Allergic diseases are less prevalent in children whose families usually wash dishes by hand instead of with a dishwasher, suggests an observational Pediatrics study.
Roughly 1000 Swedish parents of children aged 7–8 years completed questionnaires about various household habits and the child’s history of eczema, asthma, and allergic rhinoconjunctivitis. Children were at lower risk for total allergic disease development if their families reported usually handwashing dishes (adjusted odds ratio, 0.57). The association was stronger when families also reported regularly eating fermented food and buying food directly from farms.
The authors speculate that “these lifestyle factors reduce allergy development via increased or more diverse microbial exposure, stimulating the immune system to develop in a more tolerant direction.”
Editorialists note that the fact that the study’s subjects were older does not fit well with the hygiene hypothesis, which suggests that microbial exposure earlier in life (e.g., <6 months) is important for a protective effect. The study didn’t assess whether household practices changed over time.

Pediatrics article http://pediatrics.aappublications.org/content/early/2015/02/17/peds.2014-2968