Researchers at the Sahlgrenska Academy of Gothenburg University in Sweden tracked 174 babies and their parents for several years and tested them for allergies, eczema and asthma. They also asked parents how they cleaned off pacifiers, and found that nearly half of them used their mouths on occasion.
The study, published in the journal Pediatrics, found by the time the babies were 18 months old, those whose parents licked their pacifiers were less likely to have asthma and eczema, ABC News reported.
The researchers concluded this was because parents exposed their babies to bacteria in their saliva, stimulating babies' immune systems.
Dr. Erick Forno, a pediatrician at the Children's Hospital of Pittsburgh of the University of Pittsburgh Medical Center, told ABC News said the study was small and the long-term effects of this bacteria exposure are not clear, but parents with herpes or cold sores could pass the virus to their children.
Dr. Jonathan Shenkin, a pediatric dentist and American Dental Association spokesman, said the practice of using a parent's saliva to clean a pacifier might boost the immune system, but dental research showed cavity-causing bacteria from adult mouths can be transmitted to a child, thus promoting tooth decay.