Asthma Healing

Concerns over cleanliness continue to grow, but so does research showing that too much clean can be a bad thing -- at least for allergies. Researchers found that children who grow up in large families or go to day care may have fewer allergies, but more asthma as adults.

A new study shows that early exposure to many children at home or at play can have a lasting influence on respiratory health, but researchers are still unsure exactly what's responsible for those effects -- both good and bad.

Researchers interviewed more than 18,000 adults from 36 countries, including the U.S., Australia, and much of Europe. Blood samples from nearly 14,000 of the participants were analyzed for antibodies that indicate when a person may be at risk for allergies to common triggers such as dust mites, pet dander, and pollens.

They found that allergies and hay fever were less common among people who had many siblings or those who attended day care.

But the same protective effect was not found for asthma. Asthma was more common among children with more than two siblings and children enrolled in day care.

The results of the study appear in the current issue of the journal Thorax.

Although the allergy finding supports the popular "hygiene hypothesis," which holds that close contact with other children early in life programs the immune system to be less sensitive to potential allergy triggers, the heightened asthma risk found by the study suggests other factors may be at work.

The study researchers say that the same early exposure to bacteria from other children that protects them from allergies may cause permanent damage to the lungs and respiratory system and increase the risk of asthma.

In addition, researchers found that the protective effect of day care was found only among those children without siblings.

This suggests that in children who are already exposed to children in the family, exposure to more children in day care provides no further protection, according to researcher Cecilie Svanes, MD, of the department of medicine at Haraldsplass Hospital in Bergen, Norway.