LOS ANGELES, May 21 (Xinhua) -- The asthma epidemic in children is linked to a rise in cigarette use by adults, according to a new study by U.S. researchers.

    In the study, researchers at the U.S. Mailman School of Public Health (MSPH) have identified parallel increases in childhood asthma and cigarette use among adults during the past century in the United States.

    These parallel trends suggest that the increase in cigarette use may be a contributing factor to the rise in asthma among children during the same period through increased exposure to environmental tobacco smoke, said study author Renee D. Goodwin, assistant professor at the Department of Epidemiology at the MSPH.

    The prevalence of asthma has increased at least threefold during the past several decades, but the cause for this remains unknown, said the study published in the May issue of the Annals of Allergy, Asthma & Immunology, the scientific journal of the American College of Allergy, Asthma and Immunology (ACAAI).

    The increase is occurring most prominently in industrialized countries, but now developing countries are beginning to experience similar increases, said the study.

    Approximately 4.8 million children under the age of 18 have asthma in the United States. Although treatment and asthma management strategies can help control symptoms, asthma is a chronic condition with no known cure.

    Asthma most frequently begins in childhood. The cause is unknown, but allergies are a factor in the majority of children with asthma.

    Both genetic and environmental risk factors for asthma have been identified, noted Dr. Goodwin. Globally, six studies have shown environment tobacco smoke to be a risk factor of incident asthma.

    The risk for the development of childhood asthma was 2.5 times greater in young children with mothers who smoke more than 10 cigarettes per day indoors compared with mothers who smoke fewer cigarettes or not at all.

    Exposure to environmental tobacco smoke, especially in the home, increases a child's likelihood of developing asthma by 63 percent, according to the U.S. Department of Health and Human Services.

    Scientists believe that environmental tobacco smoke (ETS) inhaled unintentionally by non-smokers has a higher concentration of some toxic substances than the smoke inhaled by smokers, such as carbon monoxide and carbon dioxide.

    Children breathe more air than adults and have narrower airways, so ETS is a greater causal risk factor of asthma in children. It can also increase the severity of their asthma symptoms.