A new study suggests that parents who wash their dishes by hand, rather than in a dishwashing machine, may unwittingly lower the likelihood that their children will develop allergies.

The new study, published in the journal Pediatrics, looked at whether a number of behaviors that expose children to bacteria early in life might protect them from developing allergies later on. The idea, known as the hygiene hypothesis, is that children raised in particularly sanitized environments are less likely to develop an immune tolerance to trivial threats.

The researchers followed roughly 1,000 young children and their parents living in the Gothenburg area of Sweden. They investigated behaviors like whether the parents fed their children foods purchased directly from farms, such as eggs, meat and unpasteurized milk. They looked into whether the children ate fermented foods, which have beneficial probiotic bacteria. And they looked into whether the parents washed their dishes by hand or used a dishwasher.

Then they examined whether the children had allergic conditions including asthma, eczema and hay fever. Ultimately, the researchers found that children raised in households where dishes were always washed by hand had half the rate of allergies. They also discovered that this relationship was amplified if the children also ate fermented foods or if the families bought food directly from local farms.

The findings demonstrated only an association, not cause and effect, so it was not clear whether these behaviors directly led to fewer allergies. But it may be the case that these behaviors expose children to innocuous bacteria, which can help strengthen their immune systems, said Bill Hesselmar, an assistant professor at the University of Gothenburg and lead author of the study.

Dr. Hesselmar said that while the sanitizing effect of dishwashing machines can be a good thing, the “less efficient” method of washing dishes by hand might leave behind some bacteria that could have benefits. But he said more research was needed to see if the relationship they found was real and, if so, what was causing it.

“It’s an interesting finding and very surprising,” he said. “But we have to see if we can confirm it.”

-Anahad O’Connor Feb 23, 2015