You should expose your children to more dirt?

 Yes. It may be beneficial to let your children get dirty...

(Science Daily)- Infants exposed to rodent and pet dander, roach allergens and a wide variety of household bacteria in the first year of life appear less likely to suffer from allergies, wheezing and asthma, according to results of a study conducted by scientists at the Johns Hopkins Children's Center and other institutions.
Previous research has shown that children who grow up on farms have lower allergy and asthma rates, a phenomenon attributed to their regular exposure to microorganisms present in farm soil. Other studies, however, have found increased asthma risk among inner-city dwellers exposed to high levels of roach and mouse allergens and pollutants. The new study confirms that children who live in such homes do have higher overall allergy and asthma rates but adds a surprising twist: Those who encounter such substances before their first birthdays seem to benefit rather than suffer from them. Importantly, the protective effects of both allergen and bacterial exposure were not seen if a child's first encounter with these substances occurred after age 1, the research found.