Children with high levels of PFCs in their blood showed weak responses to tetanus and diphtheria shots, producing fewer antibodies in the blood, according to a study in the Journal of the American Medical Association. The research suggests that some of these children may still be vulnerable to the diseases even though they had been vaccinated. PFCs are found in non-stick pans, stain-resistant clothing, food packages, and fish, and have been previously linked to health problems like thyroid conditions.