In the past, studies have shown an increased asthma risk among kids who live in poorer areas of cities. Which makes a lot of sense-- the air quality’s worse and there are more rats, roaches, and allergens to contend with. But now, a new study has shaken things up by showing that kids who live in too clean of homes are more, not less, likely to develop asthma and allergies.

“What we found was somewhat surprising and somewhat contradictory to our original predictions,” said Dr. Robert Wood, co-author of the new study and head of the Johns Hopkins Children’s Center’s Division of Allergy and Immunology.

The study tracked 467 inner-city kids from Baltimore, Boston, New York City, and St. Louis from before they were born until age 3. The researchers went to each child’s home to measure the amount and types of allergens, even collecting dust to analyze for bacterial content.

They found that 41% of the kids who were completely asthma and allergy-free grew up in homes with lots of allergens/bacteria, whereas a mere 8% of the kids who had asthma or allergies grew up in homes with lots of allergens/bacteria.

These results support the hygiene hypothesis, a popular theory that says the rise in autoimmune diseases in Western nations is caused by a lack of exposure to various microbes.

Dr. Wood was careful to point out that these findings still need to be verified by other studies and should not be used as the basis for any decision making on the part of parents. “Please don’t get an intentional cockroach infestation in your house. There’s no reason to think that would help,” he said. Still, he hopes that with further study the findings might one day lead to new treatments helping to prevent the development of asthma and allergies.