Exposing newborns to dust, animal dander and germs may seem abhorrent, but a new study says it can reduce risk of developing allergies and asthma later in life.

Researchers from Johns Hopkins tracked the health of 467 inner-city newborns from Baltimore, Boston, New York and St. Louis over a three-year period.

They also measured the pollution levels in the subjects' homes.

By three years old, children who had been exposed to mouse and cat dander as well as cockroach droppings within their first year of life wheezed significantly less than those who hadn't.

It took exposure to all three allergens, cat, mouse and cockroach, to shape the children's immune responses and this had to happen within the first year to benefit from the protective effect.

"Our study shows that the timing of initial exposure may be critical," says study author Robert Wood, M.D., chief of the Division of Allergy and Immunology at the Johns Hopkins Children's Center. "What this tells us is that not only are many of our immune responses shaped in the first year of life, but also that certain bacteria and allergens play an important role in stimulating and training the immune system to behave a certain way."

Children unexposed to the three allergens in question developed wheezing problems at a rate of 51 percent, while wheezing occurred in only 17 percent of children with exposure.

In the case for good bacteria, normally associated with microbes that inhabit the digestive system, infants living in bacteria-rich homes were less likely to develop environmental allergies later on.

The study was published in the Journal of Allergy and Clinical Immunology.