It isn't clear why some people get asthma and others don't, but scientists believe it is probably due to a combination of environmental and genetic factors.

Dr. Manny Alvarez, senior managing health editor at FoxNews.com, answers this question from a concerned viewer:

Q: Do people who live in cities have asthma more than those who live out in the country?

A: Environment does seem to have some impact on whether a person will develop asthma or not.

Repeated exposure to certain substances, known as asthma triggers, can increase sensitivity of people’s airways and result in asthmatic symptoms over time.

Some of those triggers have to do with where people live, such as:
— Airborne allergens like pollen, animal dander, mold, cockroaches and dust mites

— Cold air

— Air pollution

— Irritants like cigarette smoke and perfumes

Reactions to these triggers can cause the airways to narrow and tighten. Extra mucus is also produced, which makes it even harder to breathe. These factors combined can result in an asthma attack.

Additionally, recent research has shown that children who lived in areas facing intersections with major highways or railways had a 40 to 70 percent increased risk of developing childhood asthma.

“It could be that people who live in larger cities may have a slightly higher risk of developing asthma than those who live in the country,” Alvarez said.