A concrete factory discharges pollution from its four smoke stacks just outside of Beijing, China November 2, 2006. China has set up a group to monitor air quality in Beijing and four regions around the capital as part of efforts to ensure the 2008 Olympics are pollution free, state press has reported. China's cities are ranked by international organizations, such as the United Nations and the World Bank, as among the top 10 most polluted cities in the world. (UPI Photo/Stephen Shaver)

Using data collected from the National Institute of Allergy and Infectious Diseases Inner-City Asthma Study, researchers examined 861 children with persistent asthma, ages 5 to 12, living in low-income areas in seven U.S. inner-city communities: Boston, the Bronx in New York, Chicago, Dallas, New York City, Seattle and Tucson.

For more than two years, the researchers regularly monitored the children's asthma symptoms, breathing function, school absences and obtained daily outdoor pollution measurements.

The study, published in the Journal of Allergy and Clinical Immunology, revealed that children had significantly decreased lung function following exposure to higher concentrations of the air pollutants sulfur dioxide, airborne fine particles and nitrogen dioxide.

Higher nitrogen dioxide levels and higher levels of fine particles also were associated with school absences related to asthma. Higher nitrogen dioxide levels were associated with more asthma symptoms.

Nitrogen dioxide is derived mainly from motor vehicle exhaust and the findings provide evidence that car emissions may be causing adverse respiratory health effects on urban children with asthma, the study said.