The severity of asthma in children increases with a decreased function of a particular gene due to unclean air, state researchers at Stanford University and the University of California, Berkeley.
The findings come from a study of 181 children with and without asthma in the California cities of Fresno and Palo Alto.
The researchers found that air pollution exposure suppressed the immune system's regulatory T cells (Treg), and that the decreased level of Treg function was linked to greater severity of asthma symptoms and lower lung capacity.
Treg cells are responsible for putting the brakes on the immune system so that it doesn't react to non-pathogenic substances in the body that are associated with allergy and asthma.
When Treg function is low, the cells fail to block the inflammatory responses that are the hallmark of asthma symptoms.
Forty-one participants came from the Fresno Asthmatic Children's Environment Study (FACES), a longitudinal study led by principal investigator Dr. Ira Tager, professor of epidemiology at UC Berkeley's School of Public Health.
The researchers also recruited 30 children from Fresno who did not have asthma.
Fresno was chosen because it is located in California's Central Valley, where trapped hot air mixes with high traffic and heavy agriculture to create some of the highest levels of air pollution in the country.
It is also a region known for its high incidence of asthma: Nearly one in three children there have the condition, earning Fresno the nickname, "The Asthma Capitol of California."
The researchers compared the participants from Fresno with 80 children, half with asthma and half without, in the relatively low-pollution city of Palo Alto, Calif.
The children were matched by age, gender and asthma status, among other variables. The children were tested for breathing function, allergic sensitivity and Treg cells in the blood.
The researchers calculated each child's annual average exposure to polycyclic aromatic hydrocarbons (PAH), a byproduct of fossil fuel and a major pollutant in vehicle exhaust.
The study found that the annual average exposure to PAH was 7 times greater for the children in Fresno compared with the kids in Palo Alto. Levels of ozone and particulate matter were also significantly higher in Fresno.
Not surprisingly, the study found that the children in Fresno had lower overall levels of Treg function and more severe symptoms of asthma than the children in Palo Alto.
The findings was published in the October 2010 issue of the Journal of Allergy and Clinical Immunology