Growing up around farm animals may protect children from allergies and asthma, a Canadian study presented to the American Thoracic Society says.

To find out whether exposure to livestock affects respiratory problems and allergies, University of British Columbia researcher Helen Dimich-Ward and colleague C. M. Trask surveyed 1,158 4-H Club members, aged 8 to 20.

The members were asked about current and previous residences and residential exposure to livestock, and about any problems with allergies and asthma.

Allergic symptoms were lower among those who lived on farms when the survey was taken or who had lived on farms. The findings were presented on the weekend to the thoracic society's annual international conference in Seattle.

The study confirms similar findings in Europe.

Dr. Dimich-Ward, an epidemiologist who raises llamas on a small farm in the Fraser Valley, says that the earlier studies were surprising and that because she lives on a farm she wanted to test them out herself.

The highest prevalence of allergic symptoms was found in urban or rural residents without livestock. Diagnosed asthma was lowest among those who currently lived on a farm, and former farm dwellers were the least likely ever to have wheezed.

The study, funded by the B.C. Lung Association, says a "protective farm factor" may be the reason. This means that children who grow up on farms are less likely to suffer from asthma and allergies because they have more frequent, higher exposures to endotoxins. Endotoxins, poisonous substances in bacteria, are found in livestock feces, barn and house dust and mattresses.

Scientists also call this the hygiene hypothesis: the idea that a Western lifestyle with lower birthrates, better hygiene and cleaner surroundings decreases childhood exposure to infections that may protect against allergies or asthma. But exposure to endotoxins can aggravate the condition in someone who already has asthma or allergies.

"Endotoxin exposure is an appealing explanation," Dr. Dimich-Ward says. That's true both for the lower allergic symptoms among farm children and for the results of some studies that found fewer allergic conditions in children exposed to dogs or cats at an early age.

The latter studies found that infants who lived in homes with high levels of endotoxins generally did not develop allergies to dust mites, cats, dogs, cockroaches, mice, milk, egg or soy, whereas children in homes with low levels were much more likely to develop those allergies.

The Canadian Lung Association says that about 2.5 million Canadian adults and children have asthma. A chronic lung condition, asthma is the most common chronic respiratory disease of children.

Dr. Dimich-Ward, an associate professor in UBC's department of medicine, says it is not yet absolutely clear that endotoxins are the protective mechanism. Contact with farm animals was not the only factor in her study that appeared to have a protective effect.

"Rather, lower risks for allergic symptoms were associated with living on a farm or [in a] rural area and having livestock currently and at an early age."

A study published last year in the New England Journal of Medicine found that of more than 800 Swiss, German and Austrian children evaluated, asthma and allergy rates were lower among the 319 who grew up on farms.

In 2000, a study published in the American Journal of Respiratory and Critical Care Medicine found that a lower prevalence of asthma persists in farm children, even after accounting for the fact that they smoke less than their urban counterparts.

Dr. Dimich-Ward acknowledges that other factors may explain the findings. One possibility is that children who live on a farm may be less likely to have parents who are allergic, since families subject to allergies might be inclined to leave the farm environment. Relatively few parents of the farm children she surveyed had a history of allergies, she said.