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Letting your baby get dirty could keep them healthy, researchers have claimed.

Babies who are exposed to household and even rodent bacteria in the first year of their life were found to be far less likely to develop allergies, wheezing and asthma.

Researchers say the timing of the exposure was key - with the immune system being 'set' by the age of one.

Babies who were exposed to household and even rodent bacteria in the first year of their life were found to be far less likely to develop allergies, wheezing and asthma.

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, conducted by scientists at the Johns Hopkins Children's Center and other institutions, 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.

A report on the study, published on June 6 in the Journal of Allergy and Clinical Immunology, reveals that early exposure to bacteria and certain allergens may have a protective effect by shaping children's immune responses — a finding that researchers say may help inform preventive strategies for allergies and wheezing, both precursors to asthma.

early exposure to bacteria and certain allergens may have a protective effect by shaping children¿s immune responses

'Our study shows that the timing of initial exposure may be critical,' said author Robert Wood.

'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.'

The study was conducted among 467 inner-city newborns from Baltimore, Boston, New York and St. Louis whose health was tracked over three years.

The investigators visited homes to measure the levels and types of allergens present in the infants' surroundings and tested them for allergies and wheezing via periodic blood and skin-prick tests, physical exams and parental surveys. In addition, the researchers collected and analyzed the bacterial content of dust collected from the homes of 104 of the 467 infants in the study.

Infants who grew up in homes with mouse and cat dander and cockroach droppings in the first year of life had lower rates of wheezing at age 3, compared with children not exposed to these allergens soon after birth.

The protective effect, moreover, was additive, the researchers found, with infants exposed to all three allergens having lower risk than those exposed to one, two or none of the allergens.

Specifically, wheezing was three times as common among children who grew up without exposure to such allergens (51 percent), compared with children who spent their first year of life in houses where all three allergens were present (17 percent).

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.

In addition, infants in homes with a greater variety of bacteria were less likely to develop environmental allergies and wheezing at age 3.

When researchers studied the effects of cumulative exposure to both bacteria and mouse, cockroach and cat allergens, they noticed another striking difference.

Children free of wheezing and allergies at age 3 had grown up with the highest levels of household allergens and were the most likely to live in houses with the richest array of bacterial species.

Some 41 percent of allergy-free and wheeze-free children had grown up in such allergen and bacteria-rich homes.

By contrast, only 8 percent of children who suffered from both allergy and wheezing had been exposed to these substances in their first year of life.