The cause was believed to be higher exposure to house dust mites and greater cat ownership in the West, she said. The East is quickly catching up as conditions there become better, with greater sensitization to various allergens, and consequently more asthma. Other factors that may affect or exacerbate sensitivity to allergens include industrial pollution and smoking, she said. If people are exposed to disease, their immune systems may help protect against allergy. A few studies have suggested that fetuses can be sensitized to allergens, said Waserman.
Maternal smoking is associated with a greater risk of allergy at birth.
One study of animals showed that smoking can cause greater penetration of certain allergens, such as house dust mites, into the lungs. Not by a long shot. Two studies on children and animals turned up some surprising findings, she said. One study compared children raised on a farm to children in the city. Another study showed that children exposed to cats over a lifetime had less sensitivity to cats than those whose families didn't have them.