SUNDAY, Feb. 28 (HealthDay News) -- Exposure to unclean conditions early in life doesn't affect a child's risk of developing asthma, researchers say.

"In an earlier study, we found that infants were exposed to high levels of endotoxin and allergens [mite and cockroach] in their day-care centers. Therefore, we were anxious to find out, through a birth cohort study, if infants from low-income families with high risk of asthma might be protected from the development of the disease," lead author Vera E.V. Rullo said in a news release.

The new study included 104 newborns in Brazil who were from low-income families and at high risk for asthma. The researchers assessed respiratory infections, breast-feeding, exposure to allergens and endotoxins (toxins associated with certain bacteria), and wheezing in the children.

By the time the children were 5, about 19 percent of them had persistent wheezing. Only respiratory infection in the first year of life was found to be associated with persistent wheezing. Allergen and endotoxin exposure and breast-feeding for the first six months of life had no effect on the development of asthma, the researchers found.

The study authors also reported that 27 percent of the children had sensitization to a species of house dust mite called D. pteronyssinus, but this had no association with persistent wheezing.

The study was presented Saturday at the American Academy of Allergy, Asthma & Immunology annual meeting, in New Orleans.

More information

The U.S. National Heart, Lung and Blood Institute has more about asthma.