Allergen-Free Homes Won't Stop Kids' Asthma

Study counters belief that it results from dander, dust exposure

THURSDAY, Sept. 30, 2004 (HealthDayNews) -- Reducing the levels of cat dander and dust mites in your home won't prevent your child from getting asthma, British researchers report.

This finding counters the common belief that asthma is caused by early exposure to allergens. Instead, asthma is caused by a combination of genetic and environmental factors, according to the report in the October issue of the British journal Thorax.

In fact, this finding adds to the growing body of evidence that early exposure to allergens might actually protect children from developing asthma.

"Among a group of very ordinary children born in a small town in the U.K. and followed from birth to the age of 5 to 6, levels of dust mite and cat allergens in the home were not related to their development of allergies or asthma," said lead author Dr. Paul Cullinan, a reader in respiratory epidemiology at the Imperial College School of Medicine's National Heart and Lung Institute in London.

"The most important factors were having a parent with asthma and being the first-born child in the family," he added.

In their study, Cullinan's team collected data on 625 children. The children were followed from birth. When they were 5.5 years-old, 552 were tested for allergic reactions to house dust mites, cat dander and grass pollen.

The researchers also asked the mothers whether their children had any episodes of wheezing in the past year. In addition, when the children were 8 weeks old, Cullinan's group measured the levels of allergens and dust samples taken from each home.

Testing showed that one of every 10 children was allergic to dust mites or cat dander, and that one in 14 had wheezing in the past year.

However, the researchers did not find any link between the levels of household allergens and allergies or wheezing. They did find that allergic reactions and wheezing could occur when allergen levels were low.

They also found that children whose fathers were susceptible to allergies were significantly more likely to show signs of asthma. In addition, asthma was more likely to be present in first-born children.

Based on their findings, the researchers don't see the need to reduce allergen levels to prevent asthma.

"Clinically, there seems little point in reducing allergen levels in the home as a way of preventing asthma and allergies; indeed it may even increase the risk," Cullinan said. "The issue of being the first child is intriguing. We suspect it is something to do with the way a woman's immune system deals with a first pregnancy."

"Don't be worried about dust and cats in the home as a means of preventing your child getting asthma," Cullinan advised -- cautioning, however, that "it may be a different matter if your child already has asthma." For these children, exposure to household dust mites and cat dander may trigger attacks.

There does appear to be, at very low allergen levels, sensitization and the development of tolerance to allergens, said Dr. Bruce P. Lanphear, the director of the Cincinnati Children's Environmental Health Center and a professor of pediatrics and environmental health at Cincinnati Children's Hospital Medical Center.

"This is important because it might modify how we approach prevention efforts for children," Lanphear said. "It is possible that lowering allergen levels, and then having them go up again, might cause some kids to have more severe asthma."

The finding "raises questions about the complexity of reducing allergen exposure and preventing children from developing asthma or controlling asthma," Lanphear said. "Understanding and controlling environmental risk factors for asthma is not going to be simple."

More information

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

SOURCES: Paul Cullinan, M.D., reader, respiratory epidemiology, Imperial College School of Medicine, National Heart and Lung Institute, London; Bruce P. Lanphear, M.D., M.P.H., director, Cincinnati Children's Environmental Health Center, and professor, pediatrics and environmental health, Cincinnati Children's Hospital Medical Center; October 2004 Thorax
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