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NITROGEN DIOXIDE AND RESPIRATORY ILLNESS IN CHILDREN
Impact/Purpose:
Nitrogen dioxide, a common indoor and outdoor air pollutant, is a by-product of high-temperature combustion. Motor vehicles and power plants are primarily responsible for the nitrogen dioxide in outdoor air. the U.S. Environmental Protection Agency, which establishes National Ambient Air Quality Standards (NAAQS) for nitrogen dioxide and other air pollutants, has set the NAAQS for nitrogen dioxide as an annual average of 53 parts per billion (ppb) (100 µg/m3). Although the annual average concentrations of nitrogen dioxide are well below 50 ppb in most regions of the United States, the standard is exceeded in areas of southern California, and short-term peaks of 100 ppb, and occasionally 200 ppb, occur in urban areas.
Indoor levels of nitrogen dioxide are often higher than outdoor concentrations, especially in homes where there are unvented heating and cooking appliances that utilize natural gas, kerosene, coal, or wood. Such exposures are of concern because some studies suggest that children exposed to nitrogen dioxide have more respiratory illnesses than those who are not exposed.
Although mild respiratory illnesses in infants and young children are quite common, more serious illnesses can be life-threatening and might increase the risk of developing lung disease later in life. The epidemiologic studies that have examined the role of nitrogen dioxide in childhood respiratory disease have produced inconsistent results, partly because of the difficulty of assessing exposures and measuring respiratory illness in a community setting. This study was conducted to address the limitations of previous studies and to help resolve whether exposure to nitrogen dioxide increases the incidence of duration, or both, or respiratory illness in infants.
Description:
Drs. Jonathan M. Samet, John D. Spengler, and colleagues conducted a prospective investigation of 1,205 health infants living in homes with gas or electric stoves in Albuquerque, NM. Nitrogen dioxide exposures were carefully estimated from repeated measurements in multiple locations in the subjects' homes throughout the entire 18-month observation period. Respiratory illnesses were monitored prospectively using a surveillance system based on daily parental diaries of respiratory signs and symptoms. Parental reports of illness episodes were validated in a subset of the population by comparison with clinical diagnoses and microbiological testing. Potential confounding factors that influence respiratory infections were reduced by selecting subjects whose parents did not smoke or intend to use day-care services outside the home. Rigorous quality assurance procedures were implemented in all phases of the experimental protocol.
The investigators found no association between nitrogen dioxide exposure and the incidence rates for any illness category (upper respiratory illness, lower respiratory illness, lower respiratory illness with wet cough, and lower respiratory illness with wheeze); nor was there any association between illness incidence and the presence of a gas stove. There was also no significant association between nitrogen dioxide levels and the duration of illness for the first three illness categories listed above. However, at the highest nitrogen dioxide exposure category (greater than 40 ppb), there was a nonsignificant increase in the duration of illnesses classified as lower respiratory illness with wheezing. These findings apply to healthy infants, and cannot be generalized to populations who may be more susceptible to the effects of nitrogen dioxide exposure, such as premature babies, babies with low birth weight or respiratory problems, and infants living in homes with parents who smoke. These findings also cannot be generalized to older children or adults.
A key consideration in interpreting these results is the level of nitrogen dioxide to which the children were exposed. The mean nitrogen dioxide levels in the infants' bedrooms in homes with gas stoves were 21 ppb during the winter and 14 ppb in the summer. The mean nitrogen dioxide level in bedrooms of homes with electric stoves was 7 ppb during both seasons. Over 75% of the measured nitrogen dioxide concentrations were less than 20 ppb; 5% were greater than 40 ppb. These levels, although similar to levels observed in many locations in the United States, are lower than nitrogen dioxide concentrations in heavily polluted cities and in poorly ventilated inner-city apartments. Thus, the study results cannot be generalized to infants or toddlers exposed to levels of nitrogen dioxide substantially in excess of 40 ppb.
The investigators' observations are important because they indicate that in a population of healthy infants and toddlers, no significant associations between nitrogen dioxide exposure (in the range of 0 to 40 ppb) and respiratory illness were found when every precaution was taken to make an accurate assessment of the pollutant exposures, to validate the measurements of respiratory illness, to eliminate potentially confounding variables such as exposure to environmental tobacco smoke, and to adjust for the variables that could not be eliminated.