Researchers in Sweden recently published a small study showing that children whose moms and dads placed the children’s pacifiers in their own mouths before giving it to the child—sharing some of their oral bacteria—were less likely to develop allergies like eczema and asthma later in life.

The study’s smaller size suggests that more research is needed before a link between pacifier “sharing” and reduced allergy risk can be proven, but the findings do add to a growing body of research that suggests bringing up children in a hyper-clean environment may not be the healthiest way to raise them.

“Western culture is becoming an increasingly sterile environment, but that might not be ideal for young children as their immune systems develop,” says John Lee, MD, director of Boston Children’s Hospital’s Food Allergy Clinic. “Their bodies need to learn what to attack and what to ignore. But if they’re exposed to too few, or the wrong kinds of germs, it can hinder development, sometimes confusing the immune system into attacking nonthreatening entities like pollen or food, which is what causes allergies.”

For the study, researchers followed around 180 babies and their parents. They interviewed parents about their pacifier-cleaning practices (whether they used spit, tap or boiling water) and then checked the children’s allergy sensitivities at 18 and 36 months.

Nearly half of the parents studied said they occasionally sucked their children’s pacifiers clean, and those children had a significantly reduced risk of eczema, as well as a reduced risk of asthma, compared with children whose parents did not use their mouths to clean pacifiers. Blood tests also showed that children whose parents cleaned their pacifiers with spit had lower levels of a specific type of a particular immune cell that is usually linked to allergies.

Analyses of the study participants’ saliva suggest that the children who occasionally used a pre-sucked pacifier had different microbes in their mouths. Microbes are the billions and billions of bacteria that live inside all people and affect many aspects of our health, like producing enzymes needed for digestion or anti-inflammatory agents used by our immune system.

According to the study, this direct parent-to-child microbial interaction could promote a healthier development of the child’s immune system by establishing a natural diversity of bacteria that is supposed to be present in the body.

The study also references data showing that from birth, children’s microbial exposure is linked to allergy development. (Babies delivered vaginally are exposed to different microbes than those delivered through Caesarean section—the latter are more likely to develop hay fever, asthma and food allergies.)

While all this information about bacteria exposure supports the idea that a parent cleaning a pacifier with his or her mouth might be strengthening the child’s microbial makeup and immune system, there is still much work to be done before that’s proven definitively.

“This study was too small to draw any conclusive facts about allergy and the sharing of microbes,” says Lee. “But I joke with families that a little dirt may be good for their kids, and this study certainly wouldn’t contradict that.”

Since the study’s publication, some dentists have noted that parents sharing oral bacteria with kids might lead to cavities. But Man Wai Ng, DDS, MPH, dentist in chief at Boston Children’s says it’s doubtful that occasional pacifier sharing would be overly damaging to young teeth .

“The reality is that saliva transfer is almost completely unavoidable, especially when babies get hugged and kissed a lot,” she says. “Since oral bacteria is just a part of life, parents should focus on what they can do: good brushing with a tiny smear of fluoride toothpaste, limiting exposures to sugary foods and drinks and visiting a dentist by age 1.”

Boston Children’s Hospital’s Allergy and Immunology Program is a leading center for the evaluation and treatment of children with allergies. To learn more about how we treat children of all ages with allergies, visit our website.