but not PCBs.
For example, it takes only and ounce and a half of wild-caught salmon to get the same amount of Omega 3's found in a week's worth of vitamins.