Swapping spit is usually something we try to avoid, smooches notwithstanding. But we wouldn’t be Popular Science if we didn’t tell you why you shouldn’t be grossed out by something that is, well, pretty gross.
Spit is gloopy and sticky and it has a very distinctive smell. It’s also antibacterial, and it’s a great cleaning agent (just check out this Ig Nobel Prize-winning research from 1990). And now it’s turning out to be a potential aid to preventing allergies. Recent research suggests that babies whose parents clean pacifiers with spit tend to have fewer allergies than those whose parents do what many would call “actually clean” their pacifiers.
First: why the heck would spit help babies avoid allergies? One potential explanation is that it doesn’t. Maybe this is a case of correlation, not causation. The study didn’t ask parents to clean pacifiers a certain way and then track the allergy outcomes in their babies. It asked parents to self-report how they cleaned their children’s pacifiers and tested those children to see how much IgE they had. IgE is a kind of immune system protein that mediates allergies, so having higher IgE levels generally means you’re more likely to develop allergies. It’s not a perfect test, though, and the researchers could only look at relationships between the two variables, not investigate causation. Perhaps people who use spit to clean pacifiers also lead lives that tend to reduce allergies. We know that more exposure to bacteria and general dirtiness seem to foster better immune systems, and the kind of person who doesn’t wash pacifiers (and is willing to cop to that fact in front of researchers) is probably a person who doesn’t disinfect every surface in their home or freak out if their kids roll around outside.
Another explanation, which the study authors think is fairly likely, is that there’s some health-promoting microbe in parental saliva that promotes a normal, non-allergy-prone immune system. What is that microbe? They’re not sure yet. But it’s not an unreasonable theory.
We know that vaginal birth provides an infant with important microbes that help populate their guts and could impact a child’s overall immune system. Breast feeding also seem to reduce allergies, most likely by stimulating the baby’s immune system, thus preventing the respiratory infections that often serve as triggers for new allergies. And all of this is consistent with the hygiene hypothesis: the idea that exposure to microbes and potential allergens early in life helps prevent allergic diseases. A close relative’s spit is probably a pretty safe way to get a dose of diverse germs.
There was also a prior study from 2013 that showed babies whose parents used spit to clean their pacifiers had fewer cases of asthma, allergies, and eczema. Those researchers hypothesized that something in the saliva was helping to stimulate the babies’ immune systems in a way that prevented them from misidentifying peanuts and pollen and such as dangerous invaders.
So should you, a parent, go sucking on your baby’s pacifier in order to give them a healthier life? No one’s really sure yet. It can’t hurt, assuming you’re not currently sick. And it’s probably safe to let your little one enjoy a pacifier that hasn’t been thoroughly sanitized, as long as it hasn’t fallen somewhere overtly gross. But allergists aren’t yet promoting saliva as the best cleaning agent. Right now their best advice for avoiding allergies is to breastfeed if possible, introduce babies to potentially allergenic foods early on, and not to smoke around your child. Bonus: get a pet, since they help expose babies to potential environmental allergens, thus hopefully preventing an allergy from forming in the first place. If you want to also use your saliva to clean off your baby’s pacifier, the evidence suggests it isn’t a bad idea and might even be a good one. But if the thought appalls you, don’t stress: you’re probably not dooming your child to a life of sniffles and food allergies by withholding your spit. There are other ways to ensure your kids spend their formative days surrounded by a robust variety of microbes.