According to the authors, a prior study in 2005 showed that 20 parts per billion was the average concentration of indoor nitrogen dioxide reported in an inner-city population.
At the end of the study enrollment period, a trained research assistant visited each home and collected extensive information from the study participant's mother on the family's ethnicity, education, smoking history, house characteristics and use of a household appliance fueled by natural gas. Mothers were asked the number of days during the preceding month that their child experienced respiratory symptoms. They were also questioned about medications the child took for both asthma attacks and disease maintenance.
"Children using maintenance medications were more likely to have respiratory symptoms than children who did not, indicating more severe asthma," said Dr. Belanger.
Nitrogen dioxide levels were measured in each participant's main living area for 10 to 14 days after the initial enrollment visit.
According to the authors, there are currently no U.S. standards for indoor levels of nitrogen dioxide. Moreover, the levels associated with significant health effects among the children in multi-family housing are similar to the outdoor average exposure of 21 parts per billion recommended by the World Health Organization, but below the peak exposure of 106 parts per billion set by the same organization.