No one knows exactly what causes asthma. Certain triggers cause asthmatics' airways to swell (inflame) and narrow, resulting in sticky mucus buildup. Asthma may be caused by genetics, pollution, or modern hygiene standards, but there is not enough evidence to know for sure. Certain factors seem to increase the risk of asthma in some individuals. People with asthmatic parents are three to six times more likely to develop asthma than those without asthmatic parents. Asthmatics are more likely to suffer from certain allergic conditions, such as atopic dermatitis (eczema) and allergic rhinitis (hay fever). Infancy and childhood can be affected by respiratory problems. It is possible for children to develop chronic asthma as a result of viral respiratory infections. You may experience asthma symptoms at work if you have asthma due to exposure to certain elements. Some people can also develop asthma for the first time as a result of exposure to certain dust (industrial or wood dust), chemical fumes, and molds. The airways are irritated by cigarette smoke. Those who smoke are at a higher risk of developing asthma. People exposed to secondhand smoke or whose mothers smoked during pregnancy are also more likely to suffer from asthma. Read more about the health effects of smoking on asthmatics. Asthma risks are increased by exposure to ozone (the primary component of smog). There is a greater risk of asthma in people who were born or raised in urban areas. Overweight or obese children and adults are more susceptible to developing asthma than those at a healthy weight. Experts point to low-grade inflammation in the body caused by excess weight as possible explanations. Compared to patients in a healthy weight range, obese patients require more medication, suffer worse symptoms, and have less control over their asthma.