N[O.sub.2]--the inside story: by-product may be a problem. (Science Selections).

Due to its well-documented adverse health effects, including throat irritation, cough, and difficulty breathing, nitrogen dioxide (N[O.sub.2]) has long been a prime focus of scientific investigations into indoor air pollution. Nitrous acid (HONO), a by-product of N[O.sub.2], is less well-studied in terms of human health effects of low exposures. But it has chemical characteristics that suggest that it, too, is a potential respiratory irritant and that its presence in the indoor environment is worthy of study. In this month's issue, Kiyoung Lee of the University of California at Davis School of Medicine and his coauthors investigate indoor HONO levels as well as the relationship between indoor concentrations of HONO, N[O.sub.2], and ozone [EHP 110:145-149]. Their efforts clearly indicate that HONO is indeed potentially an important factor in indoor air pollution.

Gas combustion--simply cooking on a gas range--is known to be a major source of N[O.sub.2] in the residential environment. HONO, too, is formed by gas combustion, and is also a by-product of N[O.sub.2] reactions with water on indoor surfaces. So its concentration in the indoor environment is affected by several variables, including N[O.sub.2] concentrations, ventilation, surface materials, available surface areas, temperature, and humidity. Ozone concentrations indoors appear to be determined largely by outdoor levels.

In April and May 1996, the team measured indoor and outdoor HONO, N[O.sub.2], and ozone at 119 Southern California homes. Fifty-seven of the houses were located in a valley area, and 62 were in a mountainous region. The varied nature of the outdoor environment of the houses, as well as certain characteristics of the homes themselves, such as the presence of an air conditioner, gas range, or humidifier, were thought to be significant factors contributing to the levels of the three pollutants and the relationships among them.

Analysis of the air samples taken over a six-day period both inside and outside the houses showed that average indoor HONO concentrations were higher than outdoor concentrations, which was unsurprising, given that HONO quickly breaks down outdoors in the presence of sunlight. Indoor N[O.sub.2] concentrations were also found to be significantly higher than outdoor concentrations, while indoor ozone concentrations were significantly lower than those outdoors. Indoor HONO concentrations correlated positively with indoor N[O.sub.2] concentrations, averaging about 17% of N[O.sub.2] levels, but correlated inversely with indoor ozone concentrations.

The associations explored by the researchers shed light on some of the factors contributing to what they describe as "substantial" indoor HONO concentrations. Each house's location was found to be associated with certain relationships among the three contaminants. Indoor and outdoor N[O.sub.2] levels (and therefore HONO levels) were lower in the mountain sites, which had higher ozone levels, leading the authors to speculate that "it is possible that the association between community and the three air pollutants may be different when ambient ozone levels are different." The opposite was true of valley sites, which had higher N[O.sub.2] and HONO levels and lower ozone levels.

Homes with an air conditioner and a gas range had higher indoor N[O.sub.2] concentrations, and homes with higher N[O.sub.2] also had higher HONO levels, but the investigators could not establish a direct relationship between these housing factors and high HONO levels. Similarly, homes with humidifiers had lower HONO levels, which was an anomaly in that the increased moisture on surface areas would have been expected to result in a concomitant increase in HONO.

The study represents one of the first times that indoor HONO concentrations have been measured at all. Although the authors did not find clear evidence to account for the substantial levels of indoor HONO they discovered, their efforts do indicate that the levels of HONO found make it potentially an important factor in indoor air pollution, and that further research into the complex interrelationships among the pollutants and the indoor and outdoor environments is warranted.