Nitrous Acid Formation in an Experimental Exposure Chamber

Reactive indoor chemistry has seldom been considered in investigations of the health effects of exposure to indoor contaminants although improved understanding may help to reduce exposure misclassification as well as identify previously unknown exposures. Nitrous acid (HONO), formed from the heterogeneous reaction of NO₂ with water on indoor surfaces, may be a neglected and important confounder in studies of nitrogen dioxide (NO₂) health effects. The understanding of HONO – NO₂ relationships is important since conventional measurements of NO₂ actually measure a number of nitrogen oxides including HONO. Accordingly, we examined the effects of relative humidity, chamber residence time, the presence of carpeted sufaces and the presence of human subjects on the production of HONO following the injection of NO₂ into a chamber. Increasing chamber relative humidity led to greater HONO concentrations at a given NO₂ level. At 80% relative humidity, HONO concentrations were 11% of the NO₂ concentration. Increased residence time in the chamber increased HONO levels, with a five-fold increase in HONO levels between 20 minute and 120 minute residence times. The presence of wool carpets in the chamber was not found to affect significantly the HONO production rates, although NO₂ decay rates were increased. When human subjects were present in the chamber, HONO concentrations decreased by as much as 50% of the levels present under similar conditions with no subjects in the chamber. In light of our results we argue that future investigations of NO₂ health effects and indoor air quality must recognize the importance of indoor chemistry, and in particular, the likelihood of elevated concentrations of HONO which will interfere with NO₂ measurements and potentially confound the investigation of concentration-effect relationships. Similarly, previous studies demonstrating a relationship between health endpoints and NO₂ exposure should be viewed with caution until the presence and importance of other potentially toxic nitrogenous compounds in the exposure environment are evaluated.