Bad news for mothers around the world: in a report published this week in the journal Pediatrics, researchers in Sweden found that in households where dishes are washed by hand there were fewer cases of eczema, asthma, and hay fever than in households where dishes were washed and sterilized in dishwashers.
Deborah Netburn of The Seattle Times writes that leaving some microbes on plates, utensils, and cookware may help reduce the risk of allergies in young children. The study also found that adding fermented foods, like sauerkraut and pickles, to a child’s diet, along with eggs and milk purchased from local farms, can also reduce a child’s risk of developing allergies.
These results are part of what is called the hygiene hypothesis, or the idea that extreme cleanliness is responsible for the increase in allergies. When children are exposed to germs early in their childhood this stimulates the immune system.
Researchers based their findings on a questionnaire filled out by parents of 1,029 Swedish children between the ages of 7 and 8. Eczema was reported in 23% of children in households where parents washed dishes and in 38% of homes where a dishwasher was used. Asthma was reported in 1.7% of hand-washed dishes households and 7.3% for those who used dishwashers.
A term used in the study was “traditional cooking”, meaning washing dishes by hand, eating fermented food, and buying from farm to table. They found that 19% of kids from these households reported having allergies compared to 46% of kids from non-traditional households.
Scientists are quick to point out that the findings were only an association, not cause and effect, so it could not be proven that these behaviors directly led to fewer allergies. However, it may be that these behaviors expose children to innocuous bacteria, which in turn can help strengthen the immune systems, according to Bill Hesselmar, an assistant professor at the University of Gothenburg and lead author of the study.
Anahad O’Connor of The New York Times writes that Dr. Hesselmar admits that sanitizing cookware and tableware could be a good thing, but leaving behind some bacteria by hand-washing could have its benefits – and that more research is needed.
“It’s an interesting finding and very surprising,” said Hesselmar. “But we have to see if we can confirm it.”
Rob Stein, reporting for NPR, cautions his audience not to rip out their dishwashers just yet. These findings seem to support the ever-evolving proposition that people in developed countries are growing up too clean because of things like hand sanitizers, detergents, and pet-free homes. When children are not exposed to enough bacteria and other microorganisms, it might deprive the immune system of the opportunity to recognize good germs from bad.
“The hypothesis was that these different dish-washing methods … are not equally good in reducing bacteria from eating utensils and so on,” Hesselmar says. “So we thought that perhaps hand dish-washing was less effective, so that you are exposed to more bacteria” in a way that’s helpful.