The nitrogen dioxide levels inside the homes investigated in the study were well below the Environmental Protection Agency's outdoor standards of 53 parts per billion.
The study results were detailed in the first issue for February 2006 of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.
Kathleen Belanger, Ph.D., of the Yale Center for Perinatal, Pediatric and Environmental Epidemiology in New Haven, Connecticut, along with four associates, studied 728 children with asthma, younger than 12, who were exposed to nitrogen dioxide in their home.
According to the authors, the 2000 U.S. Census indicated that more than half of U.S. households use gas and that the primary source for residential nitrogen dioxide levels was a gas-fueled cooking appliance.
In the study, the average level for nitrogen dioxide in homes with electric stoves was 8.6 parts per billion; in homes with gas stoves, the level was 25.9 parts per billion.
Among the 242 multi-family housing units studied, 111 (45.9 percent) had nitrogen dioxide concentrations greater than 20 parts per billion. Of the 486 single-family housing units examined, only 45 (9.3 percent) were above 20 parts per billion.
"The association between nitrogen dioxide exposure and respiratory symptoms was limited to children in multi-family housing," said Dr. Belanger. "To date, this is the largest study to examine the effects of nitrogen dioxide on children with asthma. The study population was quite diverse and included both white and non-white children living in single-family and multi-family homes, and children living in urban and suburban environments."
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.
Contact: Kathleen Belanger, Ph.D., Deputy Director, Yale Center for Perinatal, Pediatric, and Environmental Epidemiology, Yale School of Medicine, One Church Street, 6th floor, New Haven, Connecticut 06510 Phone: 203-764-9375 E-mail: firstname.lastname@example.org