Allergies to pollen, furry or feathery animals or house-dust mite, can be triggers for asthma, because they irritate the airways.

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She found that babies born by cesarean section are six times more likely to be sensitive to dust mite allergens than babies born naturally.

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At age four, they were also given a skin-prick test to see if they had any of the most common allergies, including dust mite, animal fur, pollen, milk, eggs or peanuts.

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